Preface

About This Document

The goal of this document is to provide you with an understanding of the C++/Serializer programming model and allow you to efficiently evaluate XSD/e against your project's technical requirements. As such, this document is intended for embedded C++ developers and software architects who are looking for an embedded XML processing solution. Prior experience with XML and C++ is required to understand this document. Basic understanding of XML Schema is advantageous but not expected or required.

More Information

Beyond this guide, you may also find the following sources of information useful:

• XSD/e Compiler Command Line Manual
• The INSTALL file in the XSD/e distribution provides build instructions for various platforms.
• The examples/cxx/serializer/ directory in the XSD/e distribution contains a collection of examples and a README file with an overview of each example.
• The xsde-users mailing list is the place to ask technical questions about XSD/e and the Embedded C++/Serializer mapping. Furthermore, the archives may already have answers to some of your questions.

1 Introduction

Welcome to CodeSynthesis XSD/e and the Embedded C++/Serializer mapping. XSD/e is a dependency-free W3C XML Schema to C++ compiler for mobile, embedded, and light-weight applications. Embedded C++/Serializer is an XML Schema to C++ mapping that represents an XML vocabulary as a set of serializer skeletons which you can implement to perform XML serialization as required by your application logic.

1.1 Mapping Overview

The Embedded C++/Serializer mapping provides event-driven, stream-oriented XML serialization, XML Schema validation, and C++ data binding. It was specifically designed and optimized for mobile and embedded systems where hardware constraints require high efficiency and economical use of resources. As a result, the generated serializers are 2-10 times faster than general-purpose validating XML serializers while at the same time maintaining extremely low static and dynamic memory footprints. For example, a validating serializer executable can be as small as 60KB in size. The size can be further reduced by disabling support for XML Schema validation.

The generated code and the runtime library are also highly-portable and, in their minimal configuration, can be used without STL, RTTI, iostream, C++ exceptions, and C++ templates.

To speed up application development, the C++/Serializer mapping can be instructed to generate sample serializer implementations and a test driver which can then be filled with the application logic code. The mapping also provides a wide range of mechanisms for controlling and customizing the generated code.

The next chapter shows how to create a simple application that uses the Embedded C++/Serializer mapping to validate and serialize simple data to an XML document. The following chapters describe the Embedded C++/Serializer mapping in more detail.

1.2 Benefits

Traditional XML serialization APIs such as Document Object Model (DOM) or XML Writer as well as general-purpose XML Schema validators have a number of drawbacks that make them less suitable for creating mobile and embedded XML processing applications. These drawbacks include:

• Text-based representation results in inefficient use of resources.
• Extra validation code that is not used by the application.
• Generic representation of XML in terms of elements, attributes, and text forces an application developer to write a substantial amount of bridging code that identifies and transforms pieces of information produced by the application logic to the text encoding used in XML.
• Resulting applications are hard to debug, change, and maintain.

In contrast, statically-typed, vocabulary-specific serializer skeletons produced by the Embedded C++/Serializer mapping use native data types (for example, integers are passed as integers, not as text) and include validation code only for XML Schema constructs that are used in the application. This results in efficient use of resources and compact object code.

Furthermore, the serializer skeletons allow you to operate in your domain terms instead of the generic elements, attributes, and text. Automatic code generation frees you for more interesting tasks (such as doing something useful with the information that needs to be stored in XML) and minimizes the effort needed to adapt your applications to changes in the document structure. To summarize, the C++/Serializer mapping has the following key advantages over generic XML serialization APIs:

• Ease of use. The generated code hides all the complexity associated with recreating the document structure, maintaining the state, and converting the data from types suitable for manipulation by the application logic to the text representation used in XML.
• Natural representation. The generated serializer skeletons implement serializer callbacks as virtual functions with names corresponding to elements and attributes in XML. As a result, you serialize the data using your domain vocabulary instead of generic elements, attributes, and text.
• Concise code. With a separate serializer skeleton for each XML Schema type, the application implementation is simpler and thus easier to read and understand.
• Safety. The data is passed by serializer callbacks as statically typed objects. The serializer callbacks themselves are virtual functions. This helps catch programming errors at compile-time rather than at runtime.
• Maintainability. Automatic code generation minimizes the effort needed to adapt the application to changes in the document structure. With static typing, the C++ compiler can pin-point the places in the application code that need to be changed.
• Efficiency. The generated serializer skeletons use native data types and combine validation and data-to-text conversion in a single step. This makes them much more efficient than traditional architectures with separate stages for validation and data conversion.

2 Hello World Example

In this chapter we will examine how to create a very simple XML document using the XSD/e-generated C++/Serializer skeletons. All the code presented in this chapter is based on the hello example which can be found in the examples/cxx/serializer/ directory of the XSD/e distribution.

2.1 Writing Schema

First, we need to get an idea about the structure of the XML document that we are going to create. The sample XML that we will try to produce with our Hello application looks like this:

<hello>

  <greeting>Hello</greeting>

  <name>sun</name>
  <name>moon</name>
  <name>world</name>

</hello>
      

Then we can write a description of the above XML in the XML Schema language and save it into hello.xsd:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:complexType name="hello">
    <xs:sequence>
      <xs:element name="greeting" type="xs:string"/>
      <xs:element name="name" type="xs:string" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  <xs:element name="hello" type="hello"/>

</xs:schema>
      

Even if you are not familiar with the XML Schema language, it should be easy to connect declarations in hello.xsd to elements in the sample XML document above. The hello type is defined as a sequence of the nested greeting and name elements. Note that the term sequence in XML Schema means that elements should appear in a particular order as opposed to appearing multiple times. The name element has its maxOccurs property set to unbounded which means it can appear multiple times in an XML document. Finally, the globally-defined hello element prescribes the root element for our vocabulary. For an easily-approachable introduction to XML Schema refer to XML Schema Part 0: Primer.

The above schema is a specification of our vocabulary; it tells everybody what valid XML instances of our vocabulary should look like. The next step is to compile this schema to generate C++ serializer skeletons.

2.2 Translating Schema to C++

Now we are ready to translate our hello.xsd to C++ serializer skeletons. To do this we invoke the XSD/e compiler from a terminal (UNIX) or a command prompt (Windows):

$ xsde cxx-serializer hello.xsd
  

The XSD/e compiler produces two C++ files: hello-sskel.hxx and hello-sskel.cxx. The following code fragment is taken from hello-sskel.hxx; it should give you an idea about what gets generated:

class hello_sskel
{
public:
  // Serializer callbacks. Override them in your implementation.
  //
  virtual void
  pre ();

  virtual std::string
  greeting () = 0;

  virtual bool
  name_next () = 0;

  virtual std::string
  name () = 0;

  virtual void
  post ();

  // Serializer construction API.
  //
  void
  greeting_serializer (xml_schema::string_sskel&);

  void
  name_serializer (xml_schema::string_sskel&);

  void
  serializers (xml_schema::string_sskel& /* greeting */,
               xml_schema::string_sskel& /* name */);

private:
  ...
};
  

The first five member functions shown above are called serializer callbacks. You would normally override them in your implementation of the serializer. Let's go through all of them one by one.

The pre() function is an initialization callback. It is called when a new element of type hello is about to be serialized. You would normally use this function to initialize data structures, such as iterators, which will be used during serialization. As we will see in subsequent chapters, there is also a way to pass an argument to this function which may be useful if you are serializing an in-memory data structure to XML. The default implementation of the initialization callback does nothing.

The post() function is a finalization callback. It is called when serialization of the element is completed. If necessary, you can use this function to perform cleanups of data structures initialized in pre() or during serialization. The default implementation of the finalization callback also does nothing.

The greeting() and name() functions are called when the greeting and name elements are about to be serialized and the values for these elements need to be provided. Because the name element can be repeated several times (note the maxOccurs="unbounded" attribute in the schema), the serializer skeleton also has the name_next() function which is called before name() to check if another name element needs to be serialized.

The last three functions are for connecting serializers to each other. For example, there is a predefined serializer for built-in XML Schema type string in the XSD/e runtime. We will be using it to serialize the values of greeting and name elements, as shown in the next section.

2.3 Implementing Application Logic

At this point we have all the parts we need to create our sample XML document. The first step is to implement the serializer:

#include <string>
#include <vector>
#include "hello-sskel.hxx"

struct hello_simpl: hello_sskel
{
  hello_simpl ()
  {
    names_.push_back ("sun");
    names_.push_back ("moon");
    names_.push_back ("world");
  }

  virtual void
  pre ()
  {
    i_ = names_.begin ();
  }

  virtual std::string
  greeting ()
  {
    return "Hello";
  }

  virtual bool
  name_next ()
  {
    return i_ != names_.end ();
  }

  virtual std::string
  name ()
  {
    return *i_++;
  }

private:
  typedef std::vector<std::string> names;

  names names_;
  names::iterator i_;
};
  

We use the hello_simpl's constructor to initialize a vector of names. Then, in the pre() initialization callback, we initialize an iterator to point to the beginning of the names vector. The greeting() callback simply returns the string representing our greeting. The name_next() callback checks if we reached the end of the names vector and returns false if that's the case. The name() callback returns the next name from the names vector and advances the iterator. Note that name() is not called if name_next() returned false. Finally, we left post() with the default implementations since we don't have anything to cleanup.

Now it is time to put this serializer implementation to work:

#include <iostream>

using namespace std;

int
main ()
{
  try
  {
    // Construct the serializer.
    //
    xml_schema::string_simpl string_s;
    hello_simpl hello_s;

    hello_s.greeting_serializer (string_s);
    hello_s.name_serializer (string_s);

    // Create the XML document.
    //
    xml_schema::document_simpl doc_s (hello_s, "hello");

    hello_s.pre ();
    doc_s.serialize (cout, xml_schema::document_simpl::pretty_print);
    hello_s.post ();
  }
  catch (const xml_schema::serializer_exception& e)
  {
    cerr << "error: " << e.text () << endl;
    return 1;
  }
}
  

The first part of this code snippet instantiates individual serializers and assembles them into a complete vocabulary serializer. xml_schema::string_simpl is an implementation of a serializer for built-in XML Schema type string. It is provided by the XSD/e runtime along with serializers for other built-in types (for more information on the built-in serializers see Chapter 7, "Built-In XML Schema Type Serializers"). We use string_simpl to serialize the greeting and name elements as indicated by the calls to greeting_serializer() and name_serializer().

Then we instantiate a document serializer (doc_s). The first argument to its constructor is the serializer for the root element (hello_s in our case). The second argument is the root element name.

The final piece is the calls to pre(), serialize(), and post(). The call to serialize() performs the actual XML serialization with the result written to std::cout. The second argument in this call is a flag that requests pretty-printing of the resulting XML document. You would normally specify this flag during testing to obtain easily-readable XML and remove it in production to get faster serialization and smaller documents. The calls to pre() and post() make sure that the serializer for the root element can perform proper initialization and cleanup.

While our serializer implementation and test driver are pretty small and easy to write by hand, for bigger XML vocabularies it can be a substantial effort. To help with this task XSD/e can automatically generate sample serializer implementations and a test driver from your schemas. To request the generation of a sample implementation with empty function bodies specify the --generate-empty-impl option. To request the generation of a test driver you can use the --generate-test-driver option. For more information on these options refer to the XSD/e Compiler Command Line Manual.

2.4 Compiling and Running

After saving all the parts from the previous section in driver.cxx, we are ready to compile and run our first application. On UNIX this can be done with the following commands:

$ c++ -I.../libxsde -c driver.cxx hello-sskel.cxx
$ c++ -o driver driver.o hello-sskel.o .../libxsde/xsde/libxsde.a
$ ./driver
<hello>
  <greeting>Hello</greeting>
  <name>sun</name>
  <name>moon</name>
  <name>world</name>
</hello>
  

Here .../libxsde represents the path to the libxsde directory in the XSD/e distribution.

We can also test XML Schema validation. We can "forget" to add any names to the vector so that name_next() returns false on the first call:

struct hello_simpl: hello_sskel
{
  hello_simpl ()
  {
    /*
    names_.push_back ("sun");
    names_.push_back ("moon");
    names_.push_back ("world");
    */
  }
  ...
};
  

This will violate our vocabulary specification which requires at least one name element to be present. If we make the above change and recompile our application, we will get the following output:

$ ./driver
error: expected element not encountered
  

3 Serializer Skeletons

As we have seen in the previous chapter, the XSD/e compiler generates a serializer skeleton class for each type defined in XML Schema. In this chapter we will take a closer look at different functions that comprise a serializer skeleton as well as the way to connect our implementations of these serializer skeletons to create a complete vocabulary serializer.

In this and subsequent chapters we will use the following schema that describes a collection of person records. We save it in people.xsd:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:simpleType name="gender">
    <xs:restriction base="xs:string">
      <xs:enumeration value="male"/>
      <xs:enumeration value="female"/>
    </xs:restriction>
  </xs:simpleType>

  <xs:complexType name="person">
    <xs:sequence>
      <xs:element name="first-name" type="xs:string"/>
      <xs:element name="last-name" type="xs:string"/>
      <xs:element name="gender" type="gender"/>
      <xs:element name="age" type="xs:short"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="people">
    <xs:sequence>
      <xs:element name="person" type="person" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  <xs:element name="people" type="people"/>

</xs:schema>
  

A sample XML instance to go along with this schema could look like this:

<people>
  <person>
    <first-name>John</first-name>
    <last-name>Doe</last-name>
    <gender>male</gender>
    <age>32</age>
  </person>
  <person>
    <first-name>Jane</first-name>
    <last-name>Doe</last-name>
    <gender>female</gender>
    <age>28</age>
  </person>
</people>
  

Compiling people.xsd with the XSD/e compiler results in three serializer skeletons being generated: gender_sskel, person_sskel, and people_sskel. We are going to examine and implement each of them in the subsequent sections.

In the previous chapter we used pre-initialized, static data to create an XML document. In this chapter we will use the standard input (std::cin) as the source of data. This approach reflects a common design theme where the data to be serialized is computed on the fly instead of being stored in, for example, an in-memory object model. The next chapter will examine mechanisms provided by the C++/Serializer mapping for serializing in-memory object models.

3.1 Implementing the Gender Serializer

The generated gender_sskel serializer skeleton looks like this:

class gender_sskel: public xml_schema::string_sskel
{
public:
  gender_sskel (xml_schema::string_sskel* base_impl)

  // Serializer callbacks. Override them in your implementation.
  //
  virtual void
  pre ();

  virtual void
  post ();
};
  

Notice that gender_sskel inherits from xml_schema::string_sskel which is a serializer skeleton for built-in XML Schema type string and is predefined in the XSD/e runtime library. This is an example of the general rule that serializer skeletons follow: if a type in XML Schema inherits from another then there will be an equivalent inheritance between the corresponding serializer skeleton classes. The gender_sskel class also declares a constructor which expects a pointer to the base serializer skeleton. We will discuss the purpose of this constructor shortly.

The pre() and post() callbacks should look familiar from the previous chapter. Let's now implement this serializer. Our implementation will simply query the gender value from the standard input stream (std::cin):

#include <string>
#include <iostream>

using namespace std;

class gender_simpl: public gender_sskel
{
public:
  gender_simpl ()
    : gender_sskel (&base_impl_)
  {
  }

  virtual void
  pre ()
  {
    string g;
    cerr << "gender (male/female): ";
    getline (cin, g);
    base_impl_.pre (g);
  }

private:
  xml_schema::string_simpl base_impl_;
};
  

While the code is quite short, there is a lot going on. First, notice that we define a member variable base_impl_ of type xml_schema::string_simpl and then pass it to the gender_sskel's constructor. We have encountered xml_schema::string_simpl already; it is an implementation of the xml_schema::string_sskel serializer skeleton for built-in XML Schema type string. By passing base_impl_ to the gender_sskel's constructor we provide an implementation for the part of the serializer skeleton that is inherited from string_sskel.

This is another common theme in the C++/Serializer programming model: reusing implementations of the base serializers in the derived ones. In our case, string_simpl will do all the dirty work of serializing the data which we pass to it with the call to base_impl_.pre(). For more information on serializer implementation reuse refer to Section 6.6, "Serializer Reuse".

In case you are curious, here are the definitions for xml_schema::string_sskel and xml_schema::string_simpl:

namespace xml_schema
{
  class string_sskel: public serializer_simple_content
  {
  public:
    virtual void
    pre (const std::string&) = 0;
  };

  class string_simpl: public string_sskel
  {
  public:
    virtual void
    pre (const std::string&);

    virtual void
    _serialize_content ();

  protected:
    std::string value_;
  };
}
  

There are two new pieces in this code that we haven't seen yet. Those are the xml_schema::serializer_simple_content class and the _serialize_content() function. The serializer_simple_content class is defined in the XSD/e runtime and is a base class for all serializer skeletons that conform to the simple content model in XML Schema. Types with the simple content model cannot have nested elements—only text and attributes. There is also the xml_schema::serializer_complex_content class which corresponds to the complex content mode (types with nested elements, for example, person from people.xsd).

The _serialize_content() function is a low-level serializer callback that is called to perform actual content serialization (that is to output text or nested elements). There is also the _serialize_attributes() callback which is called to serialize attributes in complex types. You will seldom need to use these callbacks directly. Using implementations for the built-in serializers provided by the XSD/e runtime is usually a simpler and more convenient alternative.

Another bit of information that is useful to know about is the _pre() and _post() serialization callbacks. Remember we talked about the pre() and post() callbacks in the previous chapter? The _pre() and _post have very similar but somewhat different roles. As a result, each serializer skeleton has four special callbacks:

  virtual void
  pre ();

  virtual void
  _pre ();

  virtual void
  _post ();

  virtual void
  post ();
  

pre() and _pre() are initialization callbacks. They get called in that order before a new instance of the type is about to be serialized. The difference between pre() and _pre() is conventional: pre() can be completely overridden by a derived serializer. The derived serializer can also override _pre() but has to always call the original version. This allows you to partition initialization into customizable and required parts.

Similarly, _post() and post() are finalization callbacks with exactly the same semantics: post() can be completely overridden by the derived serializer while the original _post() should always be called.

At this point you might be wondering why some pre() callbacks, for example string_sskel::pre(), have an argument with which they receive the data they need to serialize while others, for example gender_sskel::pre(), have no such argument. This is a valid concern and it will be addressed in the next chapter.

3.2 Implementing the Person Serializer

The generated person_sskel serializer skeleton looks like this:

class person_sskel: public xml_schema::serializer_complex_content
{
public:
  // Serializer callbacks. Override them in your implementation.
  //
  virtual void
  pre ();

  virtual std::string
  first_name () = 0;

  virtual std::string
  last_name () = 0;

  virtual void
  gender ();

  virtual short
  age () = 0;

  virtual void
  post ();

  // Serializer construction API.
  //
  void
  first_name_serializer (xml_schema::string_sskel&);

  void
  last_name_serializer (xml_schema::string_sskel&);

  void
  gender_serializer (gender_sskel&);

  void
  age_serializer (xml_schema::short_sskel&);

  void
  serializers (xml_schema::string_sskel& /* first-name */,
               xml_schema::string_sskel& /* last-name */,
               gender_sskel&             /* gender */,
               xml_schema::short_sskel&  /* age */);
};
  

As you can see, we have a serializer callback for each of the nested elements found in the person XML Schema type. The implementation of this serializer is straightforward:

class person_simpl: public person_sskel
{
public:
  virtual string
  first_name ()
  {
    string fn;
    cerr << "first name: ";
    getline (cin, fn);
    return fn;
  }

  virtual std::string
  last_name ()
  {
    string ln;
    cerr << "last name: ";
    getline (cin, ln);
    return ln;
  }

  virtual short
  age ()
  {
    short a;
    cerr << "age: ";
    cin >> a;
    return a;
  }
};
  

Notice that we didn't need to override the gender() callback because all the work is done by gender_simpl.

3.3 Implementing the People Serializer

The generated people_sskel serializer skeleton looks like this:

class people_sskel: public xml_schema::serializer_complex_content
{
public:
  // Serializer callbacks. Override them in your implementation.
  //
  virtual void
  pre ();

  virtual bool
  person_next () = 0;

  virtual void
  person ();

  virtual void
  post ();

  // Serializer construction API.
  //
  void
  person_serializer (person_sskel&);

  void
  serializers (person_sskel& /* person */);
};
  

The person_next() callback will be called before serializing each person element. Our implementation of person_next() asks the user whether to serialize another person record:

class people_simpl: public people_sskel
{
public:
  virtual bool
  person_next ()
  {
    string s;
    cerr << "serialize another person record (y/n): ";
    cin >> ws; // Skip leading whitespaces.
    getline (cin, s);
    return s == "y";
  }
};
  

Now it is time to put everything together.

3.4 Connecting the Serializers Together

At this point we have all the individual serializers implemented and can proceed to assemble them into a complete serializer for our XML vocabulary. The first step is to instantiate all the individual serializers that we will need:

xml_schema::short_simpl short_s;
xml_schema::string_simpl string_s;

gender_simpl gender_s;
person_simpl person_s;
people_simpl people_s;
  

Notice that our schema uses two built-in XML Schema types: string for the first-name and last-name elements as well as short for age. We will use predefined serializers that come with the XSD/e runtime to serialize these types. The next step is to connect all the individual serializers. We do this with the help of functions defined in the serializer skeletons and marked with the "Serializer Construction API" comment. One way to do it is to connect each individual serializers by calling the *_serializer() functions:

person_s.first_name_serializer (string_s);
person_s.last_name_serializer (string_s);
person_s.gender_serializer (gender_s);
person_s.age_serializer (short_s);

people_s.person_serializer (person_s);
  

You might be wondering what happens if you do not provide a serializer by not calling one of the *_serializer() functions. In that case the corresponding XML fragment will be skipped.

An alternative, shorter, way to connect the serializers is by using the serializers() functions which connects all the serializers for a given type at once:

person_s.serializers (string_s, string_s, gender_s, short_s);
people_s.serializers (person_s);
  

The following figure illustrates the resulting connections. Notice the correspondence between return types of element callbacks and argument types of the pre() functions that are connected by the arrows.

The last step is the construction of the document serializer and invocation of the complete serializer to produce an XML document:

xml_schema::document_simpl doc_s (people_s, "people");

std::ostringstream os;

people_s.pre ();
doc_s.serialize (os, xml_schema::document_simpl::pretty_print);
people_s.post ();

cout << os.str ();
  

Note that we first serialize the document into an std::ostringstream object and then write the result to the standard output stream. This is done to prevent the input prompts and output XML from interleaving. However, writing XML directly to std::cout in this example is a great way to observe the moments in the XML document construction process at which serializer callbacks are being called.

Let's consider xml_schema::document_simpl in more detail. While the exact definition of this class varies depending on the mapping configuration, here is the part relevant to our example:

namespace xml_schema
{
  class document_simpl
  {
  public:
    document_simpl (xml_schema::serializer_base&,
                    const std::string& root_element_name);

    document_simpl (xml_schema::serializer_base&,
                    const std::string& root_element_namespace,
                    const std::string& root_element_name);

    typedef unsigned short flags;
    static const flags pretty_print;

    void
    serialize (std::ostream&, flags = 0);
  };
}
  

xml_schema::document_simpl is a root serializer for the vocabulary. The first argument to its constructors is the serializer for the type of the root element (people_simpl in our case). Because a type serializer is only concerned with the element's content and not with the element's name, we need to specify the root element name somewhere. That's what is passed as the second and third arguments to the document_simpl's constructors.

There is also a number of overloaded serialize() function defined in the document_simpl class. At the moment we are only interested in the version that writes XML to a standard output stream. For more information on the xml_schema::document_simpl class refer to Chapter 8, "Document Serializer and Error Handling".

Let's now consider a step-by-step list of actions that happen as we serialize the following sample XML document:

<people>
  <person>
    <first-name>John</first-name>
    <last-name>Doe</last-name>
    <gender>male</gender>
    <age>32</age>
  </person>
</people>
  

1. people_s.pre() is called from main(). We did not provide any implementation for this callback so this call is a no-op.
2. doc_s.serialize(os) is called from main(). The document serializer writes out the <people> opening tag and calls _pre() on the root element type serializer (people_s) which is also a no-op. Serialization is delegated to people_s.
3. The people_s serializer calls person_next() to determine if another person element needs to be serialized. Our implementation ask the user (who answers "y") and returns true.
4. The people_s serializer calls person() which is a no-op. It then calls pre() on person_s (no-op), writes out the <person> opening tag, and calls _pre() on person_s (no-op). Serialization is delegated to person_s.
5. The person_s serializer calls first_name() that returns a first name which it asks the user to enter. person_s then calls pre() on string_s and passes the name returned from first_name() as its argument. It then writes out the <first-name> opening tag and calls _pre() on string_s. Serialization is delegated to string_s.
6. The _serialize_content() callback is called on string_s which writes out the string passed to it in the pre() call.
7. Control is returned to person_s which calls _post() on string_s, writes out the </first-name> closing tag, and calls post() on string_s.
8. Steps analogous to 5-7 are performed for the last-name, gender, and age elements.
9. Control is returned to people_s which calls _post() on person_s (no-op), writes out the </person> closing tag, and calls post() on person_s (no-op).
10. The people_s serializer calls person_next() to determine if another person element needs to be serialized. Our implementation ask the user (who answers "n") and returns false.
11. Control is returned to doc_s which calls _post() on people_s (no-op) and writes out the </people> closing tag.
12. Control is returned to main() which calls post() on people_s (no-op).

4 Type Maps

There are many useful things you can do inside serializer callbacks as they are right now. There are, however, times when you want to propagate some information from one serializer to another or from the caller of the serializer. One common task that would greatly benefit from such a possibility is serializing a tree-like in-memory object model to XML. During execution, each individual serializer would be responsible for disaggregating and serializing a specific portion of the tree and delegating the rest to its sub-serializers.

In this chapter we will discuss the mechanisms offered by the C++/Serializer mapping for passing information between individual serializers and see how to use them to serialize a sample object model for our people vocabulary.

4.1 Object Model

An object model for our person record example could look like this (saved in the people.hxx file):

#include <string>
#include <vector>

enum gender
{
  male,
  female
};

class person
{
public:
  person (const std::string& first,
          const std::string& last,
          ::gender gender,
          short age)
    : first_ (first), last_ (last),
      gender_ (gender), age_ (age)
  {
  }

  const std::string&
  first () const
  {
    return first_;
  }

  const std::string&
  last () const
  {
    return last_;
  }

  ::gender
  gender () const
  {
    return gender_;
  }

  short
  age () const
  {
    return age_;
  }

private:
  std::string first_;
  std::string last_;
  ::gender gender_;
  short age_;
};

typedef std::vector<person> people;
  

While it is clear which serializer is responsible for which part of the object model, it is not exactly clear how, for example, person_simpl will pass gender to gender_simpl. You might have noticed that string_simpl manages to receive its value from the first_name() callback. Let's see how we can utilize the same mechanism to propagate our own data.

There is a way to tell the XSD/e compiler that you want to exchange data between serializers. More precisely, for each type defined in XML Schema, you can tell the compiler two things. First, the argument type of the pre() callback in the serializer skeleton generated for this type. And, second, the return type for callbacks corresponding to elements and attributes of this type. For example, for XML Schema type gender we can specify the argument type for pre() in the gender_sskel skeleton and the return type for the gender() callback in the person_sskel skeleton. As you might have guessed, the generated code will then pass the return value from an element or attribute callback (person_sskel::gender() in our case) to the pre() callback of the corresponding serializer skeleton (gender_sskel::pre() in our case).

The way to tell the XSD/e compiler about these XML Schema to C++ mappings is with type map files. Here is a simple type map for the gender type from the previous paragraph.

include "people.hxx";
gender ::gender ::gender;
  

The first line indicates that the generated code must include people.hxx in order to get the definition for the gender type. The second line specifies that both argument and return types for the gender XML Schema type should be the ::gender C++ enum (we use fully-qualified C++ names to avoid name clashes). The next section will describe the type map format in more detail. We save this type map in people.map and then translate our schemas with the --type-map option to let the XSD/e compiler know about our type map:

$ xsde cxx-serializer --type-map people.map people.xsd
  

If we now look at the generated people-sskel.hxx, we will see the following changes in the gender_sskel and person_sskel skeletons:

#include "people.hxx"

class gender_sskel: public xml_schema::string_sskel
{
  virtual void
  pre (::gender) = 0;

  ...
};

class person_sskel: public xml_schema::serializer_complex_content
{
  virtual ::gender
  gender () = 0;

  ...
};
  

Notice that #include "people.hxx" was added to the generated header file from the type map to provide the definition for the gender enum.

4.2 Type Map File Format

Type map files are used to define a mapping between XML Schema and C++ types. The compiler uses this information to determine argument types of pre() callbacks in serializer skeletons corresponding to XML Schema types as well as return types for callbacks corresponding to elements and attributes of these types.

The compiler has a set of predefined mapping rules that map the built-in XML Schema types to suitable C++ types (discussed below) and all other types to void. By providing your own type maps you can override these predefined rules. The format of the type map file is presented below:

namespace <schema-namespace> [<cxx-namespace>]
{
  (include <file-name>;)*
  ([type] <schema-type> <cxx-ret-type> [<cxx-arg-type>];)*
}
  

Both <schema-namespace> and <schema-type> are regex patterns while <cxx-namespace>, <cxx-ret-type>, and <cxx-arg-type> are regex pattern substitutions. All names can be optionally enclosed in " ", for example, to include white-spaces.

<schema-namespace> determines XML Schema namespace. Optional <cxx-namespace> is prefixed to every C++ type name in this namespace declaration. <cxx-ret-type> is a C++ type name that is used as a return type for the element and attribute callbacks corresponding to this schema type. Optional <cxx-arg-type> is an argument type for the pre() callback in the serializer skeleton for this schema type. If <cxx-arg-type> is not specified, it defaults to <cxx-ret-type> if <cxx-ret-type> ends with * or & (that is, it is a pointer or a reference) and const <cxx-ret-type>& otherwise. <file-name> is a file name either in the " " or < > format and is added with the #include directive to the generated code.

The # character starts a comment that ends with a new line or end of file. To specify a name that contains # enclose it in " ". For example:

namespace http://www.example.com/xmlns/my my
{
  include "my.hxx";

  # Pass apples by value.
  #
  apple apple;

  # Pass oranges as pointers.
  #
  orange orange_t*;
}
  

In the example above, for the http://www.example.com/xmlns/my#orange XML Schema type, the my::orange_t* C++ type will be used as both return and argument types.

Several namespace declarations can be specified in a single file. The namespace declaration can also be completely omitted to map types in a schema without a namespace. For instance:

include "my.hxx";
apple apple;

namespace http://www.example.com/xmlns/my
{
  orange "const orange_t*";
}
  

The compiler has a number of predefined mapping rules for the built-in XML Schema types which can be presented as the following map files:

namespace http://www.w3.org/2001/XMLSchema
{
  boolean bool bool;

  byte "signed char" "signed char";
  unsignedByte "unsigned char" "unsigned char";

  short short short;
  unsignedShort "unsigned short" "unsigned short";

  int int int;
  unsignedInt "unsigned int" "unsigned int";

  long "long long" "long long";
  unsignedLong "unsigned long long" "unsigned long long";

  integer long long;

  negativeInteger long long;
  nonPositiveInteger long long;

  positiveInteger "unsigned long" "unsigned long";
  nonNegativeInteger "unsigned long" "unsigned long";

  float float float;
  double double double;
  decimal double double;

  NMTOKENS "const xml_schema::string_sequence*";
  IDREFS "const xml_schema::string_sequence*";

  base64Binary "const xml_schema::buffer*";
  hexBinary "const xml_schema::buffer*";

  date xml_schema::date;
  dateTime xml_schema::date_time;
  duration xml_schema::duration;
  gDay xml_schema::gday;
  gMonth xml_schema::gmonth;
  gMonthDay xml_schema::gmonth_day;
  gYear xml_schema::gyear;
  gYearMonth xml_schema::gyear_month;
  time xml_schema::time;
}
  

If STL is enabled (Section 6.1, "Standard Template Library"), the following mapping is used for the string-based XML Schema built-in types:

namespace http://www.w3.org/2001/XMLSchema
{
  include <string>;

  anySimpleType std::string;

  string std::string;
  normalizedString std::string;
  token std::string;
  Name std::string;
  NMTOKEN std::string;
  NCName std::string;
  ID std::string;
  IDREF std::string;
  language std::string;
  anyURI std::string;

  QName xml_schema::qname;
}
  

Otherwise, a C string-based mapping is used:

namespace http://www.w3.org/2001/XMLSchema
{
  anySimpleType "const char*";

  string "const char*";
  normalizedString "const char*";
  token "const char*";
  Name "const char*";
  NMTOKEN "const char*";
  NCName "const char*";
  ID "const char*";
  IDREF "const char*";
  language "const char*";
  anyURI "const char*";

  QName "const xml_schema::qname*";
}
  

For more information about the mapping of the built-in XML Schema types to C++ types refer to Chapter 7, "Built-In XML Schema Type Serializers". The last predefined rule maps anything that wasn't mapped by previous rules to void:

namespace .*
{
  .* void void;
}
  

When you provide your own type maps with the --type-map option, they are evaluated first. This allows you to selectively override any of the predefined rules. Note also that if you change the mapping of a built-in XML Schema type then it becomes your responsibility to provide the corresponding serializer skeleton and implementation in the xml_schema namespace. You can include the custom definitions into the generated header file using the --hxx-prologue-* options.

4.3 Serializer Implementations

With the knowledge from the previous section, we can proceed with creating a type map that maps types in the people.xsd schema to our object model classes in people.hxx. In fact, we already have the beginning of our type map file in people.map. Let's extend it with the rest of the types:

include "people.hxx";

gender ::gender ::gender;
person "const ::person&";
people "const ::people&";
  

A few things to note about this type map. We decided to pass the person and people objects by constant references in order to avoid unnecessary copying. We can do this because we know that our object model is present for the duration of serialization. We also did not provide any mappings for built-in XML Schema types string and short because they are handled by the predefined rules and we are happy with the result. Note also that all C++ types are fully qualified. This is done to avoid potential name conflicts in the generated code. Now we can recompile our schema and move on to implementing the serializers:

$ xsde cxx-serializer --type-map people.map people.xsd
  

Here is the implementation of our three serializers in full. One way to save typing when implementing your own serializers is to open the generated code and copy the signatures of serializer callbacks into your code. Or you could always auto generate the sample implementations and fill them with your code.

#include "people-sskel.hxx"

const char* gender_strings[] = {"male", "female"};

class gender_simpl: public gender_sskel
{
public:
  gender_simpl ()
    : gender_sskel (&base_impl_)
  {
  }

  virtual void
  pre (gender g)
  {
    base_impl_.pre (gender_strings[g]);
  }

private:
  xml_schema::string_simpl base_impl_;
};

class person_simpl: public person_sskel
{
public:
  virtual void
  pre (const person& p)
  {
    p_ = &p;
  }

  virtual std::string
  first_name ()
  {
    return p_->first ();
  }

  virtual std::string
  last_name ()
  {
    return p_->last ();
  }

  virtual ::gender
  gender ()
  {
    return p_->gender ();
  }

  virtual short
  age ()
  {
    return p_->age ();
  }

private:
  const person* p_;
};

class people_simpl: public people_sskel
{
public:
  virtual void
  pre (const people& p)
  {
    p_ = &p;
    i_ = p_->begin ();
  }

  virtual bool
  person_next ()
  {
    return i_ != p_->end ();
  }

  virtual const ::person&
  person ()
  {
    return *i_++;
  }

private:
  const people* p_;
  people::const_iterator i_;
};
  

This code fragment should look familiar by now. Just note that all the pre() callbacks now have arguments. Here is the implementation of the test driver for this example:

#include <iostream>

using namespace std;

int
main ()
{
  // Create a sample object model.
  //
  people ppl;

  ppl.push_back (person ("John", "Doe", male, 32));
  ppl.push_back (person ("Jane", "Doe", female, 28));

  // Construct the serializer.
  //
  xml_schema::short_simpl short_s;
  xml_schema::string_simpl string_s;

  gender_simpl gender_s;
  person_simpl person_s;
  people_simpl people_s;

  person_s.serializers (string_s, string_s, gender_s, short_s);
  people_s.serializers (person_s);

  // Create the XML document.
  //
  xml_schema::document_simpl doc_s (people_s, "people");

  people_s.pre (ppl);
  doc_s.serialize (cout, xml_schema::document_simpl::pretty_print);
  people_s.post ();
}
  

The serializer creation and assembly part is exactly the same as in the previous chapter. The serialization part is a bit different: people_simpl::pre() now has an argument which is the complete object model. Also we write the resulting XML directly to the standard output stream instead of first storing it in a string. We can now save the last two code fragments to driver.cxx and proceed to compile and test our new application:

$ c++ -I.../libxsde -c driver.cxx people-sskel.cxx
$ c++ -o driver driver.o people-sskel.o .../libxsde/xsde/libxsde.a
$ ./driver
<people>
  <person>
    <first-name>John</first-name>
    <last-name>Doe</last-name>
    <gender>male</gender>
    <age>32</age>
  </person>
  <person>
    <first-name>Jane</first-name>
    <last-name>Doe</last-name>
    <gender>female</gender>
    <age>28</age>
  </person>
</people>
  

5 Serializer Callbacks

In previous chapters we have learned that for each attribute and element in a schema type there is a callback in a serializer skeleton with the same name and which optionally returns this element's or attribute's value. We've also seen that elements that can appear multiple times (maxOccurs="unbounded") have an additional serializer callback in the form:

virtual bool
<name>_next ();
  

Where <name> stands for the element's name. In this chapter we will discuss other additional serializer callbacks that are generated for certain XML Schema constructs. We will also learn that besides elements and attributes, serializer callback can be generated for the all, choice, and sequence compositors as well as the any and anyAttribute wildcards.

When additional serializer callback are generated for elements and attributes, their names are derived from element's and attribute's names. Compositors and wildcards, on the other hand, do not have names and as a result the serializer callback names for these constructs are based on synthesized names in the form: all for the all compositor, sequence, sequence1, etc., for the sequence compositors, choice, choice1, etc., for the choice compositors, any, any1, etc., for the any wildcards, and any_attribute, any_attribute1, etc., for the anyAttribute wildcards. For example:

<xs:complexType name="coordinates">
  <xs:sequence maxOccurs="unbounded">
    <xs:element name="lat" type="xs:float"/>
    <xs:element name="lon" type="xs:float"/>
  </xs:sequence>
</xs:complexType>
  

The above schema fragment, when compiled, results in the following serializer skeleton:

class coordinates_sskel: public xml_schema::serializer_complex_content
{
public:
  virtual void
  pre ();

  virtual bool
  sequence_next ();

  virtual float
  lan () = 0;

  virtual float
  lon () = 0;

  virtual void
  post ();

  ...
};
  

5.1 Optional Callback

For elements, compositors, and element wildcards with the minimal occurrence constraint equals 0 (minOccurs="0") and the maximum occurrence constraint equals 1 (maxOccurs="1") as well as for optional attributes, the optional callback is generated in the form:

virtual bool
<name>_present ();
  

This callback is called before any other callbacks for this schema construct and if it returns false no further callback calls corresponding to this construct are made and the corresponding XML fragment is omitted. For example:

<xs:complexType name="name">
  <xs:sequence minOccurs="0">
    <xs:element name="first" type="xs:string"/>
    <xs:element name="initial" type="xs:string" minOccurs="0"/>
    <xs:element name="last" type="xs:string"/>
  </xs:sequence>
  <xs:attribute name="lang" type="xs:language"/>
</xs:complexType>
  

The above schema fragment, when compiled, results in the following serializer skeleton:

class name_sskel: public xml_schema::serializer_complex_content
{
public:
  virtual void
  pre ();

  virtual bool
  lang_present ();

  virtual std::string
  lang () = 0;

  virtual bool
  sequence_present ();

  virtual std::string
  first () = 0;

  virtual bool
  initial_present ();

  virtual std::string
  initial () = 0;

  virtual std::string
  last () = 0;

  virtual void
  post ();

  ...
};
  

5.2 Sequence Callback

For elements, compositors, and element wildcards with the the maximum occurrence constraint greater than 1 (for example, maxOccurs="unbounded") the sequence callback is generated in the form:

virtual bool
<name>_next ();
  

This callback is called before each new item of the sequence is about to be serialized. Returning false from this callback indicates that no more items in the sequence need to be serialized. For example:

<xs:complexType name="names">
  <xs:sequence maxOccurs="unbounded">
    <xs:element name="first" type="xs:string"/>
    <xs:element name="last" type="xs:string"/>
    <xs:element name="pseudonym" type="xs:string" maxOccurs="3"/>
  </xs:sequence>
</xs:complexType>
  

The above schema fragment, when compiled, results in the following serializer skeleton:

class names_sskel: public xml_schema::serializer_complex_content
{
public:
  virtual void
  pre ();

  virtual bool
  sequence_next () = 0;

  virtual std::string
  first () = 0;

  virtual std::string
  last () = 0;

  virtual bool
  pseudonym_next () = 0;

  virtual std::string
  pseudonym () = 0;

  virtual void
  post ();
};
  

5.3 Choice Callback

The choice compositor allows an XML document to contain one of several element or compositor options. In the Embedded C++/Serializer mapping, these options are called choice arms and are identified by the arm tags. For example:

<xs:complexType name="name">
  <xs:choice>
    <xs:element name="full-name" type="xs:string"/>
    <xs:sequence>
      <xs:element name="first-name" type="xs:string"/>
      <xs:element name="last-name" type="xs:string"/>
    </xs:sequence>
  </xs:choice>
</xs:complexType>
  

The above schema fragment, when compiled, results in the following serializer skeleton:

class name_sskel: public xml_schema::serializer_complex_content
{
public:
  virtual void
  pre ();

  enum choice_arm_tag
  {
    full_name_tag,
    sequence_tag
  };

  virtual choice_arm_tag
  choice_arm () = 0;

  virtual std::string
  full_name () = 0;

  virtual std::string
  first_name () = 0;

  virtual std::string
  last_name () = 0;

  virtual void
  post ();
};
  

The arm tags enum name (choice_arm_tag above) is derived from the choice compositor name (that is, choice, choice1, etc.) by adding the _arm_tag suffix. The tag names themselves are derived from the corresponding elements, compositors, or element wildcards.

The choice compositor callback has a name in the form choice_tag() (or choice1_tag(), etc., for subsequent choice compositors in the type). It returns the arm tag which identifies the choice arm that should be serialized. For example, if a name_sskel implementation returns full_name_tag from the choice_arm() callback, then the first choice arm is chosen and the full_name() callback is then called. Otherwise the first_name and last_name() callbacks are called.

5.4 Element Wildcard Callbacks

An element wildcard allows an arbitrary element from the specified namespace list to be present in an XML instance. Element wildcards can have the same cardinality constraints as elements and, as as a result, the optional or sequence callbacks can be generated. For example:

<xs:complexType name="name">
  <xs:sequence>
    <xs:element name="first" type="xs:string"/>
    <xs:element name="last" type="xs:string"/>
    <xs:any namespace="##other" processContents="skip" minOccurs="0"/>
  </xs:sequence>
</xs:complexType>
  

The above schema fragment, when compiled, results in the following serializer skeleton:

class name_sskel: public xml_schema::serializer_complex_content
{
public:
  virtual void
  pre ();

  virtual std::string
  first () = 0;

  virtual std::string
  last () = 0;

  virtual bool
  any_present ();

  virtual void
  any (std::string& ns, std::string& name);

  virtual void
  serialize_any ();

  virtual void
  post ();
};
  

The any() callback is called to obtain the element name and namespace. If validation is enabled, the namespace is checked against the allowed list. Then an element with these name and namespace is created and the serialize_any() callback is called to allow you to serialize the element's attributes and content. There are two common ways to serialize a wildcard element. The first approach is to use a serializer implementation. This approach is shown in the wildcard example which is part of the XSD/e distribution. The other approach is to use the low-level XML serialization API that is available to every serializer implementation via the xml_schema::serializer_base base serializer:

namespace xml_schema
{
  class serializer_base
  {
  public:
    void
    _start_element (const char* name);

    void
    _start_element (const char* ns, const char* name);

    void
    _end_element ();

    void
    _start_attribute (const char* name);

    void
    _start_attribute (const char* ns, const char* name);

    void
    _end_attribute ();

    void
    _attribute (const char* name, const char* value);

    void
    _attribute (const char* ns, const char* name, const char* value);

    void
    _characters (const char*);

    void
    _characters (const char*, size_t);

    void
    _declare_namespace (const char* ns, const char* prefix);

    void
    _declare_default_namespace (const char* ns);

    void
    _clear_default_namespace ();
  };
}
  

The following example shows how we could implement the name_sskel skeleton using this approach:

class name_simpl: public name_sskel
{
public:
  virtual std::string
  first ()
  {
    return "John";
  }

  virtual ::std::string
  last ()
  {
    return "Doe";
  }

  virtual bool
  any_present ()
  {
    return true;
  }

  virtual void
  any (std::string& ns, std::string& name)
  {
    ns = "http://www.example.com/extension";
    name = "pseudonyms";
  }

  virtual void
  serialize_any ()
  {
    _attribute ("id", "jd");

    _start_element ("pseudonym");
    _characters ("Johnny Doer");
    _end_element ();

    _start_element ("pseudonym");
    _characters ("Johnty Doo");
    _end_element ();
  }
};
  

5.5 Attribute Wildcard Callbacks

An attribute wildcard allows an arbitrary number of attributes from the specified namespace list to be present in an XML instance. As a result, the serializer callbacks for an attribute wildcard resemble those of an element with maxOccurs="unbounded". For example:

<xs:complexType name="name">
  <xs:sequence>
    <xs:element name="first" type="xs:string"/>
    <xs:element name="last" type="xs:string"/>
  </xs:sequence>
  <xs:anyAttribute namespace="##any" processContents="skip"/>
</xs:complexType>
  

The above schema fragment, when compiled, results in the following serializer skeleton:

class name_sskel: public xml_schema::serializer_complex_content
{
public:
  virtual void
  pre ();

  virtual bool
  any_attribute_next ();

  virtual void
  any_attribute (std::string& ns, std::string& name);

  virtual void
  serialize_any_attribute ();

  virtual std::string
  first () = 0;

  virtual std::string
  last () = 0;

  virtual void
  post ();
};
  

Every time the any_attribute_next() callback returns true, any_attribute() is called to obtain the attribute name and namespace. If validation is enabled, the namespace is checked against the allowed list. Then an attribute with these name and namespace is created and the serialize_any_attribute() callback is called to allow you to write the attribute value, for example using one of the serializer implementations (see the wildcard example on how to do it) or the low-level _characters() function (for more information about the low-level XML serialization API see the previous section). The following example show how we could implement the name_sskel skeleton using the latter approach:

class name_simpl: public name_sskel
{
public:
  virtual void
  pre ()
  {
    id_written_ = false;
  }

  virtual bool
  any_attribute_next ()
  {
    return !id_written_;
  }

  virtual void
  any_attribute (std::string& ns, std::string& name)
  {
    ns = "";
    name = "id";
  }

  virtual void
  serialize_any_attribute ()
  {
    _characters ("jd");
    id_written_ = true;
  }

  virtual std::string
  first ()
  {
    return "John";
  }

  virtual ::std::string
  last ()
  {
    return "Doe";
  }

private:
  bool id_written_;
};
  

6 Mapping Configuration

The Embedded C++/Serializer mapping has a number of configuration parameters that determine the overall properties and behavior of the generated code, such as the use of Standard Template Library (STL), Input/Output Stream Library (iostream), C++ exceptions, XML Schema validation, 64-bit integer types, serializer implementation reuse styles, and support for XML Schema polymorphism. Previous chapters assumed that the use of STL, iostream, C++ exceptions, and XML Schema validation were enabled. This chapter will discuss the changes in the Embedded C++/Serializer programming model that result from the changes to these configuration parameters. A complete example that uses the minimal mapping configuration is presented at the end of this chapter.

In order to enable or disable a particular feature, the corresponding configuration parameter should be set accordingly in the XSD/e runtime library as well as specified during schema compilation with the XSD/e command line options as described in the XSD/e Compiler Command Line Manual.

The Embedded C++/Serializer mapping always expects character data supplied by the application to be in the same encoding. The application encoding can either be UTF-8 (default) or ISO-8859-1. To select a particular encoding, configure the XSD/e runtime library accordingly and pass the --char-encoding option to the XSD/e compiler when translating your schemas. The underlying XML serializer used by the Embedded C++/Serializer mapping produces the resulting XML documents in the UTF-8 encoding.

6.1 Standard Template Library

To disable the use of STL you will need to configure the XSD/e runtime without support for STL as well as pass the --no-stl option to the XSD/e compiler when translating your schemas. When STL is disabled, all string-based XML Schema types are mapped to C-style const char* instead of std::string, as described in Section 4.2, "Type Map File Format". The following code fragment shows changes in the signatures of the first_name() and last_name() callbacks from the person record example.

class person_sskel
{
public:
  virtual const char*
  first_name ();

  virtual const char*
  last_name ();

  ...
};
  

When STL is disabled, the serializer implementations for the string-based built-in XML Schema types can be instructed to release the string after serialization using operator delete[]. For more information on how to do this refer to Section 7.2, "String-Based Type Serializers".

6.2 Input/Output Stream Library

To disable the use of iostream you will need to configure the XSD/e runtime library without support for iostream as well as pass the --no-iostream option to the XSD/e compiler when translating your schemas. When iostream is disabled, the following serialize() function in the xml_schema::document_simpl class become unavailable:

void
serialize (std::ostream&, flags);
  

See Section 8.1, "Document Serializer" for more information.

6.3 C++ Exceptions

To disable the use of C++ exceptions, you will need to configure the XSD/e runtime without support for exceptions as well as pass the --no-exceptions option to the XSD/e compiler when translating your schemas. When C++ exceptions are disabled, the error conditions are indicated with error codes instead of exceptions, as described in Section 8.3, "Error Codes".

6.4 XML Schema Validation

To disable support for XML Schema validation, you will need to configure the XSD/e runtime accordingly as well as pass the --suppress-validation option to the XSD/e compiler when translating your schemas. Disabling XML Schema validation allows to further increase the serialization performance and reduce footprint in cases where the data being serialized is known to be valid.

6.5 64-bit Integer Type

By default the 64-bit long and unsignedLong XML Schema built-in types are mapped to the 64-bit long long and unsigned long long fundamental C++ types. To disable the use of these types in the mapping you will need to configure the XSD/e runtime accordingly as well as pass the --no-long-long option to the XSD/e compiler when translating your schemas. When the use of 64-bit integral C++ types is disabled the long and unsignedLong XML Schema built-in types are mapped to long and unsigned long fundamental C++ types.

6.6 Serializer Reuse

When one type in XML Schema inherits from another, it is often desirable to be able to reuse the serializer implementation corresponding to the base type in the serializer implementation corresponding to the derived type. XSD/e provides support for two serializer reuse styles: the so-called mixin (generated when the --reuse-style-mixin option is specified) and tiein (generated by default) styles.

The compiler can also be instructed not to generate any support for serializer reuse with the --reuse-style-none option. This is mainly useful to further reduce the generated code size when your vocabulary does not use inheritance or when you plan to implement each serializer from scratch. Note also that the XSD/e runtime should be configured in accordance with the serializer reuse style used in the generated code. The remainder of this section discusses the mixin and tiein serializer reuse styles in more detail.

To provide concrete examples for each reuse style we will use the following schema fragment:

<xs:complexType name="person">
  <xs:sequence>
    <xs:element name="first-name" type="xs:string"/>
    <xs:element name="last-name" type="xs:string"/>
    <xs:element name="age" type="xs:short"/>
  </xs:sequence>
</xs:complexType>

<xs:complexType name="emplyee">
  <complexContent>
    <extension base="person">
      <xs:sequence>
        <xs:element name="position" type="xs:string"/>
        <xs:element name="salary" type="xs:unsignedLong"/>
      </xs:sequence>
    </extension>
  </complexContent>
</xs:complexType>
  

The mixin serializer reuse style uses the C++ mixin idiom that relies on multiple and virtual inheritance. Because virtual inheritance can result in a significant object code size increase, this reuse style should be considered when such an overhead is acceptable and/or the vocabulary consists of only a handful of types. When the mixin reuse style is used, the generated serializer skeletons use virtual inheritance, for example:

class person_sskel: public virtual serializer_complex_content
{
  ...
};

class employee_sskel: public virtual person_sskel
{
  ...
};
  

When you implement the base serializer you also need to use virtual inheritance. The derived serializer is implemented by inheriting from both the derived serializer skeleton and the base serializer implementation (that is, mixing in the base serializer implementation), for example:

class person_simpl: public virtual person_sskel
{
  ...
};

class employee_simpl: public employee_sskel,
                      public person_simpl
{
  ...
};
  

The tiein serializer reuse style uses delegation and normally results in a significantly smaller object code while being almost as convenient to use as the mixin style. When the tiein reuse style is used, the generated derived serializer skeleton declares a constructor which allows you to specify the implementation of the base serializer:

class person_sskel: public serializer_complex_content
{
  ...
};

class employee_sskel: public person_sskel
{
public:
  employee_sskel (person_sskel* base_impl)

  ...
};
  

If you pass the implementation of the base serializer to this constructor then the generated code will transparently forward all the callbacks corresponding to the base serializer skeleton to this implementation. You can also pass 0 to this constructor in which case you will need to implement the derived serializer from scratch. The following example shows how we could implement the person and employee serializers using the tiein style:

class person_simpl: public person_sskel
{
  ...
};

class employee_simpl: public employee_sskel
{
public:
  employee_simpl ()
    : employee_sskel (&base_impl_)
  {
  }

  ...

private:
  person_simpl base_impl_;
};
  

Note that you cannot use the tied in base serializer instance (base_impl_ in the above code) for serializing anything except the derived type.

The ability to override the base serializer callbacks in the derived serializer is also available in the tiein style. For example, the following code fragment shows how we can override the age() callback if we didn't like the implementation provided by the base serializer:

class employee_simpl: public employee_sskel
{
public:
  employee_simpl ()
    : employee_sskel (&base_impl_)
  {
  }

  virtual short
  age ()
  {
    ...
  }

  ...

private:
  person_simpl base_impl_;
};
  

In the above example the age element will be handled by emplyee_simpl while the first-name and last-name callbacks will still go to base_impl_.

It is also possible to inherit from the base serializer implementation instead of declaring it as a member variable. This can be useful if you need to access protected members in the base implementation or need to override a virtual function that is not part of the serializer skeleton interface. Note, however, that in this case you will need to resolve a number of ambiguities with explicit qualifications or using-declarations. For example:

class person_simpl: public person_sskel
{
public:
  virtual void
  pre (person* p)
  {
    person_ = p;
  }

  ...

protected:
  person* person_;
};

class employee_simpl: public employee_sskel,
                      public person_simpl
{
public:
  employee_simpl ()
    : employee_sskel (static_cast<person_simpl*> (this))
  {
  }

  // Resolve ambiguities.
  //
  using emplyee_sskel::serializers;

  virtual void
  pre (employee* e)
  {
    person_simpl::pre (e);
  }

  virtual std::string
  position ()
  {
    return static_cast<employee*> (person_)->position ();
  }

  virtual unsigned int
  salary ()
  {
    return static_cast<employee*> (person_)->salary ();
  }
};
  

6.7 Support for Polymorphism

By default the XSD/e compiler generates non-polymorphic code. If your vocabulary uses XML Schema polymorphism in the form of xsi:type and/or substitution groups, then you will need to configure the XSD/e runtime with support for polymorphism, compile your schemas with the --generate-polymorphic option to produce polymorphism-aware code, as well as pass true as the last argument to the xml_schema::document's constructors. If some of your schemas do not require support for polymorphism then you can compile them with the --runtime-polymorphic option and still use the XSD/e runtime configured with polymorphism support.

When using the polymorphism-aware generated code, you can specify several serializers for a single element by passing a serializer map instead of an individual serializer to the serializer connection function for the element. One of the serializers will then be looked up and used depending on the user-provided type information that can optionally be set in the callback function for the element. Consider the following schema as an example:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:complexType name="person">
    <xs:sequence>
      <xs:element name="name" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>

  <!-- substitution group root -->
  <xs:element name="person" type="person"/>

  <xs:complexType name="superman">
    <xs:complexContent>
      <xs:extension base="person">
        <xs:attribute name="can-fly" type="xs:boolean"/>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:element name="superman"
              type="superman"
              substitutionGroup="person"/>

  <xs:complexType name="batman">
    <xs:complexContent>
      <xs:extension base="superman">
        <xs:attribute name="wing-span" type="xs:unsignedInt"/>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="supermen">
    <xs:sequence>
      <xs:element ref="person" maxOccurs="unbounded"/>
    </xs:sequence>
  </xs:complexType>

  <xs:element name="supermen" type="supermen"/>

</xs:schema>
  

Conforming XML documents can use the superman and batman types in place of the person type either by specifying the type with the xsi:type attributes or by using the elements from the substitution group, for instance:

<supermen xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

  <person>
    <name>John Doe</name>
  </person>

  <superman can-fly="false">
    <name>James "007" Bond</name>
  </superman>

  <person can-fly="true" wing-span="10" xsi:type="batman">
    <name>Bruce Wayne</name>
  </person>

</supermen>
  

The C++ object model for this vocabulary might look as follows:

#include <string>
#include <vector>

enum type_id
{
  person_type,
  superman_type,
  batman_type
};

class person
{
public:
  virtual
  ~person () {}

  person (const std::string& name)
      : name_ (name)
  {
  }

  const std::string&
  name () const
  {
    return name_;
  }

  void
  name (const std::string& n)
  {
    name_ = n;
  }

  virtual type_id
  type () const
  {
    return person_type;
  }

private:
  std::string name_;
};

class superman: public person
{
public:
  superman (const std::string& name, bool can_fly)
      : person (name), can_fly_ (can_fly)
  {
  }

  bool
  can_fly () const
  {
    return can_fly_;
  }

  void
  can_fly (bool cf)
  {
    can_fly_ = cf;
  }

  virtual type_id
  type () const
  {
    return superman_type;
  }

private:
  bool can_fly_;
};

class batman: public superman
{
public:
  batman (const std::string& name, unsigned int wing_span)
      : superman (name, true), wing_span_ (wing_span)
  {
  }

  unsigned int
  wing_span () const
  {
    return wing_span_;
  }

  void
  wing_span (unsigned int ws)
  {
    wing_span_ = ws;
  }

  virtual type_id
  type () const
  {
    return batman_type;
  }

private:
  unsigned int wing_span_;
};

// Poor man's polymorphic sequence which also assumes ownership
// of the elements.
//
class supermen: public std::vector<person*>
{
public:
  ~supermen ()
  {
    for (iterator i = begin (); i != end (); ++i)
      delete *i;
  }
};
  

Here we choose to provide our own type information. We can instead use the standard C++ typeid/type_info mechanism if it is available. The type map corresponding to this object model is presented below. Notice that the superman and batman objects are passed as a reference to person:

person   "const ::person&";
superman "const ::person&";
batman   "const ::person&";
supermen "const ::supermen&";
  

The serializer implementations that serialize the above C++ object model to XML are presented next:

class person_simpl: public person_sskel
{
public:
  virtual void
  pre (const person& p)
  {
    person_ = &p;
  }

  virtual std::string
  name ()
  {
    return person_->name ();
  }

  // Derived serializer implementations need access to this
  // member variable.
  //
public:
  const person* person_;
};

class superman_simpl: public superman_sskel
{
public:
  superman_simpl ()
    : superman_sskel (&base_impl_)
  {
  }

  virtual bool
  can_fly ()
  {
    return superman_ ().can_fly ();
  }

  const superman&
  superman_ ()
  {
    return *static_cast<const superman*> (base_impl_.person_);
  }

private:
  person_simpl base_impl_;
};

class batman_simpl: public batman_sskel
{
public:
  batman_simpl ()
    : batman_sskel (&base_impl_)
  {
  }

  virtual unsigned int
  wing_span ()
  {
    return batman_ ().wing_span ();
  }

  const batman&
  batman_ ()
  {
    return static_cast<const batman&> (base_impl_.superman_ ());
  }

private:
  superman_simpl base_impl_;
};

class supermen_simpl: public supermen_sskel
{
public:
  virtual void
  pre (const supermen& s)
  {
    supermen_ = &s;
    i_ = s.begin ();
  }

  virtual bool
  person_next ()
  {
    return i_ != supermen_->end ();
  }

  virtual const ::person&
  person ()
  {
    const ::person& p = **i_++;
    xml_schema::serializer_context& ctx = _context ();

    switch (p.type ())
    {
    case person_type:
      {
        ctx.type_id (person_sskel::_static_type ());
        break;
      }
    case superman_type:
      {
        ctx.type_id (superman_sskel::_static_type ());
        break;
      }
    case batman_type:
      {
        ctx.type_id (batman_sskel::_static_type ());
        break;
      }
    }

    return p;
  }

private:
  const supermen* supermen_;
  supermen::const_iterator i_;
};
  

Most of the code in these serializer implementations is the same as in the non-polymorphic case. The only part that explicitly deals with polymorphism is the person() callback in the superman_simpl class. In it we are translating the type information as provided by the C++ object mode to the type information used in the default implementation of the serializer map (we will talk more about serializer maps as well as the _static_type() function shortly). The type_id() function from xml_schema::serializer_context allows you to specify optional type information which is used to look up the corresponding serializer. Its argument is of type const void* which allows you to pass application-specific type information as an opaque pointer.

The following code fragment shows how to connect the serializers together and then use them to serialize a sample object model. Notice that for the person element in the instance_s serializer we specify a serializer map instead of a specific serializer and we pass true as the last argument to the document serializer constructor to indicate that we are serializing potentially-polymorphic XML documents:

int
main ()
{
  // Create a sample supermen catalog. To keep things simple
  // the following code is not exception-safe.
  //
  supermen sm;

  sm.push_back (new person ("John Doe"));
  sm.push_back (new superman ("James 007 Bond", false));
  sm.push_back (new batman ("Bruce Wayne", 10));

  // Construct the serializer.
  //
  xml_schema::string_simpl string_s;
  xml_schema::boolean_simpl boolean_s;
  xml_schema::unsigned_int_simpl unsigned_int_s;

  person_simpl person_s;
  superman_simpl superman_s;
  batman_simpl batman_s;

  xml_schema::serializer_map_impl person_map (5); // 5 hashtable buckets
  supermen_simpl supermen_s;

  person_s.serializers (string_s);
  superman_s.serializers (string_s, boolean_s);
  batman_s.serializers (string_s, boolean_s, unsigned_int_s);

  // Here we are specifying several serializers that can be
  // used to serialize the person element.
  //
  person_map.insert (person_s);
  person_map.insert (superman_s);
  person_map.insert (batman_s);

  supermen_s.person_serializer (person_map);

  // Create the XML instance document. The last argument to the
  // document's constructor indicates that we are serializing
  // polymorphic XML documents.
  //
  xml_schema::document_simpl doc_s (supermen_s, "supermen", true);

  supermen_s.pre (sm);
  doc_s.serialize (std::cout, xml_schema::document_simpl::pretty_print);
  supermen_s.post ();
}
  

When polymorphism-aware code is generated, each element's *_serializer() function is overloaded to also accept an object of the xml_schema::serializer_map type. For example, the supermen_sskel class from the above example looks like this:

class supermen_sskel: public xml_schema::serializer_complex_content
{
public:

  ...

  // Serializer construction API.
  //
  void
  serializers (person_sskel&);

  // Individual element serializers.
  //
  void
  person_serializer (person_sskel&);

  void
  person_serializer (xml_schema::serializer_map&);

  ...
};
  

Note that you can specify both the individual (static) serializer and the serializer map. The individual serializer will be used when the static element type and the dynamic type of the object being serialized are the same. This is the case when the type_id() function hasn't been called or the type information pointer is set to 0. Because the individual serializer for an element is cached and no map lookup is necessary, it makes sense to specify both the individual serializer and the serializer map when most of the objects being serialized are of the static type and optimal performance is important. The following code fragment shows how to change the above example to set both the individual serializer and the serializer map:

int
main ()
{
  ...

  // Here we are specifying several serializers that can be
  // used to serialize the person element.
  //
  person_map.insert (superman_s);
  person_map.insert (batman_s);

  supermen_s.person_serializer (person_s);
  supermen_s.person_serializer (person_map);

  ...
}
  

The xml_schema::serializer_map interface and its default implementation, xml_schema::serializer_map_impl, are presented below:

namespace xml_schema
{
  class serializer_map
  {
  public:
    virtual serializer_base*
    find (const void* type_id) const = 0;

    virtual void
    reset () const = 0;
  };

  class serializer_map_impl: public serializer_map
  {
  public:
    serializer_map_impl (size_t buckets);

    // Note that the type_id string is not copied so it should
    // be valid for the lifetime of the map.
    //
    void
    insert (const char* type_id, serializer_base&);

    // This version of insert is a shortcut that uses the string
    // returned by the serializer's _dynamic_type() function.
    //
    void
    insert (serializer_base&);

    virtual serializer_base*
    find (const void* type_id) const;

    virtual void
    reset () const;

  private:
    serializer_map_impl (const serializer_map_impl&);

    serializer_map_impl&
    operator= (const serializer_map_impl&);

    ...
  };
}
  

The type_id argument in the find() virtual function is the application-specific type information for the object being serialized that is specified using the type_id() function in the element callback. It is passed as an opaque const void*. The reset() virtual function is used to reset the serializers contained in the map (as opposed to resetting or clearing the map itself). For more information on serializer resetting refer to Section 8.4, "Reusing Serializers after an Error".

The XSD/e runtime provides the default implementation for the xml_schema::serializer_map interface, xml_schema::serializer_map_impl, which uses a C string (const char*) as type information. One way to obtain a serializer's dynamic type in the form "<name> <namespace>" with the space and the namespace part absent if the type does not have a namespace is to call the _dynamic_type() function on this serializer. The static type can be obtained by calling the static _static_type() function, for example person_sskel::_static_type(). Both functions return a C string (const char*) which is valid for as long as the application is running.

The default serializer map implementation is a hashmap. It requires that you specify the number of buckets it will contain and it does not support automatic table resizing. To obtain good performance the elements to buckets ratio should be between 0.7 and 0.9. It is also recommended to use prime numbers for bucket counts: 53, 97, 193, 389, 769, 1543, 3079, 6151, 12289, 24593, 49157, 98317, 196613, 393241.

If C++ exceptions are disabled (Section 5.3, "C++ Exceptions"), the xml_schema::serializer_map_impl class has the following additional error querying API. It can be used to detect the out of memory errors after calls to the serializer_map_impl's constructor and insert() functions.

namespace xml_schema
{
  class serializer_map_impl: public serializer_map
  {
  public:
    enum error
    {
      error_none,
      error_no_memory
    };

    error
    _error () const;

    ...
  };
}
  

You can also provide your own serializer map implementation which uses custom type information. The following example shows how we can implement our own serializer map for the above example that uses the type information provided by the C++ object model:

#include <map>

class person_serializer_map: public xml_schema::serializer_map
{
public:
 void
 insert (person_sskel& p)
 {
   const char* dt = p._dynamic_type ();
   type_id ti;

   if (strcmp (dt, person_sskel::_static_type ()) == 0)
     ti = person_type;
   else if (strcmp (dt, superman_sskel::_static_type ()) == 0)
     ti = superman_type;
   else if (strcmp (dt, batman_sskel::_static_type ()) == 0)
     ti = batman_type;
   else
     return;

   map_[ti] = &p;
 }

 virtual xml_schema::serializer_base*
 find (const char* x) const
 {
   const person* p = static_cast<const person*> (x);
   map::const_iterator i = map_.find (p->type ());
   return i != map_.end () ? i->second : 0;
 }

 virtual void
 reset () const
 {
   for (map::const_iterator i (map_.begin ()), e (map_.end ());
        i != e; ++i)
   {
     person_sskel* s = i->second;
     s->_reset ();
   }
 }

private:
  typedef std::map<type_id, person_sskel*> map;
  map map_;
};
  

Our custom implementation of the serializer map expects that we pass the actual object to the find() function. To account for this will need to change the supermen_simpl::person() callback as follows:

  virtual const ::person&
  person ()
  {
    const ::person& p = **i_++;
    _context ().type_id (&p);
    return p;
  }
  

To support polymorphic serialization the XSD/e runtime and generated code maintain a number of hashmaps that contain substitution and, if XML Schema validation is enabled (Section 5.4, "XML Schema Validation"), inheritance information. Because the number of elements in these hashmaps depends on the schemas being compiled and thus is fairly static, these hashmaps do not perform automatic table resizing and instead the number of buckets is specified when the XSD/e runtime is configured. To obtain good performance the elements to buckets ratio in these hashmaps should be between 0.7 and 0.9. The recommended way to ensure this range is to add diagnostics code to your application as shown in the following example:

int
main ()
{
  // Check that the load in substitution and inheritance hashmaps
  // is not too high.
  //
#ifndef NDEBUG
  float load = xml_schema::serializer_smap_elements ();
  load /= xml_schema::serializer_smap_buckets ();

  if (load > 0.8)
  {
    cerr << "substitution hashmap load is " << load << endl;
    cerr << "time to increase XSDE_SERIALIZER_SMAP_BUCKETS" << endl;
  }

  load = xml_schema::serializer_smap_bucket_elements ();
  load /= xml_schema::serializer_smap_bucket_buckets ();

  if (load > 0.8)
  {
    cerr << "substitution inner hashmap load is " << load << endl;
    cerr << "time to increase XSDE_SERIALIZER_SMAP_BUCKET_BUCKETS" << endl;
  }

  load = xml_schema::serializer_imap_elements ();
  load /= xml_schema::serializer_imap_buckets ();

  if (load > 0.8)
  {
    cerr << "inheritance hashmap load is " << load << endl;
    cerr << "time to increase XSDE_SERIALIZER_IMAP_BUCKETS" << endl;
  }
#endif

  ...
}
  

Most of the code presented in this section is taken from the polymorphism example which can be found in the examples/cxx/serializer/ directory of the XSD/e distribution. Handling of xsi:type and substitution groups when used on root elements requires a number of special actions as shown in the polyroot example.

6.8 Custom Allocators

By default the XSD/e runtime and generated code use the standard operators new and delete to manage dynamic memory. However, it is possible to instead use custom allocator functions provided by your application. To achieve this, configure the XSD/e runtime library to use custom allocator functions as well as pass the --custom-allocator option to the XSD/e compiler when translating your schemas. The signatures of the custom allocator functions that should be provided by your application are listed below. Their semantics should be equivalent to the standard C malloc(), realloc(), and free() functions.

extern "C" void*
xsde_alloc (size_t);

extern "C" void*
xsde_realloc (void*, size_t);

extern "C" void
xsde_free (void*);
  

Note also that when custom allocators are enabled, any dynamically-allocated object of which the XSD/e runtime or generated code assume ownership should be allocated using the custom allocation function. Similarly, if your application assumes ownership of any dynamically-allocated object returned by the XSD/e runtime or the generated code, then such an object should be disposed of using the custom deallocation function. To help with these tasks the generated xml_schema namespace defines the following two helper functions and, if C++ exceptions are enabled, automatic pointer class:

namespace xml_schema
{
  void*
  alloc (size_t);

  void
  free (void*);

  struct alloc_guard
  {
    alloc_guard (void*);
    ~alloc_guard ();

    void*
    get () const;

    void
    release ();

  private:
    ...
  };
}
  

If C++ exceptions are disabled, these functions are equivalent to xsde_alloc() and xsde_free(). If exceptions are enabled, xml_schema::alloc() throws std::bad_alloc on memory allocation failure.

The following code fragment shows how to create and destroy a dynamically-allocated object with custom allocators when C++ exceptions are disabled:

void* v = xml_schema::alloc (sizeof (type));

if (v == 0)
{
  // Handle out of memory condition.
}

type* x = new (v) type (1, 2);

...

if (x)
{
  x->~type ();
  xml_schema::free (x);
}
  

The equivalent code fragment for configurations with C++ exceptions enabled is shown below:

xml_schema::alloc_guard g (xml_schema::alloc (sizeof (type)));
type* x = new (g.get ()) type (1, 2);
g.release ();

...

if (x)
{
  x->~type ();
  xml_schema::free (x);
}
  

6.9 A Minimal Example

The following example is a re-implementation of the person records example presented in Chapter 4, "Type Maps". It is intended to work without STL, iostream, and C++ exceptions. It can be found in the examples/cxx/serializer/minimal/ directory of the XSD/e distribution. The people.xsd schema is compiled with the --no-stl, --no-iostream, and --no-exceptions options. The object model types in people.hxx have also been reimplemented in order not to use STL types:

#include <stddef.h> // size_t

enum gender
{
  male,
  female
};

struct person
{
  const char* first_name_;
  const char* last_name_;
  gender gender_;
  unsigned short age_;
};

struct people
{
  person* people_;
  size_t size_;
};
  

The following listing presents the implementation of serializer skeletons and the test driver in full:

#include <stdio.h>
#include "people-sskel.hxx"

const char* gender_strings[] = {"male", "female"};

class gender_simpl: public gender_sskel
{
public:
  gender_simpl ()
    : gender_sskel (&base_impl_)
  {
  }

  virtual void
  pre (gender g)
  {
    base_impl_.pre (gender_strings[g]);
  }

private:
  public xml_schema::string_simpl base_impl_;
};

class person_simpl: public person_sskel
{
public:
  virtual void
  pre (const person& p)
  {
    person_ = &p;
  }

  virtual const char*
  first_name ()
  {
    return person_->first_name_;
  }

  virtual const char*
  last_name ()
  {
    return person_->last_name_;
  }

  virtual ::gender
  gender ()
  {
    return person_->gender_;
  }

  virtual unsigned short
  age ()
  {
    return person_->age_;
  }

private:
  const person* person_;
};

class people_simpl: public people_sskel
{
public:
  virtual void
  pre (const people& p)
  {
    i_ = 0;
    people_ = &p;
  }

  virtual bool
  person_next ()
  {
    return i_ < people_->size_;
  }

  virtual const ::person&
  person ()
  {
    return people_->people_[i_++];
  }

private:
  size_t i_;
  const people* people_;
};

class writer: public xml_schema::writer
{
public:
  virtual bool
  write (const char* s, size_t n)
  {
    return fwrite (s, n, 1, stdout) == 1;
  }

  virtual bool
  flush ()
  {
    return fflush (stdout) == 0;
  }
};

int
main ()
{
  // Create a sample person list.
  //
  people p;

  p.size_ = 2;
  p.people_ = new person[p.size_];

  if (p.people_ == 0)
  {
    fprintf (stderr, "error: no memory\n");
    return 1;
  }

  p.people_[0].first_name_ = "John";
  p.people_[0].last_name_ = "Doe";
  p.people_[0].gender_ = male;
  p.people_[0].age_ = 32;

  p.people_[1].first_name_ = "Jane";
  p.people_[1].last_name_ = "Doe";
  p.people_[1].gender_ = female;
  p.people_[1].age_ = 28;

  // Construct the serializer.
  //
  xml_schema::unsigned_short_simpl unsigned_short_s;
  xml_schema::string_simpl string_s;

  gender_simpl gender_s;
  person_simpl person_s;
  people_simpl people_s;

  person_s.serializers (string_s, string_s, gender_s, unsigned_short_s);
  people_s.serializers (person_s);

  // Serialize.
  //
  typedef xml_schema::serializer_error error;

  error e;
  writer w;

  do
  {
    xml_schema::document_simpl doc_s (people_s, "people");

    if (e = doc_s._error ())
      break;

    people_s.pre (p);

    if (e = people_s._error ())
      break;

    doc_s.serialize (w, xml_schema::document_simpl::pretty_print);

    if (e = doc_s._error ())
      break;

    people_s.post ();

    e = people_s._error ();

  } while (false);

  delete[] p.people_;

  // Handle errors.
  //
  if (e)
  {
    switch (e.type ())
    {
    case error::sys:
      {
        fprintf (stderr, "error: %s\n", e.sys_text ());
        break;
      }
    case error::xml:
      {
        fprintf (stderr, "error: %s\n", e.xml_text ());
        break;
      }
    case error::schema:
      {
        fprintf (stderr, "error: %s\n", e.schema_text ());
        break;
      }
    case error::app:
      {
        fprintf (stderr, "application error: %d\n", e.app_code ());
        break;
      }
    default:
      break;
    }

    return 1;
  }

  return 0;
}
  

7 Built-In XML Schema Type Serializers

The XSD/e runtime provides serializer implementations for all built-in XML Schema types as summarized in the following table. Declarations for these types are automatically included into each generated header file. As a result you don't need to include any headers to gain access to these serializer implementations.

XML Schema type	Serializer implementation in the xml_schema namespace	Serializer argument type
anyType and anySimpleType types
anyType	any_type_simpl	void
anySimpleType	any_simple_type_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
fixed-length integral types
byte	byte_simpl	signed char
unsignedByte	unsigned_byte_simpl	unsigned char
short	short_simpl	short
unsignedShort	unsigned_short_simpl	unsigned short
int	int_simpl	int
unsignedInt	unsigned_int_simpl	unsigned int
long	long_simpl	long long or long Section 6.5, "64-bit Integer Type"
unsignedLong	unsigned_long_simpl	unsigned long long or unsigned long Section 6.5, "64-bit Integer Type"
arbitrary-length integral types
integer	integer_simpl	long
nonPositiveInteger	non_positive_integer_simpl	long
nonNegativeInteger	non_negative_integer_simpl	unsigned long
positiveInteger	positive_integer_simpl	unsigned long
negativeInteger	negative_integer_simpl	long
boolean types
boolean	boolean_simpl	bool
fixed-precision floating-point types
float	float_simpl	float Section 7.1, "Floating-Point Type Serializers"
double	double_simpl	double Section 7.1, "Floating-Point Type Serializers"
arbitrary-precision floating-point types
decimal	decimal_simpl	double Section 7.1, "Floating-Point Type Serializers"
string-based types
string	string_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
normalizedString	normalized_string_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
token	token_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
Name	name_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
NMTOKEN	nmtoken_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
NCName	ncname_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
language	language_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
qualified name
QName	qname_simpl	const xml_schema::qname& or const xml_schema::qname* Section 7.3, "QName Serializer"
ID/IDREF types
ID	id_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
IDREF	idref_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
list types
NMTOKENS	nmtokens_simpl	const xml_schema::string_sequence* Section 7.4, "NMTOKENS and IDREFS Serializers"
IDREFS	idrefs_simpl	const xml_schema::string_sequence* Section 7.4, "NMTOKENS and IDREFS Serializers"
URI types
anyURI	uri_simpl	const std::string& or const char* Section 7.2, "String-Based Type Serializers"
binary types
base64Binary	base64_binary_simpl	const xml_schema::buffer* Section 7.5, "base64Binary and hexBinary Serializers"
hexBinary	hex_binary_simpl	const xml_schema::buffer* Section 7.5, "base64Binary and hexBinary Serializers"
date/time types
date	date_simpl	const xml_schema::date& Section 7.7, "date Serializer"
dateTime	date_time_simpl	const xml_schema::date_time& Section 7.8, "dateTime Serializer"
duration	duration_simpl	const xml_schema::duration& Section 7.9, "duration Serializer"
gDay	gday_simpl	const xml_schema::gday& Section 7.10, "gDay Serializer"
gMonth	gmonth_simpl	const xml_schema::gmonth& Section 7.11, "gMonth Serializer"
gMonthDay	gmonth_day_simpl	const xml_schema::gmonth_day& Section 7.12, "gMonthDay Serializer"
gYear	gyear_simpl	const xml_schema::gyear& Section 7.13, "gYear Serializer"
gYearMonth	gyear_month_simpl	const xml_schema::gyear_month& Section 7.14, "gYearMonth Serializer"
time	time_simpl	const xml_schema::time& Section 7.15, "time Serializer"

7.1 Floating-Point Type Serializers

The serializer implementations for the float, double, and decimal built-in XML Schema types allow you to specify the resulting notation (fixed or scientific) as well as precision. This is done by passing the corresponding arguments to their constructors:

namespace xml_schema
{
  class float_simpl: public float_sskel
  {
    enum notation
    {
      notation_auto,
      notation_fixed,
      notation_scientific
    };

    float_simpl (notation = notation_auto,
                 unsigned int precision = FLT_DIG);

    virtual void
    pre (float);

    ...
  };

  class double_simpl: public double_sskel
  {
    enum notation
    {
      notation_auto,
      notation_fixed,
      notation_scientific
    };

    double_simpl (notation = notation_auto,
                  unsigned int precision = DBL_DIG);

    virtual void
    pre (double);

    ...
  };

  class decimal_simpl: public decimal_sskel
  {
    decimal_simpl (unsigned int precision = DBL_DIG);

    virtual void
    pre (double);

    ...
  };
}
  

By default the notation for the float and double types is automatically selected to produce the shortest representation. Note that the decimal values are always serialized in the fixed-point notation.

7.2 String-Based Type Serializers

When STL is enabled (Section 6.1, "Standard Template Library"), the serializer argument type for the string, normalizedString, token, Name, NMTOKEN, NCName, ID, IDREF, language, anyURI, and anySimpleType built-in XML Schema types is const std::string&. When STL is disabled, the value is passed as a constant C-string: const char*. In this case, you can also instruct the serializer implementations for string-based types to release the string with operator delete[] by passing true to their constructors. For instance, using the person records example from the previous chapter:

class person_simpl: public person_sskel
{
public:
  virtual const char*
  first_name ()
  {
    char* r = new char[5];
    strcpy (r, "John");
    return r;
  }

  virtual const char*
  last_name ()
  {
    char* r = new char[4];
    strcpy (r, "Doe");
    return r;
  }

  ...
};

int
main ()
{
  // Construct the serializer.
  //
  xml_schema::unsigned_short_simpl unsigned_short_s;
  xml_schema::string_simpl string_s (true); // Release the string passed.

  gender_simpl gender_s;
  person_simpl person_s;
  people_simpl people_s;

  person_s.serializers (string_s, string_s, gender_s, unsigned_short_s);

  ...
}
  

7.3 QName Serializer

The argument type of the qname_simpl serializer implementation is either const xml_schema::qname& when STL is enabled (Section 6.1, "Standard Template Library") or const xml_schema::qname* when STL is disabled. The qname class represents an XML qualified name. When the argument type is const xml_schema::qname*, you can optionally instruct the serializer to release the qname object with operator delete by passing true to its constructor.

With STL enabled, the qname type has the following interface:

namespace xml_schema
{
  class qname
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    qname ();

    explicit
    qname (const std::string& name);
    qname (const std::string& prefix, const std::string& name);

    void
    swap (qname&);

    const std::string&
    prefix () const;

    std::string&
    prefix ();

    void
    prefix (const std::string&);

    const std::string&
    name () const;

    std::string&
    name ();

    void
    name (const std::string&);
  };

  bool
  operator== (const qname&, const qname&);

  bool
  operator!= (const qname&, const qname&);
}
  

When STL is disabled and C++ exceptions are enabled (Section 6.3, "C++ Exceptions"), the qname type has the following interface:

namespace xml_schema
{
  class qname
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    qname ();

    explicit
    qname (char* name);
    qname (char* prefix, char* name);

    void
    swap (qname&);

  private:
    qname (const qname&);

    qname&
    operator= (const qname&);

  public:
    char*
    prefix ();

    const char*
    prefix () const;

    void
    prefix (char*);

    void
    prefix_copy (const char*);

    char*
    prefix_detach ();

  public:
    char*
    name ();

    const char*
    name () const;

    void
    name (char*);

    void
    name_copy (const char*);

    char*
    name_detach ();
  };

  bool
  operator== (const qname&, const qname&);

  bool
  operator!= (const qname&, const qname&);
}

The modifier functions and constructors that have the char* argument assume ownership of the passed strings which should be allocated with operator new char[] and will be deallocated with operator delete[] by the qname object. If you detach the underlying prefix or name strings, then they should eventually be deallocated with operator delete[].

Finally, if both STL and C++ exceptions are disabled, the qname type has the following interface:

namespace xml_schema
{
  class qname
  {
  public:
    enum error
    {
      error_none,
      error_no_memory
    };

    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    qname ();

    explicit
    qname (char* name);
    qname (char* prefix, char* name);

    void
    swap (qname&);

  private:
    qname (const qname&);

    qname&
    operator= (const qname&);

  public:
    char*
    prefix ();

    const char*
    prefix () const;

    void
    prefix (char*);

    error
    prefix_copy (const char*);

    char*
    prefix_detach ();

  public:
    char*
    name ();

    const char*
    name () const;

    void
    name (char*);

    error
    name_copy (const char*);

    char*
    name_detach ();
  };

  bool
  operator== (const qname&, const qname&);

  bool
  operator!= (const qname&, const qname&);
}
  

7.4 NMTOKENS and IDREFS Serializers

The argument type of the nmtokens_simpl and idrefs_simpl serializer implementations is const xml_schema::string_sequence*. You can optionally instruct these serializers to release the string_sequence object with operator delete by passing true to their constructors. With STL and C++ exceptions enabled (Section 6.1, "Standard Template Library", Section 6.3, "C++ Exceptions"), the string_sequence type has the following interface:

namespace xml_schema
{
  class string_sequence
  {
  public:
    typedef std::string         value_type;
    typedef std::string*        pointer;
    typedef const std::string*  const_pointer;
    typedef std::string&        reference;
    typedef const std::string&  const_reference;

    typedef size_t              size_type;
    typedef ptrdiff_t           difference_type;

    typedef std::string*        iterator;
    typedef const std::string*  const_iterator;

  public:
    string_sequence ();

    void
    swap (string_sequence&);

  private:
    string_sequence (string_sequence&);

    string_sequence&
    operator= (string_sequence&);

  public:
    iterator
    begin ();

    const_iterator
    begin () const;

    iterator
    end ();

    const_iterator
    end () const;

    std::string&
    front ();

    const std::string&
    front () const;

    std::string&
    back ();

    const std::string&
    back () const;

    std::string&
    operator[] (size_t);

    const std::string&
    operator[] (size_t) const;

  public:
    bool
    empty () const;

    size_t
    size () const;

    size_t
    capacity () const;

    size_t
    max_size () const;

  public:
    void
    clear ();

    void
    pop_back ();

    iterator
    erase (iterator);

    void
    push_back (const std::string&);

    iterator
    insert (iterator, const std::string&);

    void
    reserve (size_t);
  };

  bool
  operator== (const string_sequence&, const string_sequence&);

  bool
  operator!= (const string_sequence&, const string_sequence&);
}
  

When STL is enabled and C++ exceptions are disabled, the signatures of the push_back(), insert(), and reserve() functions change as follows:

namespace xml_schema
{
  class string_sequence
  {
  public:
    enum error
    {
      error_none,
      error_no_memory
    };

    ...

  public:
    error
    push_back (const std::string&);

    error
    insert (iterator, const std::string&);

    error
    insert (iterator, const std::string&, iterator& result);

    error
    reserve (size_t);
  };
}
  

When STL is disabled and C++ exceptions are enabled, the string_sequence type has the following interface:

namespace xml_schema
{
  class string_sequence
  {
  public:
    typedef char*         value_type;
    typedef char**        pointer;
    typedef const char**  const_pointer;
    typedef char*         reference;
    typedef const char*   const_reference;

    typedef size_t        size_type;
    typedef ptrdiff_t     difference_type;

    typedef char** iterator;
    typedef const char* const* const_iterator;

    string_sequence ();

    void
    swap (string_sequence&);

  private:
    string_sequence (string_sequence&);

    string_sequence&
    operator= (string_sequence&);

  public:
    iterator
    begin ();

    const_iterator
    begin () const;

    iterator
    end ();

    const_iterator
    end () const;

    char*
    front ();

    const char*
    front () const;

    char*
    back ();

    const char*
    back () const;

    char*
    operator[] (size_t);

    const char*
    operator[] (size_t) const;

  public:
    bool
    empty () const;

    size_t
    size () const;

    size_t
    capacity () const;

    size_t
    max_size () const;

  public:
    void
    clear ();

    void
    pop_back ();

    iterator
    erase (iterator);

    void
    push_back (char*);

    void
    push_back_copy (const char*);

    iterator
    insert (iterator, char*);

    void
    reserve (size_t);

    // Detach a string from the sequence at a given position.
    // The string pointer at this position in the sequence is
    // set to 0.
    //
    char*
    detach (iterator);
  };

  bool
  operator== (const string_sequence&, const string_sequence&);

  bool
  operator!= (const string_sequence&, const string_sequence&);
}
  

The push_back() and insert() functions assume ownership of the passed string which should be allocated with operator new char[] and will be deallocated with operator delete[] by the string_sequence object. These two functions free the passed object if the reallocation of the underlying sequence buffer fails. The push_back_copy() function makes a copy of the passed string. If you detach the underlying element string, then it should eventually be deallocated with operator delete[].

When both STL and C++ exceptions are disabled, the signatures of the push_back(), push_back_copy(), insert(), and reserve() functions change as follows:

namespace xml_schema
{
  class string_sequence
  {
  public:
    enum error
    {
      error_none,
      error_no_memory
    };

    ...

  public:
    error
    push_back (char*);

    error
    push_back_copy (const char*);

    error
    insert (iterator, char*);

    error
    insert (iterator, char*, iterator& result);

    error
    reserve (size_t);
  };
}
  

7.5 base64Binary and hexBinary Serializers

The argument type of the base64_binary_simpl and hex_binary_simpl serializer implementations is const xml_schema::buffer*. You can optionally instruct these serializers to release the buffer object with operator delete by passing true to their constructors. With C++ exceptions enabled (Section 6.3, "C++ Exceptions"), the buffer type has the following interface:

namespace xml_schema
{
  class buffer
  {
  public:
    class bounds {}; // Out of bounds exception.

  public:
    buffer ();

    explicit
    buffer (size_t size);
    buffer (size_t size, size_t capacity);
    buffer (const void* data, size_t size);
    buffer (const void* data, size_t size, size_t capacity);

    enum ownership_value { assume_ownership };

    // This constructor assumes ownership of the memory passed.
    //
    buffer (void* data, size_t size, size_t capacity, ownership_value);

  private:
    buffer (const buffer&);

    buffer&
    operator= (const buffer&);

  public:
    void
    attach (void* data, size_t size, size_t capacity);

    void*
    detach ();

    void
    swap (buffer&);

  public:
    size_t
    capacity () const;

    bool
    capacity (size_t);

  public:
    size_t
    size () const;

    bool
    size (size_t);

  public:
    const char*
    data () const;

    char*
    data ();

    const char*
    begin () const;

    char*
    begin ();

    const char*
    end () const;

    char*
    end ();
  };

  bool
  operator== (const buffer&, const buffer&);

  bool
  operator!= (const buffer&, const buffer&);
}
  

The last constructor and the attach() member function make the buffer instance assume the ownership of the memory block pointed to by the data argument and eventually release it by calling operator delete(). The detach() member function detaches and returns the underlying memory block which should eventually be released by calling operator delete().

The capacity() and size() modifier functions return true if the underlying buffer has moved. The bounds exception is thrown if the constructor or attach() member function arguments violate the (size <= capacity) constraint.

If C++ exceptions are disabled, the buffer type has the following interface:

namespace xml_schema
{
  class buffer
  {
  public:
    enum error
    {
      error_none,
      error_bounds,
      error_no_memory
    };

    buffer ();

  private:
    buffer (const buffer&);

    buffer&
    operator= (const buffer&);

  public:
    error
    attach (void* data, size_t size, size_t capacity);

    void*
    detach ();

    void
    swap (buffer&);

  public:
    size_t
    capacity () const;

    error
    capacity (size_t);

    error
    capacity (size_t, bool& moved);

  public:
    size_t
    size () const;

    error
    size (size_t);

    error
    size (size_t, bool& moved);

  public:
    const char*
    data () const;

    char*
    data ();

    const char*
    begin () const;

    char*
    begin ();

    const char*
    end () const;

    char*
    end ();
  };

  bool
  operator== (const buffer&, const buffer&);

  bool
  operator!= (const buffer&, const buffer&);
}
  

7.6 Time Zone Representation

The date, dateTime, gDay, gMonth, gMonthDay, gYear, gYearMonth, and time XML Schema built-in types all include an optional time zone component. The following xml_schema::time_zone base class is used to represent this information:

namespace xml_schema
{
  class time_zone
  {
  public:
    time_zone ();
    time_zone (short hours, short minutes);

    bool
    zone_present () const;

    void
    zone_reset ();

    short
    zone_hours () const;

    void
    zone_hours (short);

    short
    zone_minutes () const;

    void
    zone_minutes (short);
  };

  bool
  operator== (const time_zone&, const time_zone&);

  bool
  operator!= (const time_zone&, const time_zone&);
}
  

The zone_present() accessor function returns true if the time zone is specified. The zone_reset() modifier function resets the time zone object to the not specified state. If the time zone offset is negative then both hours and minutes components are represented as negative integers.

7.7 date Serializer

The argument type of the date_simpl serializer implementation is const xml_schema::date&. The date class represents a year, a day, and a month with an optional time zone. Its interface is presented below. For more information on the base xml_schema::time_zone class refer to Section 7.6, "Time Zone Representation".

namespace xml_schema
{
  class date: public time_zone
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    date ();

    date (int year, unsigned short month, unsigned short day);

    date (int year, unsigned short month, unsigned short day,
          short zone_hours, short zone_minutes);

    int
    year () const;

    void
    year (int);

    unsigned short
    month () const;

    void
    month (unsigned short);

    unsigned short
    day () const;

    void
    day (unsigned short);
  };

  bool
  operator== (const date&, const date&);

  bool
  operator!= (const date&, const date&);
}
  

7.8 dateTime Serializer

The argument type of the date_time_simpl serializer implementation is const xml_schema::date_time&. The date_time class represents a year, a month, a day, hours, minutes, and seconds with an optional time zone. Its interface is presented below. For more information on the base xml_schema::time_zone class refer to Section 7.6, "Time Zone Representation".

namespace xml_schema
{
  class date_time: public time_zone
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    date_time ();

    date_time (int year, unsigned short month, unsigned short day,
               unsigned short hours, unsigned short minutes,
               double seconds);

    date_time (int year, unsigned short month, unsigned short day,
               unsigned short hours, unsigned short minutes,
               double seconds, short zone_hours, short zone_minutes);

    int
    year () const;

    void
    year (int);

    unsigned short
    month () const;

    void
    month (unsigned short);

    unsigned short
    day () const;

    void
    day (unsigned short);

    unsigned short
    hours () const;

    void
    hours (unsigned short);

    unsigned short
    minutes () const;

    void
    minutes (unsigned short);

    double
    seconds () const;

    void
    seconds (double);
  };

  bool
  operator== (const date_time&, const date_time&);

  bool
  operator!= (const date_time&, const date_time&);
}
  

7.9 duration Serializer

The argument type of the duration_simpl serializer implementation is const xml_schema::duration&. The duration class represents a potentially negative duration in the form of years, months, days, hours, minutes, and seconds. Its interface is presented below.

namespace xml_schema
{
  class duration
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    duration ();

    duration (bool negative,
              unsigned int years, unsigned int months, unsigned int days,
              unsigned int hours, unsigned int minutes, double seconds);

    bool
    negative () const;

    void
    negative (bool);

    unsigned int
    years () const;

    void
    years (unsigned int);

    unsigned int
    months () const;

    void
    months (unsigned int);

    unsigned int
    days () const;

    void
    days (unsigned int);

    unsigned int
    hours () const;

    void
    hours (unsigned int);

    unsigned int
    minutes () const;

    void
    minutes (unsigned int);

    double
    seconds () const;

    void
    seconds (double);
  };

  bool
  operator== (const duration&, const duration&);

  bool
  operator!= (const duration&, const duration&);
}
  

7.10 gDay Serializer

The argument type of the gday_simpl serializer implementation is const xml_schema::gday& The gday class represents a day of the month with an optional time zone. Its interface is presented below. For more information on the base xml_schema::time_zone class refer to Section 7.6, "Time Zone Representation".

namespace xml_schema
{
  class gday: public time_zone
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    gday ();

    explicit
    gday (unsigned short day);

    gday (unsigned short day, short zone_hours, short zone_minutes);

    unsigned short
    day () const;

    void
    day (unsigned short);
  };

  bool
  operator== (const gday&, const gday&);

  bool
  operator!= (const gday&, const gday&);
}
  

7.11 gMonth Serializer

The argument type of the gmonth_simpl serializer implementation is const xml_schema::gmonth&. The gmonth class represents a month of the year with an optional time zone. Its interface is presented below. For more information on the base xml_schema::time_zone class refer to Section 7.6, "Time Zone Representation".

namespace xml_schema
{
  class gmonth: public time_zone
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    gmonth ();

    explicit
    gmonth (unsigned short month);

    gmonth (unsigned short month,
            short zone_hours, short zone_minutes);

    unsigned short
    month () const;

    void
    month (unsigned short);
  };

  bool
  operator== (const gmonth&, const gmonth&);

  bool
  operator!= (const gmonth&, const gmonth&);
}
  

7.12 gMonthDay Serializer

The argument type of the gmonth_day_simpl serializer implementation is const xml_schema::gmonth_day&. The gmonth_day class represents a day and a month of the year with an optional time zone. Its interface is presented below. For more information on the base xml_schema::time_zone class refer to Section 7.6, "Time Zone Representation".

namespace xml_schema
{
  class gmonth_day: public time_zone
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    gmonth_day ();

    gmonth_day (unsigned short month, unsigned short day);

    gmonth_day (unsigned short month, unsigned short day,
                short zone_hours, short zone_minutes);

    unsigned short
    month () const;

    void
    month (unsigned short);

    unsigned short
    day () const;

    void
    day (unsigned short);
  };

  bool
  operator== (const gmonth_day&, const gmonth_day&);

  bool
  operator!= (const gmonth_day&, const gmonth_day&);
}
  

7.13 gYear Serializer

The argument type of the gyear_simpl serializer implementation is const xml_schema::gyear&. The gyear class represents a year with an optional time zone. Its interface is presented below. For more information on the base xml_schema::time_zone class refer to Section 7.6, "Time Zone Representation".

namespace xml_schema
{
  class gyear: public time_zone
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    gyear ();

    explicit
    gyear (int year);

    gyear (int year, short zone_hours, short zone_minutes);

    int
    year () const;

    void
    year (int);
  };

  bool
  operator== (const gyear&, const gyear&);

  bool
  operator!= (const gyear&, const gyear&);
}
  

7.14 gYearMonth Serializer

The argument type of the gyear_month_simpl serializer implementation is const xml_schema::gyear_month&. The gyear_month class represents a year and a month with an optional time zone. Its interface is presented below. For more information on the base xml_schema::time_zone class refer to Section 7.6, "Time Zone Representation".

namespace xml_schema
{
  class gyear_month: public time_zone
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    gyear_month ();

    gyear_month (int year, unsigned short month);

    gyear_month (int year, unsigned short month,
                 short zone_hours, short zone_minutes);

    int
    year () const;

    void
    year (int);

    unsigned short
    month () const;

    void
    month (unsigned short);
  };

  bool
  operator== (const gyear_month&, const gyear_month&);

  bool
  operator!= (const gyear_month&, const gyear_month&);
}
  

7.15 time Serializer

The argument type of the time_simpl serializer implementation is const xml_schema::time&. The time class represents hours, minutes, and seconds with an optional time zone. Its interface is presented below. For more information on the base xml_schema::time_zone class refer to Section 7.6, "Time Zone Representation".

namespace xml_schema
{
  class time: public time_zone
  {
  public:
    // The default constructor creates an uninitialized object.
    // Use modifiers to initialize it.
    //
    time ();

    time (unsigned short hours, unsigned short minutes, double seconds);

    time (unsigned short hours, unsigned short minutes, double seconds,
          short zone_hours, short zone_minutes);

    unsigned short
    hours () const;

    void
    hours (unsigned short);

    unsigned short
    minutes () const;

    void
    minutes (unsigned short);

    double
    seconds () const;

    void
    seconds (double);
  };

  bool
  operator== (const time&, const time&);

  bool
  operator!= (const time&, const time&);
}
  

8 Document Serializer and Error Handling

In this chapter we will discuss the xml_schema::document_simpl type, the error handling mechanisms provided by the mapping, as well as how to reuse a serializer after an error has occurred.

There are four categories of errors that can result from running a serializer to produce an XML instance: system, xml, schema, and application. The system category contains memory allocation and input/output operation errors. The xml category is for XML serialization and well-formedness checking errors. Similarly, the schema category is for XML Schema validation errors. Finally, the application category is for application logic errors that you may want to propagate from serializer implementations to the caller of the serializer.

The C++/Serializer mapping supports two methods of reporting errors: using C++ exceptions and with error codes. The method used depends on whether or not you have configured the XSD/e runtime and the generated code with C++ exceptions enabled, as described in Section 6.3, "C++ Exceptions".

8.1 Document Serializer

The xml_schema::document_simpl serializer is a root serializer for the vocabulary. As mentioned in Section 3.4, "Connecting the Serializer Together", its interface varies depending on the mapping configuration (Chapter 6, "Mapping Configuration"). When STL, C++ exceptions, and the iostream library are enabled, the xml_schema::document_simpl class has the following interface:

namespace xml_schema
{
  class serializer_base;

  class writer
  {
  public:
    // The first write function is called to write a '\0'-terminated
    // string. Its default implementation calls the second versions:
    // write (s, strlen (s)). These functions use exceptions to
    // indicate a write failure.
    //
    virtual void
    write (const char* s);

    virtual void
    write (const char* s, size_t n) = 0;

    virtual void
    flush () = 0;
  };

  class document_simpl
  {
  public:
    document_simpl (serializer_base&,
                    const char* root_element_name);

    document_simpl (serializer_base&,
                    const char* root_element_namespace,
                    const char* root_element_name);

    document_simpl (serializer_base&,
                    const std::string& root_element_name);

    document_simpl (serializer_base&,
                    const std::string& root_element_namespace,
                    const std::string& root_element_name);

  public:
    void
    add_prefix (const char* prefix, const char* namespace_);

    void
    add_default_prefix (const char* namespace_);

    void
    add_schema (const char* namespace_, const char* location);

    void
    add_no_namespace_schema (const char* location);

    void
    add_prefix (const std::string& prefix,
                const std::string& namespace_);

    void
    add_default_prefix (const std::string& namespace_);

    void
    add_schema (const std::string& namespace_,
                const std::string& location);

    void
    add_no_namespace_schema (const std::string& location);

  public:
    // Serialization flags.
    //
    typedef unsigned short flags;

    static const flags pretty_print;

  public:
    // Serialize to std::ostream. The std::ios_base::failure
    // exception is used to report io errors (badbit and failbit)
    // if C++ exceptions are enabled. Otherwise error codes are
    // used.
    //
    void
    serialize (std::ostream&, flags = 0);

  public:
    // Serialize by calling writer::write() and writer::flush() to
    // output XML.
    //
    void
    serialize (writer&, flags = 0);

    // Serialize by calling the write and flush functions. If the
    // unbounded write function is not provided, the bounded version
    // is called: write_bound_func (s, strlen (s)). user_data is
    // passed as a first argument to these functions. These functions
    // use exceptions to indicate a write failure.
    //
    typedef void (*write_func) (void*, const char*);
    typedef void (*write_bound_func) (void*, const char*, size_t);
    typedef void (*flush_func) (void*);

    void
    serialize (write_bound_func,
               flush_func,
               void* user_data,
               flags = 0);

    void
    serialize (write_func,
               write_bound_func,
               flush_func,
               void* user_data,
               flags = 0);
  public:
    // Low-level, genx-specific serialization. With this method
    // it is your responsibility to call genxStartDoc*() and
    // genxEndDocument().
    //
    void
    serialize (genxWriter);
  };
}
  

When the use of STL is disabled, the constructors, as well as the add_prefix() and add_schema() functions that use std::string in their signatures are not available. When the use of iostream is disabled, the serialize() functions that serializes to std::ostream is not available.

When C++ exceptions are disabled, the write() and flush() virtual functions in the writer interface as well as write_func, write_bound_func, and flush_func function pointers use bool return type for error reporting. These functions should return true if the operation was successful and false otherwise. The relevant parts in the writer and document_simpl interfaces change as follows:

namespace xml_schema
{
  class serializer_base;

  class writer
  {
  public:
    // The first write function is called to write a '\0'-terminated
    // string. Its default implementation calls the second versions:
    // write (s, strlen (s)). These functions return true if the
    // operation was successful and false otherwise.
    //
    // indicate a write failure.
    //
    virtual bool
    write (const char* s);

    virtual bool
    write (const char* s, size_t n) = 0;

    virtual bool
    flush () = 0;
  };

  class document_simpl
  {
    ...

    // Serialize by calling the write and flush functions. If the
    // unbounded write function is not provided, the bounded version
    // is called: write_bound_func (s, strlen (s)). user_data is
    // passed as a first argument to these functions. These functions
    // return true if the operation was successful and false otherwise.
    //
    typedef bool (*write_func) (void*, const char*);
    typedef bool (*write_bound_func) (void*, const char*, size_t);
    typedef bool (*flush_func) (void*);

    ...

  public:
    const serializer_error&
    _error () const;
  };
}
  

For more information on error handling with C++ exceptions and error codes see Section 8.2, "Exceptions" and Section 8.3, "Error Codes" below.

When support for XML Schema polymorphism is enabled, the overloaded document_simpl constructors have additional arguments which control polymorphic serialization. For more information refer to Section 6.7, "Support for Polymorphism".

The first argument to all overloaded constructors is the serializer for the type of the root element. The serializer_base class is the base type for all serializer skeletons. The second and third arguments to the document_simpl's constructors are the root element's name and namespace.

The add_prefix() and add_default_prefix() functions allow you to establish custom prefixes for XML namespaces. If none is provided, and namespaces are used by your vocabulary, the serializer will automatically assign namespace prefixes in an implementation-specific manner. For example:

xml_schema::document_simpl doc_s (
  root_s,
  "http://www.example.com/example",
  "root");

doc_s.add_prefix ("ex", "http://www.example.com/example");
  

The resulting XML will have the following namespace declaration:

<ex:root xmlns:ex="http://www.example.com/example" ...>
  ...
</ex:root>
  

Similarly, the add_schema() and add_no_namespace_schema() functions allow you to embed schema location information for a particular namespace into resulting XML. The schema location information is placed into the xsi:schemaLocation and xsi:noNamespaceSchemaLocation attributes. For example:

xml_schema::document_simpl doc_s (
  root_s,
  "http://www.example.com/example",
  "root");

doc_s.add_prefix ("ex", "http://www.example.com/example");
doc_s.add_schema ("http://www.example.com/example", "example.xsd");
  

The resulting XML will have the following namespace declaration:

<ex:root
  xmlns:ex="http://www.example.com/example"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.example.com/example example.xsd" ...>
  ...
</ex:root>
  

8.2 Exceptions

When C++ exceptions are used for error reporting, the system errors are mapped to the standard exceptions. The out of memory condition is indicated by throwing an instance of std::bad_alloc. The stream operation errors are reported by throwing an instance of std::ios_base::failure.

The xml and schema errors are reported by throwing the xml_schema::serializer_xml and xml_schema::serializer_schema exceptions, respectively. These two exceptions derive from xml_schema::serializer_exception which, in turn, derives from std::exception. As a result, you can handle any error from these two categories by either catching std::exception, xml_schema::serializer_exception, or individual exceptions. The further down the hierarchy you go the more detailed error information is available to you. The following listing shows the definitions of these exceptions:

namespace xml_schema
{
  class serializer_exception: public std::exception
  {
  public:
    virtual const char*
    text () const = 0;

    ...
  };

  std::ostream&
  operator<< (std::ostream&, const serializer_exception&);


  typedef <implementation-details> serializer_xml_error;

  class serializer_xml: public serializer_exception
  {
  public:
    serializer_xml_error
    code () const;

    virtual const char*
    text () const;

    virtual const char*
    what () const throw ();

    ...
  };


  typedef <implementation-details> serializer_schema_error;

  class serializer_schema: public serializer_exception
  {
  public:
    serializer_schema_error
    code () const;

    virtual const char*
    text () const;

    virtual const char*
    what () const throw ();

    ...
  };
}
  

The serializer_xml_error and serializer_schema_error are implementation-specific error code types. The operator<< defined for the serializer_exception class simply prints the error description as returned by the text() function. The following example shows how we can catch these exceptions:

int
main ()
{
  try
  {
    // Serialize.
  }
  catch (const xml_schema::serializer_exception& e)
  {
    cout << "error: " << e.text () << endl;
    return 1;
  }
}
  

Finally, for reporting application errors from serializer callbacks, you can throw any exceptions of your choice. They are propagated to the caller of the serializer without any alterations.

8.3 Error Codes

When C++ exceptions are not available, error codes are used to report error conditions. Each serializer skeleton and the root document_simpl serializer have the following member function for querying the error status:

xml_schema::serializer_error
_error () const;
  

To handle all possible error conditions, you will need to obtain the error status after calls to: the document_simpl's constructor (it performs memory allocations which may fail), calls to add_prefix() and add_schema() functions if any, the call to the root serializer pre() callback, the call to the serialize() function, and, finally, the call to the root serializer post() callback. The definition of xml_schema::serializer_error class is presented below:

namespace xml_schema
{
  class sys_error
  {
  public:
    enum value
    {
      none,
      no_memory,
      open_failed,
      read_failed,
      write_failed
    };

    sys_error (value);

    operator value () const;

    static const char*
    text (value);

    ...
  };

  typedef <implementation-details> serializer_xml_error;
  typedef <implementation-details> serializer_schema_error;

  class serializer_error
  {
  public:
    enum error_type
    {
      none,
      sys,
      xml,
      schema,
      app
    };

    error_type
    type () const;

    // Returns true if there is an error so that you can write
    // if (s.error ()) or if (error e = s.error ()).
    //
    typedef void (error::*bool_convertible) ();
    operator bool_convertible () const;

    // system
    //
    sys_error
    sys_code () const;

    const char*
    sys_text () const;

    // xml
    //
    serializer_xml_error
    xml_code () const;

    const char*
    xml_text () const;

    // schema
    //
    serializer_schema_error
    schema_code () const;

    const char*
    schema_text () const;

    // app
    //
    int
    app_code () const;

    ...
  };
}
  

The serializer_xml_error and serializer_schema_error are implementation-specific error code types. The serializer_error class incorporates four categories of errors which you can query by calling the type() function. The following example shows how to handle error conditions with error codes. It is based on the person record example presented in Chapter 3, "Serializer Skeletons".

int
main ()
{
  // Construct the serializer.
  //
  xml_schema::short_simpl short_s;
  xml_schema::string_simpl string_s;

  gender_simpl gender_s;
  person_simpl person_s;
  people_simpl people_s;

  person_s.serializers (string_s, string_s, gender_s, short_s);
  people_s.serializers (person_s);

  // Serialize.
  //
  using xml_schema::serializer_error;
  serializer_error e;

  do
  {
    xml_schema::document_simpl doc_s (people_s, "people");
    if (e = doc_s._error ())
      break;

    people_s.pre ();
    if (e = people_s._error ())
      break;

    doc_s.serialize (cout);
    if (e = doc_s._error ())
      break;

    people_s.post ();
    e = people_s._error ();

  } while (false);

  // Handle errors.
  //
  if (e)
  {
    switch (e.type ())
    {
    case serializer_error::sys:
      {
        cerr << "system error: " << e.sys_text () << endl;
        break;
      }
    case serializer_error::xml:
      {
        cerr << "xml error: " << e.xml_text () << endl;
        break;
      }
    case serializer_error::schema:
      {
        cerr << "schema error: " << e.schema_text () << endl;
        break;
      }
    case serializer_error::app:
      {
        cerr << "application error: " << e.app_code () << endl;
        break;
      }
    }
    return 1;
  }
}
  

The error type for application errors is int with the value 0 indicated the absence of error. You can set the application error by calling the _app_error() function inside a serializer callback. For example, if it was invalid to have a person younger than 18 in our people catalog, then we could have implemented this check as follows:

class person_simpl: public person_sskel
{
public:
  virtual short
  age ()
  {
    short a = ...;

    if (a < 18)
      _app_error (1);

    return a;
  }
};
  

You can also set a system error by calling the _sys_error() function inside a serializer callback. This function has one argument of type xml_schema::sys_error which was presented above. For example:

class person_simpl: public person_sskel
{
public:
  virtual const char*
  first_name ()
  {
    char* r = new char[5];

    if (r == 0)
    {
      _sys_error (xml_schema::sys_error::no_memory);
      return 0;
    }

    strcpy (r, "John");
    return r;
  }
};
  

8.4 Reusing Serializers after an Error

After a successful execution a serializer returns into the initial state and can be used to serialize another document without any extra actions. On the other hand, if an error occurred during serialization and you would like to reuse the serializer to serialize another document, you need to explicitly reset it into the initial state as shown in the following code fragment:

int
main ()
{
  ...

  xml_schema::document_simpl doc_s (people_s, "people");

  for (size_t i = 0; i < 4; ++i)
  {
    try
    {
      people_s.pre ();
      doc_s.serialize (cout);
      people_s.post ();
    }
    catch (const xml_schema::serializer_exception&)
    {
      doc_s.reset ();
    }
  }
}
  

If you do not need to reuse serializers after an error for example because your application terminates or you create a new serializer instance in such situations, then you can avoid generating serializer reset code by specifying the --suppress-reset XSD/e compiler option.

Your individual serializer implementations may also require extra actions in order to bring them into a usable state after an error. To accomplish this you can override the _reset() virtual function as shown below. Notice that when you override the _reset() function in your implementation, you should always call the base skeleton version to allow it to reset its state:

class person_simpl: public person_sskel
{
public:
  virtual void
  pre (person* p)
  {
    p_ = p;
  }

  virtual void
  post ()
  {
    delete p_;
    p_ = 0;
  }

  virtual void
  _reset ()
  {
    person_sskel::_reset ();
    delete p_;
    p_ = 0;
  }

  ...

private:
  person* p_;
};
  

Note also that the _reset() mechanism is used only when an error has occurred. To make sure that your serializer implementations arrive at the initial state during successful execution, use the initialization (pre() and _pre()) and finalization (post_*() and _post()) callbacks.

Appendix A — Supported XML Schema Constructs

The Embedded C++/Serializer mapping supports validation of the following W3C XML Schema constructs in the generated code.

Construct	Notes
Structure
element
attribute
any
anyAttribute
all
sequence
choice
complex type, empty content
complex type, mixed content
complex type, simple content extension
complex type, simple content restriction
complex type, complex content extension
complex type, complex content restriction
list
Facets
length	String-based types.
minLength	String-based types.
maxLength	String-based types.
pattern	String-based types.
enumeration	String-based types.
minExclusive	Integer and floating-point types.
minInclusive	Integer and floating-point types.
maxExclusive	Integer and floating-point types.
maxInclusive	Integer and floating-point types.
fractionDigits	decimal
Datatypes
byte
unsignedByte
short
unsignedShort
int
unsignedInt
long
unsignedLong
integer
nonPositiveInteger
nonNegativeInteger
positiveInteger
negativeInteger
boolean
float
double
decimal
string
normalizedString
token
Name
NMTOKEN
NCName
language
anyURI
ID	Identity constraint is not enforced.
IDREF	Identity constraint is not enforced.
NMTOKENS
IDREFS	Identity constraint is not enforced.
QName
base64Binary
hexBinary
date
dateTime
duration
gDay
gMonth
gMonthDay
gYear
gYearMonth
time