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<title>C++ Object Persistence with ODB</title>
<meta name="copyright" content="© 2009-2010 Code Synthesis Tools CC"/>
<meta name="keywords" content="odb,c++,object,persistence,ORM,relational,database,RDBMS,ODBMS,OODBMS"/>
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If you make changes to this document, follow these stylistic rules
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- Don't use 'object' for instances of non-persistent classes. Use
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for example The persist() function has two overloaded versions.
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<div class="noprint">
<div id="titlepage">
<div class="title">C++ Object Persistence with ODB</div>
<p>Copyright © 2009-2010 Code Synthesis Tools CC</p>
<p>Permission is granted to copy, distribute and/or modify this
document under the terms of the
<a href="http://www.codesynthesis.com/licenses/fdl-1.3.txt">GNU Free
Documentation License, version 1.3</a>; with no Invariant Sections,
no Front-Cover Texts and no Back-Cover Texts.</p>
<!-- REMEMBER TO CHANGE VERSIONS IN THE META TAGS ABOVE! -->
<p id="revision">Revision 1.0, September 2010</p>
<p>This revision of the manual describes ODB 1.0.0 and is available
in the following formats:
<a href="http://www.codesynthesis.com/products/odb/doc/manual.xhtml">XHTML</a>,
<a href="http://www.codesynthesis.com/products/odb/doc/odb-manual.pdf">PDF</a>, and
<a href="http://www.codesynthesis.com/products/odb/doc/odb-manual.ps">PostScript</a>.</p>
</div>
<h1>Table of Contents</h1>
<table class="toc">
<tr>
<th></th><td><a href="#0">Preface</a>
<table class="toc">
<tr><th></th><td><a href="#0.1">About This Document</a></td></tr>
<tr><th></th><td><a href="#0.2">More Information</a></td></tr>
</table>
</td>
</tr>
<tr>
<th>1</th><td><a href="#1">Introduction</a>
<table class="toc">
<tr><th>1.1</th><td><a href="#1.1">Architecture and Workflow</a></td></tr>
<tr><th>1.2</th><td><a href="#1.2">Benefits</a></td></tr>
</table>
</td>
</tr>
<tr>
<th>2</th><td><a href="#2">Hello World Example</a>
<table class="toc">
<tr><th>2.1</th><td><a href="#2.1">Declaring a Persistent Class</a></td></tr>
<tr><th>2.2</th><td><a href="#2.2">Generating Database Support Code</a></td></tr>
<tr><th>2.3</th><td><a href="#2.3">Compiling and Running</a></td></tr>
<tr><th>2.4</th><td><a href="#2.4">Making Objects Persistent</a></td></tr>
<tr><th>2.5</th><td><a href="#2.5">Querying the Database for Objects</a></td></tr>
<tr><th>2.6</th><td><a href="#2.6">Updating Persistent Objects</a></td></tr>
<tr><th>2.7</th><td><a href="#2.7">Deleting Persistent Objects</a></td></tr>
<tr><th>2.8</th><td><a href="#2.8">Summary</a></td></tr>
</table>
</td>
</tr>
<tr>
<th>3</th><td><a href="#3">Working with Persistent Objects</a>
<table class="toc">
<tr><th>3.1</th><td><a href="#3.1">Concepts and Terminology</a></td></tr>
<tr><th>3.2</th><td><a href="#3.2">Database</a></td></tr>
<tr><th>3.3</th><td><a href="#3.3">Transactions</a></td></tr>
<tr><th>3.4</th><td><a href="#3.4">Making Objects Persistent</a></td></tr>
<tr><th>3.5</th><td><a href="#3.5">Loading Persistent Objects</a></td></tr>
<tr><th>3.6</th><td><a href="#3.6">Updating Persistent Objects</a></td></tr>
<tr><th>3.7</th><td><a href="#3.7">Deleting Persistent Objects</a></td></tr>
<tr><th>3.8</th><td><a href="#3.8">ODB Exceptions</a></td></tr>
</table>
</td>
</tr>
<tr>
<th>4</th><td><a href="#4">Querying the Database</a>
<table class="toc">
<tr><th>4.1</th><td><a href="#4.1">ODB Query Language</a></td></tr>
<tr><th>4.2</th><td><a href="#4.2">Parameter Binding</a></td></tr>
<tr><th>4.3</th><td><a href="#4.3">Executing a Query</a></td></tr>
<tr><th>4.4</th><td><a href="#4.4">Query Result</a></td></tr>
</table>
</td>
</tr>
<tr>
<th>5</th><td><a href="#5">ODB Pragma Language</a>
<table class="toc">
<tr>
<th>5.1</th><td><a href="#5.1">C++ Compiler Warnings</a>
<table class="toc">
<tr><th>5.1.1</th><td><a href="#5.1.1">GNU C++</a></td></tr>
<tr><th>5.1.2</th><td><a href="#5.1.2">Visual C++</a></td></tr>
<tr><th>5.1.3</th><td><a href="#5.1.3">Sun C++</a></td></tr>
<tr><th>5.1.4</th><td><a href="#5.1.4">IBM XL C++</a></td></tr>
</table>
</td>
</tr>
<tr>
<th>5.2</th><td><a href="#5.2">Object Type Pragmas</a>
<table class="toc">
<tr><th>5.2.1</th><td><a href="#5.2.1"><code>table</code></a></td></tr>
</table>
</td>
</tr>
<tr>
<th>5.3</th><td><a href="#5.3">Value Type Pragmas</a>
<table class="toc">
<tr><th>5.3.1</th><td><a href="#5.3.1"><code>type</code></a></td></tr>
</table>
</td>
</tr>
<tr>
<th>5.4</th><td><a href="#5.4">Data Member Pragmas</a>
<table class="toc">
<tr><th>5.4.1</th><td><a href="#5.4.1"><code>id</code></a></td></tr>
<tr><th>5.4.2</th><td><a href="#5.4.2"><code>auto</code></a></td></tr>
<tr><th>5.4.3</th><td><a href="#5.4.3"><code>type</code></a></td></tr>
<tr><th>5.4.4</th><td><a href="#5.4.4"><code>column</code></a></td></tr>
<tr><th>5.4.5</th><td><a href="#5.4.5"><code>transient</code></a></td></tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
<tr>
<th>6</th><td><a href="#6">Database Systems</a>
<table class="toc">
<tr>
<th>6.1</th><td><a href="#6.1">MySQL Database</a>
<table class="toc">
<tr><th>6.1.1</th><td><a href="#6.1.1">MySQL Type Mapping</a></td></tr>
<tr><th>6.1.2</th><td><a href="#6.1.2">MySQL Database Class</a></td></tr>
<tr><th>6.1.3</th><td><a href="#6.1.3">Connection Factory</a></td></tr>
<tr><th>6.1.4</th><td><a href="#6.1.4">MySQL Exceptions</a></td></tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
</table>
</div>
<h1><a name="0">Preface</a></h1>
<p>As more critical aspects of our lives become dependant on software
systems, more and more applications are required to save the data
they work on in persistent and reliable storage. Database management
systems and, in particular, relational database management systems
(RDBMS) are commonly used for such storage. However, while the
application development techniques and programming languages have
evolved significantly over the past decades, the relational database
technology in this area stayed relatively unchanged. In particular,
this led to the now infamous mismatch between the object-oriented
model used by many modern applications and the relational model still
used by RDBMS.</p>
<p>While relational databases may be inconvenient to use from modern
programming languages, they are still the main choice for many
applications due to their maturity, reliability, as well as the
availability of tools and alternative implementations.</p>
<p>To allow application developers to utilize relational databases
from their object-oriented applications, a technique called
object-relational mapping (ORM) is often used. It involves a
conversion layer that maps between objects in the application's
memory and their relational representation in the database. While
the object-relational mapping code can be written manually,
automated ORM systems are available for most object-oriented
programming languages in use today.</p>
<p>ODB is an ORM system for the C++ programming language. It was
designed and implemented with the following main goals:</p>
<ul class="list">
<li>Provide a fully-automatic ORM system. In particular, the
application developer should not have to manually write any
mapping code, neither for persistent classes nor for their
data member. </li>
<li>Provide clean and easy to use object-oriented persistence
model and database APIs that support the development of realistic
applications for a wide variety of domains.</li>
<li>Provide a portable and thread-safe implementation. ODB should be
written in standard C++ and capable of persisting any standard
C++ classes.</li>
<li>Provide profiles that integrate ODB with type systems of
widely-used frameworks and libraries such as Qt and Boost.</li>
<li>Provide a high-performance and low overhead implementation. ODB
should make efficient use of database and application resources.</li>
</ul>
<h2><a name="0.1">About This Document</a></h2>
<p>The goal of this manual is to provide you with an understanding
of the object persistence model and APIs which are implemented by ODB.
As such, this document is intended for C++ application developers and
software architects who are looking for a C++ object persistence
solution. Prior experience with C++ is required to understand
this document. A basic understanding of relational database systems
is advantageous but not expected or required.</p>
<h2><a name="0.2">More Information</a></h2>
<p>Beyond this manual, you may also find the following sources of
information useful:</p>
<ul class="list">
<li><a href="http://www.codesynthesis.com/products/odb/doc/odb.xhtml">ODB
Compiler Command Line Manual.</a></li>
<li>The <code>INSTALL</code> files in the ODB source packages provide
build instructions for various platforms.</li>
<li>The <code>odb-examples</code> package contains a collection of
examples and a README file with an overview of each example.</li>
<li>The <a href="http://www.codesynthesis.com/mailman/listinfo/odb-users">odb-users</a>
mailing list is the place to ask technical questions about ODB.
Furthermore, the searchable
<a href="http://www.codesynthesis.com/pipermail/odb-users/">archives</a>
may already have answers to some of your questions.</li>
</ul>
<!-- CHAPTER -->
<h1><a name="1">1 Introduction</a></h1>
<p>ODB is an object-relational mapping (ORM) system for C++. It provides
tools, APIs, and library support that allow you to persist C++ objects
to a relational database (RDBMS) without having to deal with tables,
columns, or SQL and without manually writing any of the mapping code.</p>
<p>ODB is highly flexible and customizable. It can either completely
hide the relational nature of the underlying database or expose
some of the details as required. For example, you can automatically
map basic C++ types to suitable SQL types, generate the relational
database schema for your persistent classes, and use simple, safe,
and yet powerful object query language instead of SQL. Or you can
assign SQL types to individual data members, use the existing
database schema, and run native SQL <code>SELECT</code> queries.</p>
<p>ODB is not a framework. It does not dictate how you should write
your application. Rather, it is designed to fit into your
style and architecture by only handling object persistence
and not interfering with any other functionality. There is
no common base type that all persistent classes should derive
from nor are there any restrictions on the data member types
in persistent classes. Existing classes can be made persistent
with a few or no modifications.</p>
<p>ODB has been designed for high performance and low memory
overhead. Prepared statements are used to send and receive
object state in binary format instead of text which reduces
the load on the application and the database server. Extensive
caching of connections, prepared statements, and buffers saves
time and resources on connection establishment, statement parsing
and memory allocations. For each supported database system the
native C API is used instead of ODBC or higher-level wrapper
APIs to reduce overhead and provide the most efficient implementation
for each database operation. Finally, persistent classes have
zero memory overhead. There are no hidden "database" members
that each class must have nor are there per-object data structures
allocated by ODB.</p>
<p>In this chapter we present a high-level overview of ODB.
We will start with the ODB architecture and then outline the
workflow of building an application that uses ODB. We will
conclude the chapter by contrasting the drawbacks of
the traditional way of saving C++ objects to relational
databases with the benefits of using ODB for object
persistence. The next chapter takes a more hands-on approach
and shows the concrete steps necessary to implement object
persistence in a simple "Hello World" application.</p>
<h2><a name="1.1">1.1 Architecture and Workflow</a></h2>
<p>From the application developer's perspective, ODB
consists of three main components: the ODB compiler, the common
runtime library, called <code>libodb</code>, and the
database-specific runtime libraries, called
<code>libodb-<database></code>, where <database> is
the name of the database system this runtime
is for, for example, <code>libodb-mysql</code>. For instance,
if the application is going to use the MySQL database for
object persistence, then the three ODB components that this
application will use are the ODB compiler, <code>libodb</code>
and <code>libodb-mysql</code>.</p>
<p>The ODB compiler generates the database support code for
persistent classes in your application. The input to the ODB
compiler is one or more C++ header files defining C++ classes
that you want to make persistent. For each input header file
the ODB compiler generates a set of C++ source files implementing
conversion between persistent C++ classes defined in this
header and their database representation. The ODB compiler
can also generate a database schema file that creates tables
necessary to store the persistent classes.</p>
<p>The ODB compiler is a real C++ compiler except that it produces
C++ instead of assembly or machine code. In particular, it is not
an ad-hoc header pre-processor that is only capable of recognizing
a subset of C++. ODB is capable of parsing any standard C++ code.</p>
<p>The common runtime library defines database system-independent
interfaces that your application can use to manipulate persistent
objects. The database-specific runtime library provides implementations
of these interfaces for a concrete database as well as other
database-specific utilities that are used by the generated code.
Normally, the application does not use the database-specific
runtime library directly but rather works with it via the common
interfaces from <code>libodb</code>. The following diagram shows
the object persistence architecture of an application that uses
MySQL as the underlying database system:</p>
<!-- align=center is needed for html2ps -->
<div class="img" align="center"><img src="odb-arch.png"/></div>
<p>The ODB system also defines two special-purpose languages:
the ODB Pragma Language and ODB Query Language. The ODB Pragma
Language is used to communicate various properties of persistent
classes to the ODB compiler by means of special <code>#pragma</code>
directives embedded in the C++ header files. It controls aspects
of the object-relational mapping such as names of tables and columns
that are used for persistent classes and their members or mapping between
C++ types and database types.</p>
<p>The ODB Query Language is an object-oriented database query
language that can be used to search for objects matching
certain criteria. It is modeled after and is integrated into
C++ allowing you to write expressive and safe queries that look
and feel like ordinary C++.</p>
<p>The use of the ODB compiler to generate database support code
adds an additional step to your application build sequence. The
following diagram outlines the typical build workflow of an
application that uses ODB:</p>
<!-- align=center is needed for html2ps -->
<div class="img" align="center"><img src="odb-flow.png"/></div>
<h2><a name="1.2">1.2 Benefits</a></h2>
<p>The traditional way of saving C++ objects to relational databases
requires that you manually write code which converts between the database
and C++ representations of each persistent class. The actions that
such code usually performs include conversion between C++ values and
strings or database types, preparation and execution of SQL queries,
as well as handling the result sets. Writing this code manually has
the following drawbacks:</p>
<ul class="list">
<li><b>Difficult and time consuming.</b> Writing database conversion
code for any non-trivial application requires extensive
knowledge of the specific database system and its APIs.
It can also take a considerable amount of time to write
and maintain. Supporting multi-threaded applications can
complicate this task even further.</li>
<li><b>Suboptimal performance.</b> Optimal conversion often
requires writing large amounts of extra code, such as
parameter binding for prepared statements and caching
of connections, statements, and buffers. Writing code
like this in an ad-hoc manner is often too difficult
and time consuming.</li>
<li><b>Database vendor lock-in.</b> The conversion code is written for
a specific database which makes it hard to switch to another
database vendor.</li>
<li><b>Lack of type safety.</b> It is easy to misspell column names or
pass incompatible values in SQL queries. Such errors will
only be detected at runtime.</li>
<li><b>Complicates the application.</b> The database conversion code
often ends up interspersed throughout the application making it
hard to debug, change, and maintain.</li>
</ul>
<p>In contrast, using ODB for C++ object persistence has the
following benefits:</p>
<ul class="list">
<li><b>Ease of use.</b> ODB automatically generates database conversion
code from your C++ class declarations and allows you to manipulate
persistent objects using simple and thread-safe object-oriented
database APIs.</li>
<li><b>Concise code.</b> With ODB hiding the details of the underlying
database, the application logic is written using the natural object
vocabulary instead of tables, columns and SQL. The resulting code
is simpler and thus easier to read and understand.</li>
<li><b>Optimal performance.</b> ODB has been designed for high performance
and low memory overhead. All the available optimization techniques,
such as prepared statements and extensive connection, statement,
and buffer caching, are used to provide the most efficient
implementation for each database operation.</li>
<li><b>Database portability.</b> Because the database conversion code
is automatically generated, it is easy to switch from one database
vendor to another. In fact, it is possible to test your application
on several database systems before making a choice.</li>
<li><b>Safety.</b> The ODB object persistence and query APIs are
statically typed. You use C++ identifiers instead of strings
to refer to object members and the generated code makes sure
database and C++ types are compatible. All this helps catch
programming errors at compile-time rather than at runtime.</li>
<li><b>Maintainability.</b> Automatic code generation minimizes the
effort needed to adapt the application to changes in persistent
classes. The database support code is kept separately from the
class declarations and application logic. This makes the
application easier to debug and maintain.</li>
</ul>
<p>Overall, ODB provides an easy to use yet flexible and powerful
object-relational mapping (ORM) system for C++. Unlike other
ORM implementations for C++ that still require you to write
database conversion or member registration code for each
persistent class, ODB keeps persistent classes purely
declarative. The functional part, the database conversion
code, is automatically generated by the ODB compiler from
these declarations.</p>
<!-- CHAPTER -->
<h1><a name="2">2 Hello World Example</a></h1>
<p>In this chapter we will show how to create a simple C++
application that relies on ODB for object persistence using
the traditional "Hello World" example. In particular, we will
discuss how to declare persistent classes, generate database
support code, as well as compile and run our application. We
will also learn how to make objects persistent, load, update
and delete persistent objects, as well as query the database
for persistent objects that match certain criteria.</p>
<p>The code presented in this chapter is based on the
<code>hello</code> example which can be found in the
<code>odb-examples</code> package of the ODB distribution.</p>
<h2><a name="2.1">2.1 Declaring a Persistent Class</a></h2>
<p>In our "Hello World" example we will depart slightly from
the norm and say hello to people instead of the world. People
in our application will be represented as objects of C++ class
<code>person</code> which is saved in <code>person.hxx</code>:</p>
<pre class="c++">
// person.hxx
//
#include <string>
class person
{
public:
person (const std::string& first,
const std::string& last,
unsigned short age);
const std::string&
first () const;
const std::string&
last () const;
unsigned short
age () const;
void
age (unsigned short);
private:
std::string first_;
std::string last_;
unsigned short age_;
};
</pre>
<p>In order not to miss anyone whom we need to greet, we would like
to save the <code>person</code> objects in a database. To achieve this
we declare the <code>person</code> class as persistent:</p>
<pre class="c++">
// person.hxx
//
#include <string>
#include <odb/core.hxx> // (1)
#pragma db object // (2)
class person
{
...
private:
person () {} // (3)
friend class odb::access; // (4)
#pragma db id auto // (5)
unsigned long id_; // (5)
std::string first_;
std::string last_;
unsigned short age_;
};
</pre>
<p>To be able to save the <code>person</code> objects in the database
we had to make five changes, marked with (1) to (5), to the original
class definition. The first change is the inclusion of the ODB
header <code><odb/core.hxx></code>. This header provides a number
of core ODB declarations, such as <code>odb::access</code>, that
are used to define persistent classes.</p>
<p>The second change is the addition of <code>db object</code>
pragma just before the class definition. This pragma tells the
ODB compiler that the class that follows is persistent. Note
that making a class persistent does not mean that all objects
of this class will automatically be stored in the database.
You would still create ordinary or <em>transient</em> instances
of this class just as you would before. The difference is that
now you can make such transient instances persistent, as we will
see shortly.</p>
<p>The third change is the addition of the default constructor.
The ODB-generated database support code will use this constructor
when instantiating an object from the persistent state. Just as we have
done for the <code>person</code> class, you can make the default
constructor private or protected if you don't want to make it
available to the users of your class.</p>
<p>With the fourth change we make the <code>odb::access</code> class a
friend of our <code>person</code> class. This is necessary to make
the default constructor and the data members accessible to the
ODB support code. If your class has public default constructor and
public data members, then the <code>friend</code> declaration is
unnecessary.</p>
<p>The final change adds a data member called <code>id_</code> which
is preceded by another pragma. In ODB every persistent object must
have a unique, within its class, identifier. Or, in other words,
no two persistent instances of the same type have equal
identifiers. For our class we use an integer id. The
<code>db id auto</code> pragma that precedes the <code>id_</code>
member tells the ODB compiler that the following member is the
object's identifier. The <code>auto</code> specifier indicates that it
is a database-assigned id. A unique id will be automatically generated
by the database and assigned to the object when it is made
persistent.</p>
<p>In this example we chose to add an identifier because none of
the existing members could serve the same purpose. However, if
a class already has a member with suitable properties, then it
is natural to use that member as an identifier. For example,
if our <code>person</code> class contained some form of personal
identification (SSN in the United States or ID/passport number
in other countries), then we could use that as an id. Or, if
we stored an email associated with each person, then we could
have used that since each person is presumed to have a unique
email address, for example:</p>
<pre class="c++">
class person
{
...
#pragma db id
std::string email_;
std::string first_;
std::string last_;
unsigned short age_;
};
</pre>
<p>Now that we have the header file with the persistent class, let's
see how we can generate that database support code.</p>
<h2><a name="2.2">2.2 Generating Database Support Code</a></h2>
<p>The persistent class definition that we created in the previous
section was particularly light on any code that could actually
do the job and store the person's data to a database. There
was no serialization or deserialization code, not even data member
registration, that you would normally have to write by hand in
other ORM libraries for C++. This is because in ODB code
that translates between the database and C++ representations
of an object is automatically generated by the ODB compiler.</p>
<p>To compile the <code>person.hxx</code> header we created in the
previous section and generate the support code for the MySQL
database, we invoke the ODB compiler from a terminal (UNIX) or
a command prompt (Windows):</p>
<pre class="terminal">
odb -d mysql --generate-query person.hxx
</pre>
<p>We will use MySQL as the database of choice in the remainder of
this chapter, though other supported database systems can be used
instead.</p>
<p>If you haven't installed the common ODB runtime library
(<code>libodb</code>) or installed it into a directory where
C++ compilers don't search for headers by default,
then you may get the following error:</p>
<pre class="terminal">
person.hxx:10:24: fatal error: odb/core.hxx: No such file or directory
</pre>
<p>To resolve this you will need to specify the <code>libodb</code> headers
location with the <code>-I</code> preprocessor option, for example:</p>
<pre class="terminal">
odb -I.../libodb -d mysql --generate-query person.hxx
</pre>
<p>Here <code>.../libodb</code> represents the path to the
<code>libodb</code> directory.</p>
<p>The above invocation of the ODB compiler produces three C++ files:
<code>person-odb.hxx</code>, <code>person-odb.ixx</code>,
<code>person-odb.cxx</code>. You normally don't use types
or functions contained in these files directly. Rather, all
you have to do is include <code>person-odb.hxx</code> in
C++ files where you are performing database operations
with classes from <code>person.hxx</code> as well as compile
<code>person-odb.cxx</code> and link the resulting object
file to your application.</p>
<p>You may be wondering what the <code>--generate-query</code>
option is for. It instructs the ODB compiler to generate
optional query support code that we will use later in our
"Hello World" example. Another option that we will find
useful is <code>--generate-schema</code>. This option
makes the ODB compiler generate a fourth file,
<code>person.sql</code>, which is the database schema
for the persistent classes defined in <code>person.hxx</code>:</p>
<pre class="terminal">
odb -d mysql --generate-query --generate-schema person.hxx
</pre>
<p>The database schema file contains SQL statements that creates
tables necessary to store the persistent classes. We will learn
how to use it in the next section.</p>
<p>If you would like to see a list of all the available ODB compiler
options, refer to the
<a href="http://www.codesynthesis.com/products/odb/doc/odb.xhtml">ODB
Compiler Command Line Manual</a>.</p>
<p>Now that we have the persistent class and the database support
code, the only part that is left is the application code that
does something useful with all of this. But before we move on to
the fun part, let's first learn how to build and run an application
that uses ODB. This way when we have some application code
to try, there are no more delays before we can run it.</p>
<h2><a name="2.3">2.3 Compiling and Running</a></h2>
<p>Assuming that the <code>main()</code> function with the application
code is saved in <code>driver.cxx</code> and the database support
code and schema are generated as described in the previous section,
to build our application we will first need to compile all the C++
source files and then link them with two ODB runtime libraries.</p>
<p>On UNIX, the compilation part can be done with the following commands
(substitute <code>c++</code> with your C++ compiler name; for Microsoft
Visual Studio setup, see the <code>odb-examples</code> package):</p>
<pre class="terminal">
c++ -c driver.cxx
c++ -c person-odb.cxx
</pre>
<p>Similar to the ODB compilation, if you get an error stating that
a header in <code>odb/</code> or <code>odb/mysql</code> directory
is not found, you will need to use the <code>-I</code>
preprocessor option to specify the location of the common ODB runtime
library (<code>libodb</code>) and MySQL ODB runtime library
(<code>libodb-mysql</code>).</p>
<p>Once the compilation is done, we can link the application with
the following command:</p>
<pre class="terminal">
c++ -o driver driver.o person-odb.o -lodb-mysql -lodb
</pre>
<p>Notice that we link our application with two ODB libraries:
<code>libodb</code> which is a common runtime library and
<code>libodb-mysql</code> which is a MySQL runtime library
(if you use another database, then the name of this library
will change accordingly). If you get an error saying that
one of these libraries could not be found, then you will need
to use the <code>-L</code> linker option to specify their locations.</p>
<p>Before we can run our application we need to create a database
schema using the generated <code>person.sql</code> file. For MySQL
we can use the <code>mysql</code> client program, for example:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test < person.sql
</pre>
<p>The above command will log in to a local MySQL server as user
<code>odb_test</code> without a password and use the database
named <code>odb_test</code>. Note that after executing this
command, all the data stored in the <code>odb_test</code> database
will be deleted.</p>
<p>Once the database schema is ready, we run our application
using the same login and database name:</p>
<pre class="terminal">
./driver --user odb_test --database odb_test
</pre>
<h2><a name="2.4">2.4 Making Objects Persistent</a></h2>
<p>Now that we have the infrastructure work out of the way, it
is time to see our first code fragment that interacts with the
database. In this section we will learn how to make <code>person</code>
objects persistent:</p>
<pre class="c++">
// driver.cxx
//
#include <memory> // std::auto_ptr
#include <iostream>
#include <odb/database.hxx>
#include <odb/transaction.hxx>
#include <odb/mysql/database.hxx>
#include "person.hxx"
#include "person-odb.hxx"
using namespace std;
using namespace odb;
int
main (int argc, char* argv[])
{
try
{
auto_ptr<database> db (new mysql::database (argc, argv));
unsigned long john_id, jane_id, joe_id;
// Create a few persistent person objects.
//
{
person john ("John", "Doe", 33);
person jane ("Jane", "Doe", 32);
person joe ("Joe", "Dirt", 30);
transaction t (db->begin ());
db->persist (john);
db->persist (jane);
db->persist (joe);
t.commit ();
// Save object ids for later use.
//
john_id = john.id ();
jane_id = jane.id ();
joe_id = joe.id ();
}
}
catch (const odb::exception& e)
{
cerr << e.what () << endl;
return 1;
}
}
</pre>
<p>Let's examine this code piece by piece. At the beginning we include
a bunch of headers. After the standard C++ headers we include
<code><odb/database.hxx></code>
and <code><odb/transaction.hxx></code> which define database
system-independent <code>odb::database</code> and
<code>odb::transaction</code> interfaces. Then we include
<code><odb/mysql/database.hxx></code> which defines the
MySQL implementation of the <code>database</code> interface. Finally,
we include <code>person.hxx</code> and <code>person-odb.hxx</code>
which define our persistent <code>person</code> class.</p>
<p>Once we are in <code>main()</code>, the first thing we do is create
the MySQL database object. Notice that this is the last line in
<code>driver.cxx</code> that mentions MySQL explicitly; the rest
of the code works through the common interfaces and is database
system-independent. We use the <code>argc</code>/<code>argv</code>
<code>mysql::database</code> constructor which automatically
extract the database parameters, such as login name, password,
database name, etc., from the command line. In your own applications
you may prefer to use other <code>mysql::database</code>
constructors which allow you to pass this information directly
(see <a href="#6.1.2">Section 6.1.2, "MySQL Database Class"</a>).</p>
<p>Next, we create three <code>person</code> objects. Right now they are
transient objects, which means that if we terminate the application
at this point, they will be gone without any evidence of them ever
existing. The next line starts a database transaction. We discuss
transactions in detail later in this manual. For now, all we need
to know is that all ODB database operations must be performed within
a transaction and that a transaction is an atomic unit of work; all
database operations performed within a transaction either succeed
(committed) together or are automatically undone (rolled back).</p>
<p>Once we are in a transaction, we call the <code>persist()</code>
database function on each of our <code>person</code> objects.
At this point the state of each object is saved in the database.
However, note that this state is not permanent until and unless
the transaction is committed. If, for example, our application
crashes at this point, there will still be no evidence of our
objects ever existing.</p>
<p>In our case, one more thing happens when we call <code>persist()</code>.
Remember that we decided to use database-assigned identifiers for our
<code>person</code> objects. The call to <code>persist()</code> is
where this assignment happens. Once this function returns, the
<code>id_</code> member contains this object's unique identifier.</p>
<p>After we have persisted our objects, it is time to commit the
transaction and make the changes permanent. Only after the
<code>commit()</code> function returns successfully, are we
guaranteed that the objects are made persistent. Continuing
with the crash example, if our application terminates after
the commit for whatever reason, the objects' state in the
database will remain intact. In fact, as we will discover
shortly, our application can be restarted and load the
original objects from the database. Note also that a
transaction must be committed explicitly with the
<code>commit()</code> call. If the <code>transaction</code>
object leaves scope without the transaction being
explicitly committed or rolled back, it will automatically be
rolled back. This behavior allows you not to worry about
exceptions being thrown within a transaction; if they
cross the transaction boundary, the transaction will
automatically be rolled back and all the changes made
to the database undone.</p>
<p>After the transaction has been committed, we save the
objects' identifiers in local variables. We will use them later in this
chapter to perform other database operations on our persistent
objects. You might have noticed that our <code>person</code>
class doesn't have the <code>id()</code> function that we use
here. To make our code compile we need to add a simple accessor
with this name that returns the value of the <code>id_</code>
data member.</p>
<p>The final bit of code in our example is the <code>catch</code>
block that handles the database exceptions. We do this by catching
the base ODB exception (see <a href="#3.8">Section 3.8, "ODB
Exceptions"</a>) and printing the diagnostics.</p>
<p>Let's now compile (see <a href="#2.3">Section 2.3, "Compiling and
Running"</a>) and then run our first ODB application:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test < person.sql
./driver --user odb_test --database odb_test
</pre>
<p>Our first application doesn't print anything except for error
messages so we can't really tell whether it actually stored the
objects' state in the database. While we will make our application
more entertaining shortly, for now we can use the <code>mysql</code>
client to examine the database content. It will also give us a feel
for how the objects are stored:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test
Welcome to the MySQL monitor.
mysql> select * from person;
+----+-------+------+-----+
| id | first | last | age |
+----+-------+------+-----+
| 1 | John | Doe | 33 |
| 2 | Jane | Doe | 32 |
| 3 | Joe | Dirt | 30 |
+----+-------+------+-----+
3 rows in set (0.00 sec)
mysql> quit
</pre>
<p>In the next section we will see how to access persistent objects
from our application.</p>
<h2><a name="2.5">2.5 Querying the Database for Objects</a></h2>
<p>So far our application doesn't resemble a typical "Hello World"
example. It doesn't print anything except for error messages.
Let's change that and teach our application to say hello to
people from our database. To make it a bit more interesting,
let's say hello only to people over 30:</p>
<pre class="c++">
// driver.cxx
//
...
int
main (int argc, char* argv[])
{
try
{
...
// Create a few persistent person objects.
//
{
...
}
typedef odb::query<person> query;
typedef odb::result<person> result;
// Say hello to those over 30.
//
{
transaction t (db->begin ());
result r (db->query<person> (query::age > 30));
for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
cout << "Hello, " << i->first () << "!" << endl;
}
t.commit ();
}
}
catch (const odb::exception& e)
{
cerr << e.what () << endl;
return 1;
}
}
</pre>
<p>The first half of our application is the same as before and is
replaced with "..." in the above listing for brevity. Again, let's
examine the rest of it piece by piece.</p>
<p>The two <code>typedef</code>s create convenient aliases for two
template instantiations that will be used a lot in our application.
The first is the query type for the <code>person</code> objects
and the second is the result type for that query.</p>
<p>Then we begin a new transaction and call the <code>query()</code>
database function. We pass a query expression
(<code>query::age > 30</code>) which limits the returned objects
only to those with the age greater than 30. We also save the result
of the query in a local variable.</p>
<p>The next few lines perform a standard for-loop iteration
over the result sequence printing hello for every returned person.
Then we commit the transaction and that's it. Let's see what
this application will print:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test < person.sql
./driver --user odb_test --database odb_test
Hello, John!
Hello, Jane!
</pre>
<p>That looks about right, but how do we know that the query actually
used the database instead of just using some in-memory artifacts of
the earlier <code>persist()</code> calls? One way to test this
would be to comment out the first transaction in our application
and re-run it without re-creating the database schema. This way the
objects that were persisted during the previous run will be returned.
Alternatively, we can just re-run the same application without
re-creating the schema and notice that we now show duplicate
objects:</p>
<pre class="terminal">
./driver --user odb_test --database odb_test
Hello, John!
Hello, Jane!
Hello, John!
Hello, Jane!
</pre>
<p>What happens here is that the previous run of our application
persisted a set of <code>person</code> objects and when we re-run
the application, we persist another set with the same names but
with different ids. When we later run the query, matches from
both sets are returned. We can change the line where we print
the "Hello" string as follows to illustrate this point:</p>
<pre class="c++">
cout << "Hello, " << i->first () << " (" << i->id () << ")!" << endl;
</pre>
<p>If we now re-run this modified program, again without re-creating
the database schema, we will get the following output:</p>
<pre class="terminal">
./driver --user odb_test --database odb_test
Hello, John (1)!
Hello, Jane (2)!
Hello, John (4)!
Hello, Jane (5)!
Hello, John (7)!
Hello, Jane (8)!
</pre>
<p>The identifiers 3, 6, and 9 that are missing from the above list belong
to the "Joe Dirt" objects which are not selected by this query.</p>
<h2><a name="2.6">2.6 Updating Persistent Objects</a></h2>
<p>While making objects persistent and then selecting some of them using
queries are two useful operations, most applications will also need
to change the object's state and then make these changes persistent.
Let's illustrate this by updating Joe's age who just had a birthday:</p>
<pre class="c++">
// driver.cxx
//
...
int
main (int argc, char* argv[])
{
try
{
...
unsigned long john_id, jane_id, joe_id;
// Create a few persistent person objects.
//
{
...
// Save object ids for later use.
//
john_id = john.id ();
jane_id = jane.id ();
joe_id = joe.id ();
}
// Joe Dirt just had a birthday, so update his age.
//
{
transaction t (db->begin ());
auto_ptr<person> joe (db->load<person> (joe_id));
joe->age (joe->age () + 1);
db->update (*joe);
t.commit ();
}
// Say hello to those over 30.
//
{
...
}
}
catch (const odb::exception& e)
{
cerr << e.what () << endl;
return 1;
}
}
</pre>
<p>The beginning and the end of the new transaction are the same as
the previous two. Once within a transaction, we call the
<code>load()</code> database function to instantiate a
<code>person</code> object with Joe's persistent state. We
pass Joe's object identifier that we stored earlier when we
made this object persistent.</p>
<p>With the instantiated object in hand we increment the age
and call the <code>update()</code> function to update
the object's state in the database. Once the transaction is
committed, the changes are made permanent.</p>
<p>If we now run this application, we will see Joe in the output
since he is now over 30:</p>
<pre class="terminal">
mysql --user=odb_test --database=odb_test < person.sql
./driver --user odb_test --database odb_test
Hello, John!
Hello, Jane!
Hello, Joe!
</pre>
<p>What if we didn't have an identifier for Joe? Maybe this object
was made persistent in another run of our application or by another
application altogether. Provided that we only have one Joe Dirt
in the database, we can use the query facility to come up with
an alternative implementation of the above transaction:</p>
<pre class="c++">
// Joe Dirt just had a birthday, so update his age. An
// alternative implementation without using the object id.
//
{
transaction t (db->begin ());
result r (db->query<person> (query::first == "Joe" &&
query::last == "Dirt"));
result::iterator i (r.begin ());
if (i != r.end ())
{
auto_ptr<person> joe (i.load ());
joe->age (joe->age () + 1);
db->update (*joe);
}
t.commit ();
}
</pre>
<h2><a name="2.7">2.7 Deleting Persistent Objects</a></h2>
<p>The last operation that we will discuss in this chapter is deleting
the persistent object from the database. The following code
fragment shows how we can delete an object given its identifier:</p>
<pre class="c++">
// John Doe is no longer in our database.
//
{
transaction t (db->begin ());
db->erase<person> (john_id);
t.commit ();
}
</pre>
<p>To delete John from the database we start a transaction, call
the <code>erase()</code> database function with John's object
id, and commit the transaction. After the transaction is committed,
the erased object is no longer persistent.</p>
<p>If we don't have an object id handy, we can use queries to find
and delete the object:</p>
<pre class="c++">
// John Doe is no longer in our database. An alternative
// implementation without using the object id.
//
{
transaction t (db->begin ());
result r (db->query<person> (query::first == "John" &&
query::last == "Doe"));
result::iterator i (r.begin ());
if (i != r.end ())
{
auto_ptr<person> john (i.load ());
db->erase (*john);
}
t.commit ();
}
</pre>
<h2><a name="2.8">2.8 Summary</a></h2>
<p>This chapter presented a very simple application which, nevertheless,
exercised all of the core database functions: <code>persist()</code>,
<code>query()</code>, <code>load()</code>, <code>update()</code>,
and <code>erase()</code>. We also saw that writing an application
that uses ODB involves the following steps:</p>
<ol>
<li>Declare persistent classes in header files.</li>
<li>Compile these headers to generate database support code.</li>
<li>Link the application with the generated code and two ODB runtime
libraries.</li>
</ol>
<p>Do not be concerned if, at this point, much appears unclear. The intent
of this chapter is to give you only a general idea of how to persist C++
objects with ODB. We will cover all the details throughout the remainder
of this manual.</p>
<!-- CHAPTER -->
<h1><a name="3">3 Working with Persistent Objects</a></h1>
<p>The previous chapters gave us a high-level overview of ODB and
showed how to use it to store C++ objects in a database. In this
chapter we will examine the ODB object persistence model as
well as the core database APIs in greater detail. We will
start with basic concepts and terminology in <a href="#3.1">Section 3.1</a>
and continue with the discussion of the <code>odb::database</code>
class in <a href="#3.2">Section 3.2</a> and transactions in
<a href="#3.3">Section 3.3</a>. The remainder of this chapter
deals with the core database operations and concludes with
the discussion of ODB exceptions.</p>
<p>In this chapter we will continue to use and expand the
<code>person</code> persistent class that we have developed in the
previous chapter.</p>
<h2><a name="3.1">3.1 Concepts and Terminology</a></h2>
<p>The term <em>database</em> can refer to three distinct things:
a general notion of a place where an application stores its data,
a software implementation for managing this data (for example
MySQL), and, finally, some database software implementations
may manage several data stores which are usually distinguished
by name. This name is also commonly referred to as a database.</p>
<p>In this manual, when we use the word <em>database</em>, we
refer to the first meaning above, for example,
"The <code>update()</code> function saves the object's state to
the database." The term Database Management System (DBMS) is
often used to refer to the second meaning of the word database.
In this manual we will use the term <em>database system</em>
for short, for example, "Database system-independent
application code." Finally, to distinguish the third meaning
from the other two, we will use the term <em>database name</em>,
for example, "The second option specifies the database name
that the application should use to store its data."</p>
<p>In C++ there is only one notion of a type and an instance
of a type. For example, a fundamental type, such as <code>int</code>,
is, for the most part, treated the same as a user defined class
type. However, when it comes to persistence, we have to place
certain restrictions and requirements on certain C++ types that
can be stored in the database. As a result, we divide persistent
C++ types into two groups: <em>object types</em> and <em>value
types</em>. An instance of an object type is called an <em>object</em>
and an instance of a value type — a <em>value</em>.</p>
<p>An object is an independent entity. It can be stored, updated,
and deleted in the database independent of other objects or values.
An object has an identifier, called <em>object id</em>, that is
unique among all instances of an object type within a database.
An object consists of data members which are either values or
references to other objects. In contrast, a value can only be
stored in the database as part of an object and doesn't have
its own unique identifier.</p>
<p>An object type is a C++ class. Because of this one-to-one
relationship, we will use terms <em>object type</em>
and <em>object class</em> interchangeably. In contrast,
a value type can be a fundamental C++ type, such as
<code>int</code> or a class type, such as <code>std::string</code>.
If a value consists of other values, then it is called a
<em>composite value</em> and its type — a
<em>composite value type</em>. Otherwise, the value is
called <em>simple value</em> and its type — a
<em>simple value type</em>. Note that the distinction between
simple and composite values is conceptual rather than
representational. For example, <code>std::string</code>
is a simple value type because conceptually string is a
single value even though the representation of the string
class may contain several data members each of which could be
considered a value. In fact, the same value type can be
viewed (and mapped) as both simple and composite by different
applications.</p>
<p>Seeing how all these concepts map to the relational model
will hopefully make these distinctions clearer. In a relational
database an object type is mapped to a table and a value type is
mapped to one or more columns. A simple value type is mapped
to a single column while a composite value type is mapped to
several columns. An object is stored as a row in this
table and a value is stored as one or more cells in this row.
A simple value is stored in a single cell while a composite
value occupies several cells.</p>
<p>Going back to the distinction between simple and composite
values, consider a date type which has three integer
members: year, month, and day. In one application it can be
considered a composite value and each member will get its
own column in a relational database. In another application
it can be considered a simple value and stored in a single
column as a number of days from some predefined date.</p>
<p>Until now, we have been using the term <em>persistent class</em>
to refer to object classes. We will continue to do so even though
a value type can also be a class. The reason for this asymmetry
is the subordinate nature of value types when it comes to
database operations. Remember that values are never stored
directly but rather as part of an object that contains them.
As a result, when we say that we want to make a C++ class
persistent or persist an instance of a class in the database,
we invariably refer to an object class rather than a value
class.</p>
<p>To make a C++ class a persistent object class we declare
it as such using the <code>db object</code> pragma, for
example:</p>
<pre class="c++">
#pragma db object
class person
{
...
};
</pre>
<p>The other pragma that we often use is <code>db id</code>
which designates one of the data members as an object id, for
example:</p>
<pre class="c++">
#pragma db object
class person
{
private:
#pragma db id
unsigned long id_;
};
</pre>
<p>These two pragmas are the minimum required to declare a
persistent class. Other pragmas can be used to fine-tune
the database-related properties of a class and its
members (see <a href="#5">Chapter 5, "ODB Pragma Language"</a>).</p>
<p>You may be wondering whether we also have to declare value types
as persistent. We don't need to do anything special for simple value
types such as <code>int</code> or <code>std::string</code> since the
ODB compiler knows how to map them to suitable database system types and
how to convert between the two. On the other hand, if a simple value
is unknown to the ODB compiler then you will need to provide the
mapping to the database system type and, possibly, the code to
convert between the two. For more information on this refer to
<a href="#5.3">Section 5.3, "Value Type Pragmas"</a>. Composite
value types are not yet supported by ODB and we will not discuss
them further in this revision of the manual.</p>
<p>Normally, you would use object types to model real-world entities,
things that have their own identity. For example, in the
previous chapter we created a <code>person</code> class to model
a person, which is a real-world entity. Name and age, which we
used as data members in our <code>person</code> class are clearly
values. It is hard to think of age 31 or name "Joe" as having their
own identities.</p>
<p>A good test to determine whether something is an object or
a value, is to consider if other objects might reference
it. A person is clearly an object because it can be referred
to by other objects such as a spouse, an employer, or a
bank. On the other hand, a person's age or name is not
something that other objects would normally refer to.</p>
<p>Also, when an object represents a real entity, it is easy to
choose a suitable object id. For example, for a
person there is an established notion of an identifier
(SSN, student id, passport number, etc). Another alternative
is to use a person's email address as an identifier.</p>
<p>Note, however, that these are only guidelines. There could
be good reasons to make something that would normally be
a value an object. Consider, for example, a database that
stores a vast number of people. Many of the <code>person</code>
objects in this database have the same names and surnames and
the overhead of storing them in every object may negatively
affect the performance. In this case, we could make the first name
and last name each an object and only store references to
these objects in the <code>person</code> class.</p>
<p>An instance of a persistent class can be in one of two states:
<em>transient</em> and <em>persistent</em>. A transient
instance only has a representation in the application's
memory and will cease to exist when the application terminates,
unless it is explicitly made persistent. In other words, a
transient instance of a persistent class behaves just like an
instance of any ordinary C++ class. A persistent instance
has a representation in both the application's memory and the
database. A persistent instance will remain even after the
application terminates unless and until it is explicitly
deleted from the database.</p>
<h2><a name="3.2">3.2 Database</a></h2>
<p>Before an application can make use of persistence services
offered by ODB, it has to create a database class instance. A
database instance is the representation of the place where
the application stores its persistent objects. You create
a database instance by instantiating one of the database
system-specific classes. For example, <code>odb::mysql::database</code>
would be such a class for the MySQL database system. You will
also normally pass a database name as an argument to the
class' constructor. The following code fragment
shows how we can create a database instance for the MySQL
database system:</p>
<pre class="c++">
#include <odb/database.hxx>
#include <odb/mysql/database.hxx>
auto_ptr<odb::database> db (
new odb::mysql::database (
"test_user" // database login name
"test_password" // database password
"test_database" // database name
));
</pre>
<p>The <code>odb::database</code> class is a common interface for
all database system-specific classes provided by ODB. You
would normally work with the database
instance via this interface unless there is a specific
functionality that your application depends on and which is
only exposed by a particular system's <code>database</code>
class. You will need to include the <code><odb/database.hxx></code>
header file to make this class available in your application.</p>
<p>The <code>odb::database</code> interface defines functions for
starting transactions and manipulating persistent objects.
These are discussed in detail in the remainder of this chapter
as well as the next chapter which is dedicated to the topic of
querying the database for persistent objects. For details on the
system-specific <code>database</code> classes, refer to
<a href="#6">Chapter 6, "Database Systems"</a>.</p>
<h2><a name="3.3">3.3 Transactions</a></h2>
<p>A transaction is an atomic, consistent, isolated and durable
(ACID) unit of work. Database operations can only be
performed within a transaction and each thread of execution
in an application can have only one active transaction at a
time.</p>
<p>By atomicity we mean that when it comes to making changes to
the database state within a transaction,
either all the changes are applied or none at all. Consider,
for example, a transaction that transfers funds between two
objects representing bank accounts. If the debit function
on the first object succeeds but the credit function on
the second fails, the transaction is rolled back and the
database state of the first object remains unchanged.</p>
<p>By consistency we mean that a transaction must take all the
objects stored in the database from one consistent state
to another. For example, if a bank account object must
reference a person object as its owner and we forget to
set this reference before making the object persistent,
the transaction will be rolled back and the database
will remain unchanged.</p>
<p>By isolation we mean that the changes made to the database
state during a transaction are only visible inside this
transaction until and unless it is committed. Using the
above example with the bank transfer, the results of the
debit operation performed on the first object is not
visible to other transactions until the credit operation
is successfully completed and the transaction is committed.</p>
<p>By durability we mean that once the transaction is committed,
the changes that it made to the database state are permanent
and will survive failures such as an application crash. From
now on the only way to alter this state is to execute and commit
another transaction.</p>
<p>A transaction is started by calling the
<code>database::begin()</code>
function. The returned transaction handle is stored in
an instance of the <code>odb::transaction</code> class.
You will need to include the <code><odb/transaction.hxx></code>
header file to make this class available in your application.
The <code>odb::transaction</code> class has the following
interface:</p>
<pre class="c++">
namespace odb
{
class transaction
{
public:
typedef odb::database database_type;
void
commit ();
void
rollback ();
database_type&
database ();
static transaction&
current ();
static bool
has_current ();
};
}
</pre>
<p>The <code>commit()</code> function commits a transaction and
<code>rollback()</code> rolls it back. Unless the transaction
has been <em>finalized</em>, that is, explicitly committed or rolled
back, the destructor of the <code>odb::transaction</code> class will
automatically roll it back when the transaction instance goes
out of scope. If you try to commit or roll back a finalized
transaction, the <code>odb::transaction_already_finalized</code>
exception is thrown.</p>
<p>The <code>database()</code> function returns the database this
transaction is working on. The <code>current()</code> static
function returns the currently active transaction for this
thread. If there is no active transaction, this function
throws the <code>odb::not_in_transaction</code> exception.
You can check whether there is a transaction in effect in
this thread using the <code>has_current()</code> static function.</p>
<p>If two or more transactions access or modify more than one object
and are executed concurrently by different applications or by
different threads within the same application, then it is possible
that these transactions will try to access objects in an incompatible
order and deadlock. The canonical example of a deadlock are
two transactions in which the first has modified <code>object1</code>
and is waiting for the second transaction to commit its changes to
<code>object2</code> so that it can also update <code>object2</code>.
At the same time the second transaction has modified <code>object2</code>
and is waiting for the first transaction to commit its changes to
<code>object1</code> because it also needs to modify <code>object1</code>.
As a result, none of the two transactions can be completed.</p>
<p>The database system detects such situations and automatically
aborts the waiting operation in one of the deadlocked transactions.
In ODB this translates to the <code>odb::deadlock</code> exception
being thrown from one of the database functions. You would normally
handle a deadlock by restarting the transaction, for example:</p>
<pre class="c++">
for (;;)
{
try
{
transaction t (db.begin ());
...
t.commit ();
break;
}
catch (const odb::deadlock&)
{
continue;
}
}
</pre>
<p>Note that in the above discussion of atomicity, consistency,
isolation, and durability, all of those guarantees only apply
to the object's state in the database as opposed to the object's
state in the application's memory. It is possible to roll
a transaction back but still have changes from this
transaction in the application's memory. An easy way to
avoid this potential inconsistency is to instantiate
persistent objects only within the transaction scope. Consider,
for example, these two implementations of the same transaction:</p>
<pre class="c++">
void
update_age (database& db, person& p)
{
transaction t (db.begin ());
p.age (p.age () + 1);
db.update (p);
t.commit ();
}
</pre>
<p>In the above implementation, if the <code>update()</code> call fails
and the transaction is rolled back, the state of the <code>person</code>
object in the database and the state of the same object in the
application's memory will differ. Now consider an
alternative implementation which only instantiates the
<code>person</code> object for the duration of the transaction:</p>
<pre class="c++">
void
update_age (database& db, unsigned long id)
{
transaction t (db.begin ());
auto_ptr<person> p (db.load<person> (id));
p.age (p.age () + 1);
db.update (p);
t.commit ();
}
</pre>
<p>Of course, it may not always be possible to write the
application in this style. Oftentimes we need to access and
modify the application's state of persistent objects out of
transactions. In this case it may make sense to try to
roll back the changes made to the application state if
the transaction was rolled back and the database state
remains unchanged. One way to do this is to re-load
the object's state from the database, for example:</p>
<pre class="c++">
void
update_age (database& db, person& p)
{
try
{
transaction t (db.begin ());
p.age (p.age () + 1);
db.update (p);
t.commit ();
}
catch (...)
{
transaction t (db.begin ());
db.load (p.id (), p);
t.commit ();
throw;
}
}
</pre>
<h2><a name="3.4">3.4 Making Objects Persistent</a></h2>
<p>A newly created instance of a persistent class is transient.
We use the <code>database::persist()</code> function template
to make a transient instance persistent. This function has two
overloaded versions with the following signatures:</p>
<pre class="c++">
template <typename T>
typename object_traits<T>::id_type
persist (const T& object);
template <typename T>
typename object_traits<T>::id_type
persist (T& object);
</pre>
<p>The first <code>persist()</code> function expects a constant reference
to an instance being persisted and is used on objects with
application-assigned object ids (see <a href="#5.4">Section 5.4,
"Data Member Pragmas"</a>). The second
function expects an unrestricted reference and, if the object id is
assigned by the database, it updates the id member of the passed instance
with the assigned value. Both functions return the object id of the
newly persistent object.</p>
<p>If the database already contains an object of this type with this
identifier, the <code>persist()</code> functions throw the
<code>odb::object_already_persistent</code> exception. This should
never happen for database-assigned object ids as long as the
number of objects persisted does not exceed the value space of
the id type.</p>
<p>When calling the <code>persist()</code> functions, we don't need to
explicitly specify the template type since it will be automatically
deduced from the argument being passed. The <code>odb::object_traits</code>
template used in the signature above is part of the database support
code generated by the ODB compiler.</p>
<p>The following example shows how we can call these functions:</p>
<pre class="c++">
person john ("John", "Doe", 33);
person jane ("Jane", "Doe", 32);
transaction t (db->begin ());
db->persist (john);
unsigned long jane_id (db->persist (jane));
t.commit ();
cerr << "Jane's id: " << jane_id << endl;
</pre>
<p>Notice that in the above code fragment we have created instances
that we were planning to make persistent before starting the
transaction. Likewise, we printed Jane's id after we have committed
the transaction. As a general rule, you should avoid performing
operations within the transaction scope that can be performed
before the transaction starts or after it terminates. An active
transaction consumes both your application's resources, such as
a database connection, as well as the database server's
resources, such as object locks. By following the above rule you
make sure these resources are made available to other threads
in your application and to other applications as soon as
possible.</p>
<h2><a name="3.5">3.5 Loading Persistent Objects</a></h2>
<p>Once an object is made persistent, and you know its object id, it
can be loaded by the application using the <code>database::load()</code>
function template. This function has two overloaded versions with
the following signatures:</p>
<pre class="c++">
template <typename T>
typename object_traits<T>::pointer_type
load (const typename object_traits<T>::id_type& id);
template <typename T>
void
load (const typename object_traits<T>::id_type& id, T& object);
</pre>
<p>Given an object id, the first function allocates a new instance
of the object class in the dynamic memory, loads its state from
the database, and returns the pointer to the new instance. The
second function loads the object's state into an existing instance.
Both functions throw <code>odb::object_not_persistent</code> if
there is no object of this type with this id in the database.</p>
<p>When we call the first <code>load()</code> function, we need to
explicitly specify the object type. We don't need to do this for
the second function because the object type will be automatically
deduced from the second argument, for example:</p>
<pre class="c++">
transaction t (db->begin ());
auto_ptr<person> jane (db->load<person> (jane_id));
db->load (jane_id, *jane);
t.commit ();
</pre>
<p>If we don't know for sure whether an object with a given id
is persistent, we can use the <code>find()</code> function
instead of <code>load()</code>, for example:</p>
<pre class="c++">
template <typename T>
typename object_traits<T>::pointer_type
find (const typename object_traits<T>::id_type& id);
template <typename T>
bool
find (const typename object_traits<T>::id_type& id, T& object);
</pre>
<p>If an object with this id is not found in the database, the first
<code>find()</code> function returns a <code>NULL</code> pointer
while the second function leaves the passed instance unmodified and
returns <code>false</code>.</p>
<p>If we don't know the object id, then we can use queries to
find the object (or objects) matching some criteria (see
<a href="#4">Chapter 4, "Querying the Database"</a>). Note,
however, that loading an object's state using its
identifier can be significantly faster than executing a query.</p>
<h2><a name="3.6">3.6 Updating Persistent Objects</a></h2>
<p>If a persistent object has been modified, we can store the updated
state in the database using the <code>database::update()</code>
function template:</p>
<pre class="c++">
template <typename T>
void
update (const T& object);
</pre>
<p>If the object passed to this function does not exist in the
database, <code>update()</code> throws the
<code>odb::object_not_persistent</code> exception.</p>
<p>Below is an example of the funds transfer that we talked about
in the earlier section on transactions. It uses the hypothetical
<code>bank_account</code> persistent class:</p>
<pre class="c++">
void
transfer (database& db,
unsigned long from_acc,
unsigned long to_acc,
unsigned int amount)
{
bank_account from, to;
transaction t (db.begin ());
db.load (from_acc, from);
if (from.balance () < amount)
throw insufficient_funds ();
db.load (to_acc, to);
to.balance (to.balance () + amount);
from.balance (from.balance () - amount);
db.update (to);
db.update (from);
t.commit ();
}
</pre>
<h2><a name="3.7">3.7 Deleting Persistent Objects</a></h2>
<p>To delete a persistent object's state from the database we use the
<code>database::erase()</code> function template. If the application
still has an instance of the erased object, this instance becomes
transient. The <code>erase()</code> function has the following
overloaded versions:</p>
<pre class="c++">
template <typename T>
void
erase (const T& object);
template <typename T>
void
erase (const typename object_traits<T>::id_type& id);
</pre>
<p>The first <code>erase()</code> function uses an object itself to
delete its state from the database. Note that the passed object
is unchanged. It simply becomes transient. The second function uses
the object id to identify the object to be deleted. If the object
does not exist in the database, both functions throw the
<code>odb::object_not_persistent</code> exception.</p>
<p>We have to specify the object type when calling the second
<code>erase()</code> function. The same is unnecessary for the
first function because the object type will be automatically
deduced from its argument. The following example shows how we
can call these functions:</p>
<pre class="c++">
const person& john = ...
transaction t (db->begin ());
db->erase (john);
db->erase<person> (jane_id);
t.commit ();
</pre>
<h2><a name="3.8">3.8 ODB Exceptions</a></h2>
<p>In the previous sections we have already mentioned some of the
exceptions that can be thrown by the database functions. In this
section we will discuss the ODB exception hierarchy and document
all the exceptions that can be thrown by the common ODB
runtime.</p>
<p>The root of the ODB exception hierarchy is the abstract
<code>odb::exception</code> class. This class inherits
from <code>std::exception</code> and has the following
interface:</p>
<pre class="c++">
namespace odb
{
struct exception: std::exception
{
virtual const char*
what () const throw () = 0;
};
}
</pre>
<p>Catching this exception guarantees that you will catch all the
exceptions thrown by ODB. The <code>what()</code> function
returns a human-readable description of the condition that
triggered the exception.</p>
<p>The concrete exceptions that can be thrown by ODB are presented
in the following listing:</p>
<pre class="c++">
namespace odb
{
struct already_in_transaction: odb::exception
{
virtual const char*
what () const throw ();
};
struct not_in_transaction: odb::exception
{
virtual const char*
what () const throw ();
};
struct transaction_already_finalized: odb::exception
{
virtual const char*
what () const throw ();
};
struct deadlock: odb::exception
{
virtual const char*
what () const throw ();
};
struct object_not_persistent: odb::exception
{
virtual const char*
what () const throw ();
};
struct object_already_persistent: odb::exception
{
virtual const char*
what () const throw ();
};
struct result_not_cached: odb::exception
{
virtual const char*
what () const throw ();
};
struct database_exception: odb::exception
{
};
}
</pre>
<p>The first four exceptions (<code>already_in_transaction</code>,
<code>not_in_transaction</code>,
<code>transaction_already_finalized</code>, and
<code>deadlock</code>) are thrown by the
<code>odb::transaction</code> class and are discussed
in <a href="#3.3">Section 3.3, "Transactions"</a>.</p>
<p>The <code>object_already_persistent</code> exception is thrown
by the <code>persist()</code> database function. See
<a href="#3.4">Section 3.4, "Making Objects Persistent"</a>
for details.</p>
<p>The <code>object_not_persistent</code> exception is thrown
by the <code>load()</code> and <code>update()</code>
database functions. Refer to
<a href="#3.5">Section 3.5, "Loading Persistent Objects"</a> and
<a href="#3.6">Section 3.6, "Updating Persistent Objects"</a> for
more information.</p>
<p>The <code>result_not_cached</code> exception is thrown by
the query result class. Refer to <a href="#4.4">Section 4.4,
"Query Result"</a> for details.</p>
<p>The <code>database_exception</code> is a base class for all
database system-specific exceptions that are thrown by the
database system-specific runtime library. See <a href="#6">Chapter
6, "Database Systems"</a> for more information.</p>
<p>The <code>odb::exception</code> class is defined in the
<code><odb/exception.hxx></code> header file. All the
concrete ODB exceptions are defined in
<code><odb/exceptions.hxx></code> which also includes
<code><odb/exception.hxx></code>. Normally you don't
need to include either of these two headers because they are
automatically included by <code><odb/database.hxx></code>.
However, if the source file that handles ODB exceptions
does not include <code><odb/database.hxx></code>, then
you will need to explicitly include one of these headers.</p>
<!-- CHAPTER -->
<h1><a name="4">4 Querying the Database</a></h1>
<p>If you don't know the identifiers of the objects that you are looking
for, you can use queries to search the database for objects matching
certain criteria. The ODB query facility is optional and you need to
explicitly request the generation of the necessary database support
code with the <code>--generate-query</code> ODB compiler option.</p>
<p>ODB provides a flexible query API that offers two distinct levels of
abstraction from the database system query language such as SQL.
At the high level you are presented with an easy to use yet powerful
object-oriented query language, called ODB Query Language. This
query language is modeled after and is integrated into C++ allowing
you to write expressive and safe queries that look and feel like
ordinary C++. We have already seen examples of these queries in the
introductory chapters. Below is another, more interesting, example:</p>
<pre class="c++">
typedef odb::query<person> query;
typedef odb::result<person> result;
unsigned short age;
query q (query::first == "John" && query::age < query::_ref (age));
for (age = 10; age < 100; age += 10)
{
result r (db->query<person> (q));
...
}
</pre>
<p>At the low level, queries can be written as predicates using
the database system-native query language such as the
<code>WHERE</code> predicate from the SQL <code>SELECT</code>
statement. This language will be referred to as native query
language. At this level ODB still takes care of converting
query parameters from C++ to the database system format. Below
is the re-implementation of the above example using SQL as
the native query language:</p>
<pre class="c++">
query q ("first = 'John' AND age = " + query::_ref (age));
</pre>
<p>Note that at this level you lose the static typing of
query expressions. For example, if we wrote something like this:</p>
<pre class="c++">
query q (query::first == 123 && query::agee < query::_ref (age));
</pre>
<p>We would get two errors during the C++ compilation. The first would
indicate that we cannot compare <code>query::first</code> to an
integer and the second would pick the misspelling in
<code>query::agee</code>. On the other hand, if we wrote something
like this:</p>
<pre class="c++">
query q ("first = 123 AND agee = " + query::_ref (age));
</pre>
<p>It would compile fine and would trigger an error only when executed
by the database system.</p>
<p>You can also combine the two query languages in a single query, for
example:</p>
<pre class="c++">
query q ("first = 'John'" + (query::age < query::_ref (age)));
</pre>
<h2><a name="4.1">4.1 ODB Query Language</a></h2>
<p>An ODB query is an expression that tells the database system whether
any given object matches the desired criteria. As such, a query expression
always evaluates as <code>true</code> or <code>false</code>. At
the higher level, an expression consists of other expressions
combined with logical operators such as <code>&&</code> (AND),
<code>||</code> (OR), and <code>!</code> (NOT). For example:</p>
<pre class="c++">
typedef odb::query<person> query;
query q (query::first == "John" || query::age == 31);
</pre>
<p>At the core of every query expression lie simple expressions which
involve one or more object members, values, or parameters. To
refer to an object member you use an expression such as
<code>query::first</code> above. The names of members in the
<code>query</code> class are derived from the names of data members
in the object class by removing the common member name decorations,
such as leading and trailing underscores, the <code>m_</code> prefix,
etc.</p>
<p>In a simple expression an object member can be compared to a value,
parameter, or another member using a number of predefined operators
and functions. The following table gives an overview of the available
expressions:</p>
<!-- border="1" is necessary for html2ps -->
<table id="operators" border="1">
<tr>
<th>Operator</th>
<th>Description</th>
<th>Example</th>
</tr>
<tr>
<td><code>==</code></td>
<td>equal</td>
<td><code>query::age == 31</code></td>
</tr>
<tr>
<td><code>!=</code></td>
<td>unequal</td>
<td><code>query::age != 31</code></td>
</tr>
<tr>
<td><code><</code></td>
<td>less than</td>
<td><code>query::age < 31</code></td>
</tr>
<tr>
<td><code>></code></td>
<td>greater than</td>
<td><code>query::age > 31</code></td>
</tr>
<tr>
<td><code><=</code></td>
<td>less than or equal</td>
<td><code>query::age <= 31</code></td>
</tr>
<tr>
<td><code>>=</code></td>
<td>greater than or equal</td>
<td><code>query::age >= 31</code></td>
</tr>
<tr>
<td><code>in()</code></td>
<td>one of the values</td>
<td><code>query::age.in (30, 32, 34)</code></td>
</tr>
<tr>
<td><code>in_range()</code></td>
<td>one of the values in range</td>
<td><code>query::age.in_range (begin, end)</code></td>
</tr>
<tr>
<td><code>is_null()</code></td>
<td>value is NULL</td>
<td><code>query::age.is_null ()</code></td>
</tr>
<tr>
<td><code>is_not_null()</code></td>
<td>value is not NULL</td>
<td><code>query::age.is_not_null ()</code></td>
</tr>
</table>
<p>The <code>in()</code> function accepts a maximum of five arguments.
Use the <code>in_range()</code> function if you need to compare
to more than five values. This function accepts a pair of
standard C++ iterators and compares to all the values from
the <code>begin</code> position inclusive and until and
excluding the <code>end</code> position. The following
code fragment shows how we can use these functions:</p>
<pre class="c++">
std::vector<string> names;
names.push_back ("John");
names.push_back ("Jack");
names.push_back ("Jane");
query q1 (query::first.in ("John", "Jack", "Jane"));
query q2 (query::first.in_range (names.begin (), names.end ()));
</pre>
<p>The operator precedence in the query expressions are the same
as for equivalent C++ operators. You can use parentheses to
make sure the expression is evaluated in the desired order.
For example:</p>
<pre class="c++">
query q ((query::first == "John" || query::first == "Jane") &&
query::age < 31);
</pre>
<h2><a name="4.2">4.2 Parameter Binding</a></h2>
<p>An instance of the <code>odb::query</code> class encapsulates two
parts of information about the query: the query expression and
the query parameters. Parameters can be bound to C++ variables
either by value or by reference.</p>
<p>If a parameter is bound by value, then the value for this parameter
is copied from the C++ variable to the query instance at the query
construction time. On the other hand, if a parameter is bound by
reference, then the query instance stores a reference to the
bound variable. The actual value of the parameter is only extracted
at the query execution time. Consider, for example, the following
two queries:</p>
<pre class="c++">
string name ("John");
query q1 (query::first == query::_val (name));
query q2 (query::first == query::_ref (name));
name = "Jane";
db->query<person> (q1); // Find John.
db->query<person> (q2); // Find Jane.
</pre>
<p>The <code>odb::query</code> class provides two special functions,
<code>_val()</code> and <code>_ref()</code>, that allow you to
bind the parameter either by value or by reference, respectively.
In the ODB query language, if the binding is not specified
explicitly, the value semantic is used by default. In the
native query language, binding must always be specified
explicitly. For example:</p>
<pre class="c++">
query q1 (query::age < age); // By value.
query q2 (query::age < query::_val (age)); // By value.
query q3 (query::age < query::_ref (age)); // By reference.
query q4 ("age < " + age); // Error.
query q5 ("age < " + query::_val (age)); // By value.
query q6 ("age < " + query::_ref (age)); // By reference.
</pre>
<p>A query that only has by-value parameters does not depend on any
other variables and is self-sufficient once constructed. A query
that has one or more by-reference parameters depends on the
bound variables until the query is executed. If one such variable
goes out of scope and you execute the query, the behavior is
undefined.</p>
<h2><a name="4.3">4.3 Executing a Query</a></h2>
<p>Once we have the query instance ready and by-reference parameters
initialized, we can execute the query using the
<code>database::query()</code> function template. It has two
overloaded versions:</p>
<pre class="c++">
template <typename T>
result<T>
query (bool cache = true);
template <typename T>
result<T>
query (const odb::query<T>&, bool cache = true);
</pre>
<p>The first <code>query()</code> function is used to return all the
persistent objects of a given type stored in the database.
The second function uses the passed query instance to only return
objects matching the query criteria. The <code>cache</code> argument
determines whether the objects' states should be cached in the
application's memory or if they should be returned by the database
system one by one as the iteration over the result progresses. The
result caching is discussed in detail in the next section.</p>
<p>When calling the <code>query()</code> function, we have to
explicitly specify the object type we are querying. For example:</p>
<pre class="c++">
typedef odb::query<person> query;
typedef odb::result<person> result;
result all (db->query<person> ());
result johns (db->query<person> (query::first == "John"));
</pre>
<p>Note that it is not required to explicitly create a named
query variable before executing it. For example, the following
two queries are equivalent:</p>
<pre class="c++">
query q (query::first == "John");
result r1 (db->query<person> (q));
result r1 (db->query<person> (query::first == "John"));
</pre>
<p>Normally you would create a named query instance if you are
planning to run the same query multiple times and would use the
in-line version for those that are executed only once.</p>
<p>It is also possible to create queries from other queries by
combining them using logical operators. For example:</p>
<pre class="c++">
result
find_minors (database& db, const query& name_query)
{
return db.query<person> (name_query && query::age < 18);
}
result r (find_underage (db, query::first == "John"));
</pre>
<h2><a name="4.4">4.4 Query Result</a></h2>
<p>The result of executing a query is zero, one, or more objects
matching the query criteria. The result is returned as an instance
of the <code>odb::result</code> class template, for example:</p>
<pre class="c++">
typedef odb::query<person> query;
typedef odb::result<person> result;
result johns (db->query<person> (query::first == "John"));
</pre>
<p>It is best to view an instance of <code>odb::result</code>
as a handle to a stream, such as a file stream. While you can
make a copy of a result or assign one result to another, the
two instances will refer to the same result stream. Advancing
the current position in one instance will also advance it in
another. The result instance is only usable within the transaction
it was created in. Trying to manipulate the result after the
transaction has terminated leads to undefined behavior.</p>
<p>The <code>odb::result</code> class template conforms to the
standard C++ sequence requirements and has the following
interface:</p>
<pre class="c++">
namespace odb
{
template <typename T>
class result
{
public:
typedef odb::result_iterator<T> iterator;
public:
result ();
result (const result&);
result&
operator= (const result&);
void
swap (result&)
public:
iterator
begin ();
iterator
end ();
public:
void
cache ();
bool
empty () const;
std::size_t
size () const;
};
}
</pre>
<p>The default constructor creates an empty result set. The
<code>cache()</code> function caches the returned objects'
state in the application's memory. We have already mentioned
result caching when we talked about query execution. As you
may remember the <code>database::query()</code> function
caches the result unless instructed not to by the caller.
The <code>cache()</code> function allows you to
cache the result at a later stage if it wasn't already
cached during query execution.</p>
<p>If the result is cached, the database state of all the returned
objects is stored in the application's memory. Note that
the actual objects are still only instantiated on demand
during result iteration. It is the raw database state that
is cached in memory. In contrast, for uncached results
the object's state is sent by the database system one object
at a time as the iteration progresses.</p>
<p>Uncached results can improve the performance of both the application
and the database system in situations where you have a large
number of objects in the result or if you will only examine
a small portion of the returned objects. However, uncached
results have a number of limitations. There can only be one
uncached result in a transaction. Creating another result
(cached or uncached) by calling <code>database::query()</code>
will invalidate the existing uncached result. Furthermore,
calling any other database functions, such as <code>update()</code>
or <code>erase()</code> will also invalidate the uncached result.</p>
<p>The <code>empty()</code> function returns <code>true</code> if
there are no objects in the result and <code>false</code> otherwise.
The <code>size()</code> function can only be called for cached results.
It returns the number of objects in the result. If you call this
function on an uncached result, the <code>odb::result_not_cached</code>
exception is thrown.</p>
<p>To iterate over the objects in a result we use the
<code>begin()</code> and <code>end()</code> functions
together with the <code>odb::result<T>::iterator</code>
type, for example:</p>
<pre class="c++">
result r (db->query<person> (query::first == "John"));
for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
...
}
</pre>
<p>The result iterator is an input iterator which means that the
only two position operations that it supports are to move to the
next object and to determine whether the end of the result stream
has been reached. In fact, the result iterator can only be in two
states: the current position and the end position. If you have
two iterators pointing to the current position and then you
advance one of them, the other will advance as well. This,
for example, means that it doesn't make sense to store an
iterator that points to some object of interest in the result
stream with the intent of dereferencing it after the iteration
is over. Instead, you would need to store the object itself.</p>
<p>The result iterator has the following dereference functions
that can be used to access the pointed-to object:</p>
<pre class="c++">
namespace odb
{
template <typename T>
class result_iterator
{
public:
T*
operator-> () const;
T&
operator* () const;
typename object_traits<T>::pointer_type
load ();
void
load (T& x);
};
}
</pre>
<p>When you call the <code>*</code> or <code>-></code> operator,
the iterator will allocate a new instance of the object class
in the dynamic memory, load its state from the database
state, and return a reference or pointer to the new instance. The
iterator maintains the ownership of the returned object and will
return the same pointer for subsequent calls to either of these
operators until it is advanced to the next object or you call
the first <code>load()</code> function (see below). For example:</p>
<pre class="c++">
result r (db->query<person> (query::first == "John"));
for (result::iterator i (r.begin ()); i != r.end ();)
{
cout << i->last () << endl; // Create an object.
person& p (*i); // Reference to the same object.
cout << p.age () << endl;
++i; // Free the object.
}
</pre>
<p>The overloaded <code>result_iterator::load()</code> functions are
similar to <code>database::load()</code>. The first function
returns a dynamically allocated instance of the current
object. As an optimization, if the iterator already owns an object
as a result of an earlier
call to the <code>*</code> or <code>-></code> operator, then it
relinquishes the ownership of this object and returns it instead.
This allows you to write code like this without worrying about
a double allocation:</p>
<pre class="c++">
result r (db->query<person> (query::first == "John"));
for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
if (i->last == "Doe")
{
auto_ptr p (i.load ());
...
}
}
</pre>
<p>Note, however, that because of this optimization, a subsequent
to <code>load()</code> call to the <code>*</code> or <code>-></code>
operator results in the allocation of a new object.</p>
<p>The second <code>load()</code> function allows
you to load the current object's state into an existing instance.
For example:</p>
<pre class="c++">
result r (db->query<person> (query::first == "John"));
person p;
for (result::iterator i (r.begin ()); i != r.end (); ++i)
{
i.load (p);
cout << p.last () << endl;
cout << i.age () << endl;
}
</pre>
<!-- CHAPTER -->
<h1><a name="5">5 ODB Pragma Language</a></h1>
<p>As we have already seen in previous chapters, ODB uses a pragma-based
language to capture database-specific information about C++ types.
This chapter describes the ODB pragma language in more detail. It
can be read together with other chapters in the manual to get a
sense of what kind of configurations and mapping fine-tuning are
possible. You can also use this chapter as a reference at a later
stage.</p>
<p>An ODB pragma has the following syntax:</p>
<p><code>#pragma db <i>qualifier</i> [<i>specifier</i> <i>specifier</i> ...]</code></p>
<p>The <em>qualifier</em> tells the ODB compiler what kind of C++ construct
this pragma describes. Valid qualifiers are <code>object</code>,
<code>value</code>, and <code>member</code>. Pragmas with the
<code>object</code> qualifier describe persistent object types.
It tells the ODB compiler that the C++ class it describes is a
persistent class. Similarly, pragmas with the <code>value</code>
qualifier describes value types and the <code>member</code>
qualifier is used to describe data members of persistent object
and value types.</p>
<p>The <em>specifier</em> informs the ODB compiler about a particular
database-related property of the C++ declaration. For example, the
<code>id</code> member specifier tells the ODB compiler that this
member contains this object's identifier. Below is the declaration
of the <code>person</code> class that shows how we can use ODB
pragmas:</p>
<pre class="c++">
#pragma db object
class person
{
...
private:
#pragma db member id
unsigned long id_;
...
};
</pre>
<p>In the above example we don't explicitly specify which C++ class or
data member the pragma belongs to. Rather, the pragma applies to
a C++ declaration that immediately follows the pragma. Such pragmas
are called <em>positioned pragmas</em>. In positioned pragmas that
apply to data members, the <code>member</code> qualifier can be
omitted for brevity, for example:</p>
<pre class="c++">
#pragma db id
unsigned long id_;
</pre>
<p>Note also that if the C++ declaration immediately following a
position pragma is incompatible with the pragma qualifier, an
error will be issued. For example:</p>
<pre class="c++">
#pragma db object // Error: expected class instead of data member.
unsigned long id_;
</pre>
<p>While keeping the C++ declarations and database declarations close
together eases maintenance and increases readability, you can also
place them in different parts of the same header file or even
factor them to a separate file. To achieve this we use the so called
<em>named pragmas</em>. Unlike positioned pragmas, named pragmas
explicitly specify the C++ declaration to which they apply by
adding the declaration name after the pragma qualifier. For example:</p>
<pre class="c++">
class person
{
...
private:
unsigned long id_;
...
};
#pragma db object(person)
#pragma db member(person::id_) id
</pre>
<p>Note that in the named pragmas for data members the <code>member</code>
qualifier is no longer optional. The C++ declaration name in the
named pragmas is resolved using the standard C++ name resolution
rules, for example:</p>
<pre class="c++">
namespace db
{
class person
{
...
private:
unsigned long id_;
...
};
}
namespace db
{
#pragma db object(person) // Resolves db::person.
}
#pragma db member(db::person::id_) id
</pre>
<p>As another example, the following code fragment shows how to use the
named value type pragma to map a C++ type to a native database type:</p>
<pre class="c++">
#pragma db value(bool) type("INT NOT NULL")
#pragma db object
class person
{
...
private:
bool married_; // Mapped to INT NOT NULL database type.
...
};
</pre>
<h2><a name="5.1">5.1 C++ Compiler Warnings</a></h2>
<p>The C++ header file that defines your persistent classes and
normally contains one or more ODB pragmas is compiled by both
the ODB compiler to generate the database support code and
the C++ compiler to build your application. Some C++ compilers
issue warnings about pragmas that they do not recognize. There
are several ways to deal with this problem. The easiest is to
disable such warnings using one of the compiler-specific command
line options or warning control pragmas. This method is described
in the following sub-section for popular C++ compilers.</p>
<p>There are also several C++ compiler-independent methods that you
can employ. The first is to use the <code>PRAGMA_DB</code> macro,
defined in <code><odb/core.hxx></code>, instead of using
<code>#pragma db</code> directly. This macro expands to the
ODB pragma when compiled with the ODB compiler and to an empty
declaration when compiled with other compilers. The following example
shows how we can use this macro:</p>
<pre class="c++">
#include <odb/core.hxx>
PRAGMA_DB(object)
class person
{
...
private:
PRAGMA_DB(id)
unsigned long id_;
...
};
</pre>
<p>An alternative to using the <code>PRAGMA_DB</code> macro is to
group the <code>#pragma db</code> directives in blocks that are
conditionally included into compilation only when compiled with the
ODB compiler. For example:</p>
<pre class="c++">
class person
{
...
private:
unsigned long id_;
...
};
#ifdef ODB_COMPILER
# pragma db object(person)
# pragma db member(person::id_) id
#endif
</pre>
<p>The disadvantage of this approach is that it can quickly become
overly verbose when positioned pragmas are used.</p>
<h3><a name="5.1.1">5.1.1 GNU C++</a></h3>
<p>GNU g++ does not issue warnings about unknown pragmas
unless requested with the <code>-Wall</code> command line option.
To disable only the unknown pragma warning, you can add the
<code>-Wno-unknown-pragmas</code> option after <code>-Wall</code>,
for example:</p>
<pre class="terminal">
g++ -Wall -Wno-unknown-pragmas ...
</pre>
<h3><a name="5.1.2">5.1.2 Visual C++</a></h3>
<p>Microsoft Visual C++ issues an unknown pragma warning (C4068) at
warning level 1 or higher. This means that unless you have disabled
the warnings altogether (level 0), you will see this warning.</p>
<p>To disable this warning via the compiler command line, you can add
the <code>/wd4068</code> C++ compiler option in Visual Studio 2008
and earlier. In Visual Studio 2010 there is now a special GUI field
where you can enter warning numbers that should be disabled. Simply
enter 4068 into this field.</p>
<p>You can also disable this warning for only a specific header or
a fragment of a header using the warning control pragma. For
example:</p>
<pre class="c++">
#include <odb/core.hxx>
#pragma warning (push)
#pragma warning (disable:4068)
#pragma db object
class person
{
...
private:
#pragma db id
unsigned long id_;
...
};
#pragma warning (pop)
</pre>
<h3><a name="5.1.3">5.1.3 Sun C++</a></h3>
<p>The Sun C++ compiler does not issue warnings about unknown pragmas
unless the <code>+w</code> or <code>+w2</code> option is specified.
To disable only the unknown pragma warning you can add the
<code>-erroff=unknownpragma</code> option anywhere on the
command line, for example:</p>
<pre class="terminal">
CC +w -erroff=unknownpragma ...
</pre>
<h3><a name="5.1.4">5.1.4 IBM XL C++</a></h3>
<p>IBM XL C++ issues an unknown pragma warning (1540-1401) by default.
To disable this warning you can add the <code>-qsuppress=1540-1401</code>
command line option, for example:</p>
<pre class="terminal">
xlC -qsuppress=1540-1401 ...
</pre>
<h2><a name="5.2">5.2 Object Type Pragmas</a></h2>
<p>A pragma with the <code>object</code> qualifier declares a C++ class
as a persistent object type. The qualifier can be optionally followed
by the <code>table</code> specifier.</p>
<h3><a name="5.2.1">5.2.1 <code>table</code></a></h3>
<p>The <code>table</code> specifier specifies the table name that should
be used to store objects of this class in a relational database. For
example:</p>
<pre class="c++">
#pragma db object table("people")
class person
{
...
};
</pre>
<p>If the table name is not specified, the class name is used as the
table name.</p>
<h2><a name="5.3">5.3 Value Type Pragmas</a></h2>
<p>A pragma with the <code>value</code> qualifier describes a value
type and can be optionally followed by the <code>type</code>
specifier.</p>
<h3><a name="5.3.1">5.3.1 <code>type</code></a></h3>
<p>The <code>type</code> specifier specifies the native database type
that should be used for data members of this type. For example:</p>
<pre class="c++">
#pragma db value(bool) type("INT NOT NULL")
#pragma db object
class person
{
...
private:
bool married_; // Mapped to INT NOT NULL database type.
...
};
</pre>
<p>The ODB compiler provides the default mapping between common C++
types, such as <code>bool</code>, <code>int</code>, and
<code>std::string</code> and the database types for each supported
database system. For more information on the default mapping,
refer to <a href="#6">Chapter 6, "Database Systems"</a>.</p>
<p>In the above example we changed the mapping for the <code>bool</code>
type which is now mapped to the <code>INT</code> database type. In
this case, the <code>value</code> pragma is all that is necessary
since the ODB compiler will be able to figure out how to store
a boolean value as an integer in the database. However, there
could be situations where the ODB compiler will not know how to
handle the conversion between the C++ and database representations
of a value. Consider, as an example, a situation where the
boolean value is stored in the database as a string:</p>
<pre class="c++">
#pragma db value(bool) type("VARCHAR(5) NOT NULL")
</pre>
<p>The possible database values for the C++ <code>true</code> value could
be <code>"true"</code>, or <code>"TRUE"</code>, or <code>"True"</code>.
Or, maybe, all of the above are valid. The ODB compiler has no way
of knowing how your application wants to convert <code>bool</code>
to a string and back. To support such custom value type mappings,
ODB allows you to provide your own database conversion functions
by specializing the <code>value_traits</code> class template. The
<code>mapping</code> example in the <code>odb-examples</code>
package shows how to do this for all the supported database systems.</p>
<p>It is also possible to change the database type mapping for individual
members, as described in <a href="#5.4">Section 5.4, "Data Member
Pragmas"</a>.</p>
<h2><a name="5.4">5.4 Data Member Pragmas</a></h2>
<p>A pragma with the <code>member</code> qualifier or a positioned
pragma without a qualifier describes a data member. It can
be optionally followed, in any order, by one or more specifiers
summarized in the table below:</p>
<!-- border="1" is necessary for html2ps -->
<table id="specifiers" border="1">
<tr>
<th>Specifier</th>
<th>Summary</th>
<th>Section</th>
</tr>
<tr>
<td><code>id</code></td>
<td>the member is an object id</td>
<td><a href="#5.4.1">5.4.1</a></td>
</tr>
<tr>
<td><code>auto</code></td>
<td>id is assigned by the database</td>
<td><a href="#5.4.2">5.4.2</a></td>
</tr>
<tr>
<td><code>type</code></td>
<td>the database type for the member</td>
<td><a href="#5.4.3">5.4.3</a></td>
</tr>
<tr>
<td><code>column</code></td>
<td>the column name for the member</td>
<td><a href="#5.4.4">5.4.4</a></td>
</tr>
<tr>
<td><code>transient</code></td>
<td>the member is not stored in the database</td>
<td><a href="#5.4.5">5.4.5</a></td>
</tr>
</table>
<h3><a name="5.4.1">5.4.1 <code>id</code></a></h3>
<p>The <code>id</code> specifier specifies that the data member contains
the object id. Every persistent class must have a member designated
as an object's identifier. For example:</p>
<pre class="c++">
#pragma db object
class person
{
...
private:
#pragma db id
std::string email_;
...
};
</pre>
<p>In a relational database, an identifier member is mapped to a
primary key.</p>
<h3><a name="5.4.2">5.4.2 <code>auto</code></a></h3>
<p>The <code>auto</code> specifier specifies that the object's identifier
is automatically assigned by the database. Only a member that was
designated as an object id can have this specifier. For example:</p>
<pre class="c++">
#pragma db object
class person
{
...
private:
#pragma db id auto
unsigned long id_;
...
};
</pre>
<p>Note that automatically-assigned object ids are not reused.
If you have a high object turnover (that is, objects are routinely
made persistent and then erased), then care must be taken not to
run out of object ids. In such situations, using
<code>unsigned long long</code> as the identifier type is a safe
choice.</p>
<p>For additional information on the automatic identifier assignment,
refer to <a href="#3.4">Section 3.4, "Making Objects Persistent"</a>.</p>
<h3><a name="5.4.3">5.4.3 <code>type</code></a></h3>
<p>The <code>type</code> specifier specifies the native database type
that should be used for this data member. For example:</p>
<pre class="c++">
#pragma db object
class person
{
...
private:
#pragma db type("INT NOT NULL")
bool married_;
...
};
</pre>
<p>The behavior of this specifier for members is similar to that
for value types. The only difference is the scope. The value
type pragma applies to all members with this value type that
don't have their own <code>type</code> specifiers, while the
member pragma applies only to a single member. For more
information on the semantics of this specifier, refer to
<a href="#5.3">Section 5.3, "Value Type Pragmas"</a>.</p>
<h3><a name="5.4.4">5.4.4 <code>column</code></a></h3>
<p>The <code>column</code> specifier specifies the column name
that should be used to store this member in a relational database.
For example:</p>
<pre class="c++">
#pragma db object
class person
{
...
private:
#pragma db id column("person_id")
unsigned long id_;
...
};
</pre>
<p>If the column name is not specified, it is derived from the member
name by removing the common member name decorations, such as leading
and trailing underscores, the <code>m_</code> prefix, etc.</p>
<h3><a name="5.4.5">5.4.5 <code>transient</code></a></h3>
<p>The <code>transient</code> specifier instructs the ODB compiler
not to store the data member in the database. For example:</p>
<pre class="c++">
#pragma db object
class person
{
...
private:
date born_;
#pragma db transient
unsigned short age_; // Computed from born_.
...
};
</pre>
<p>This pragma is usually used on computed members, pointers and
references that are only meaningful in the application's
memory, as well as utility members such as mutexes, etc.</p>
<!-- CHAPTER -->
<h1><a name="6">6 Database Systems</a></h1>
<p>This chapter covers topics specific to the database system
implementations and their support in ODB. In particular, it
describes the system-specific <code>database</code> classes
as well as the default mapping between basic C++ value types
and native database types.</p>
<h2><a name="6.1">6.1 MySQL Database</a></h2>
<p>To generate support code for the MySQL database you will need
to pass the "<code>--database mysql</code>"
(or "<code>-d mysql</code>") option to the ODB compiler.
Your application will also need to link to the MySQL ODB runtime
library (<code>libodb-mysql</code>). All MySQL-specific ODB
classes are defined in the <code>odb::mysql</code> namespace.</p>
<h3><a name="6.1.1">6.1.1 MySQL Type Mapping</a></h3>
<p>The following table summarizes the default mapping between basic
C++ value types and MySQL database types. This mapping can be
customized on the per-type and per-member basis using the ODB
Pragmas Language (see <a href="#5">Chapter 5, "ODB Pragma
Language"</a>).</p>
<!-- border="1" is necessary for html2ps -->
<table id="mapping" border="1">
<tr>
<th>C++ Type</th>
<th>MySQL type</th>
</tr>
<tr>
<td><code>bool</code></td>
<td><code>TINYINT(1) NOT NULL</code></td>
</tr>
<tr>
<td><code>char</code></td>
<td><code>TINYINT NOT NULL</code></td>
</tr>
<tr>
<td><code>signed char</code></td>
<td><code>TINYINT NOT NULL</code></td>
</tr>
<tr>
<td><code>unsigned char</code></td>
<td><code>TINYINT UNSIGNED NOT NULL</code></td>
</tr>
<tr>
<td><code>short</code></td>
<td><code>SMALLINT NOT NULL</code></td>
</tr>
<tr>
<td><code>unsigned short</code></td>
<td><code>SMALLINT UNSIGNED NOT NULL</code></td>
</tr>
<tr>
<td><code>int</code></td>
<td><code>INT NOT NULL</code></td>
</tr>
<tr>
<td><code>unsigned int</code></td>
<td><code>INT UNSIGNED NOT NULL</code></td>
</tr>
<tr>
<td><code>long</code></td>
<td><code>BIGINT NOT NULL</code></td>
</tr>
<tr>
<td><code>unsigned long</code></td>
<td><code>BIGINT UNSIGNED NOT NULL</code></td>
</tr>
<tr>
<td><code>long long</code></td>
<td><code>BIGINT NOT NULL</code></td>
</tr>
<tr>
<td><code>unsigned long long</code></td>
<td><code>BIGINT UNSIGNED NOT NULL</code></td>
</tr>
<tr>
<td><code>float</code></td>
<td><code>FLOAT NOT NULL</code></td>
</tr>
<tr>
<td><code>double</code></td>
<td><code>DOUBLE NOT NULL</code></td>
</tr>
<tr>
<td><code>std::string</code></td>
<td><code>TEXT NOT NULL/VARCHAR(255) NOT NULL</code></td>
</tr>
</table>
<p>Note that the <code>std::string</code> type is mapped
differently depending on whether the member of this type
is an object id or not. If the member is an object id,
then for this member <code>std::string</code> is mapped
to <code>VARCHAR(255) NOT NULL</code> MySQL type. Otherwise,
it is mapped to <code>TEXT NOT NULL</code>.</p>
<h3><a name="6.1.2">6.1.2 MySQL Database Class</a></h3>
<p>The MySQL <code>database</code> class has the following
interface:</p>
<pre class="c++">
namespace odb
{
namespace mysql
{
class database: public odb::database
{
public:
database (const char* user,
const char* passwd,
const char* db,
const char* host = 0,
unsigned int port = 0,
const char* socket = 0,
unsigned long client_flags = 0,
std::auto_ptr<connection_factory> = 0);
database (const std::string& user,
const std::string& passwd,
const std::string& db,
const std::string& host = "",
unsigned int port = 0,
const std::string* socket = 0,
unsigned long client_flags = 0,
std::auto_ptr<connection_factory> = 0);
database (const std::string& user,
const std::string* passwd,
const std::string& db,
const std::string& host = "",
unsigned int port = 0,
const std::string* socket = 0,
unsigned long client_flags = 0,
std::auto_ptr<connection_factory> = 0);
database (const std::string& user,
const std::string& passwd,
const std::string& db,
const std::string& host,
unsigned int port,
const std::string& socket,
unsigned long client_flags = 0,
std::auto_ptr<connection_factory> = 0);
database (const std::string& user,
const std::string* passwd,
const std::string& db,
const std::string& host,
unsigned int port,
const std::string& socket,
unsigned long client_flags = 0,
std::auto_ptr<connection_factory> = 0);
database (int& argc,
char* argv[],
bool erase = false,
unsigned long client_flags = 0,
std::auto_ptr<connection_factory> = 0);
static void
print_usage (std::ostream&);
public:
const char*
user () const;
const char*
password () const;
const char*
db () const;
const char*
host () const;
unsigned int
port () const;
const char*
socket () const;
unsigned long
client_flags () const;
public:
details::shared_ptr<mysql::connection>
connection ();
};
}
}
</pre>
<p>You will need to include the <code><odb/mysql/database.hxx></code>
header file to make this class available in your application.</p>
<p>The overloaded <code>database</code> constructors allow you
to specify MySQL database parameters that should be used when
connecting to the database. In MySQL <code>NULL</code> and an
empty string are treated as the same values for all the
string parameters except <code>password</code> and
<code>socket</code>. The <code>client_flags</code> argument
allows you to specify various MySQL client library flags. For more
information on the possible values, refer to the MySQL C API
documentation. The <code>CLIENT_FOUND_ROWS</code> flag is always set
by the MySQL ODB runtime regardless of whether it was passed in the
<code>client_flags</code> argument.</p>
<p>The last constructor extracts the database parameters
from the command line. The following options are recognized:</p>
<pre class="terminal">
--user <login>
--password <password>
--database <name>
--host <host>
--port <integer>
--socket <socket>
--options-file <file>
</pre>
<p>The <code>--options-file</code> option allows you to specify some
or all of the database options in a file with each option appearing
on a separate line followed by space and an option value.</p>
<p>If the <code>erase</code> argument to this constructor is true,
then the above options are removed from the <code>argv</code>
array and the <code>argc</code> count is updated accordingly.
This is primarily useful if your application accepts other
options or arguments and you would like to get the MySQL
options out of the <code>argv</code> array.</p>
<p>This constructor throws the <code>odb::mysql::cli_exception</code>
exception if the MySQL option values are missing or invalid.
See section <a href="#6.1.4">Section 6.1.4, "MySQL Exceptions"</a>
for more information on this exception.</p>
<p>The static <code>print_usage()</code> function prints the list of options
with short descriptions that are recognized by this constructor.</p>
<p>The last argument to all of the constructors is the
pointer to the connection factory. If you pass a
non-<code>NULL</code> value, the database instance assumes
ownership of the factory instance. The connection factory
interface as well as the available implementations are described
in the next section.</p>
<p>The set of accessor functions following the constructors allow you
to query the parameters of the <code>database</code> instance.</p>
<p>The <code>connection()</code> function returns the MySQL database
connection encapsulated by the <code>odb::mysql::connection</code>
class. Normally, you wouldn't call this function directly and
instead let the ODB runtime manage the database connections. However,
if for some reason you need to access the underlying MySQL connection
handle, refer to the MySQL ODB runtime source code for the interface
of the <code>connection</code> class.</p>
<h3><a name="6.1.3">6.1.3 Connection Factory</a></h3>
<p>The <code>connection_factory</code> abstract class has the
following interface:</p>
<pre class="c++">
namespace odb
{
namespace mysql
{
class connection_factory
{
public:
virtual void
database (mysql::database&) = 0;
virtual details::shared_ptr<mysql::connection>
connect () = 0;
};
}
}
</pre>
<p>The <code>database()</code> function is called when a connection
factory is associated with a database instance. This happens in
the <code>odb::mysql::database</code> class constructors. The
<code>connect()</code> function is called whenever a database
connection is requested.</p>
<p>The two implementations of the <code>connection_factory</code>
interface provided by the MySQL ODB runtime are
the <code>new_connection_factory</code> and
<code>connection_pool_factory</code>. You will need to include
the <code><odb/mysql/connection-factory.hxx></code>
header file to make the <code>connection_factory</code> interface
and these implementation classes available in your application.</p>
<p>The <code>new_connection_factory</code> class creates a new
connection whenever one is requested. When a connection is no
longer needed, it is released and closed. The
<code>connection_pool_factory</code> class implements a
connection pool. It has the following interface:</p>
<pre class="c++">
namespace odb
{
namespace mysql
{
class connection_pool_factory: public connection_factory
{
connection_pool_factory (std::size_t max_connections = 0,
std::size_t min_connections = 0)
};
};
</pre>
<p>The <code>max_connections</code> argument specifies the maximum
number of concurrent connections that this pool factory will
maintain. Similarly, the <code>min_connections</code> argument
specifies the minimum number of available connections that
should be kept open.</p>
<p>Whenever a connection is requested, the pool factory first
checks if there is an unused connection that can be returned.
If there is none, the pool factory checks the
<code>max_connections</code> value to see if a new connection
can be created. If the total number of connections maintained
by the pool is less than this value, then a new connection is
created and returned. Otherwise, the calling thread is blocked
until a connection becomes available.</p>
<p>When a connection is released, the pool factory first checks
if there are blocked threads waiting for a connection. If so,
one of them is unblocked and is given the connection. Otherwise,
the pool factory checks whether the total number of connections
maintained by the pool is greater than the <code>min_connections</code>
value. If that's the case, the connection is closed. Otherwise, the
connection is added to the pool of available connections to be
returned on the next request. In other words, if the number of
connections maintained by the pool exceeds the <code>min_connections</code>
number and there are no threads waiting for a new connection,
then the pool will close the excess connections.</p>
<p>If the <code>max_connections</code> value is 0, then the pool will
create a new connection whenever all of the existing connections
are in use. If the <code>min_connections</code> value is 0, then
the pool will never close a connection and instead maintain all
the connections that were ever created.</p>
<p>If you pass <code>NULL</code> as the connection factory to
one of the <code>database</code> constructors, then the
<code>connection_pool_factory</code> instance will be
created by default with the min and max connections values
set to 0. The following code fragment shows how we can
pass our own connection factory instance:</p>
<pre class="c++">
#include <odb/database.hxx>
#include <odb/mysql/database.hxx>
#include <odb/mysql/connection-factory.hxx>
int
main (int argc, char* argv[])
{
auto_ptr<odb::mysql::connection_factory> f (
new odb::mysql::connection_pool_factory (20));
auto_ptr<odb::database> db (
new mysql::database (argc, argv, false, 0, f));
}
</pre>
<h3><a name="6.1.4">6.1.4 MySQL Exceptions</a></h3>
<p>The MySQL ODB runtime library defines the following MySQL-specific
exceptions:</p>
<pre class="c++">
namespace odb
{
namespace mysql
{
class database_exception: odb::database_exception
{
public:
unsigned int
error () const;
const std::string&
sqlstate () const;
const std::string&
message () const;
virtual const char*
what () const throw ();
};
class cli_exception: odb::exception
{
public:
virtual const char*
what () const throw ();
};
}
}
</pre>
<p>You will need to include the <code><odb/mysql/exceptions.hxx></code>
header file to make these exceptions available in your application.</p>
<p>The <code>odb::mysql::database_exception</code> is thrown if
a MySQL database operation fails. The MySQL-specific error
information is accessible via the <code>error()</code>,
<code>sqlstate()</code>, and <code>message()</code> functions.
All this information is also combined and returned in a
human-readable form by the <code>what()</code> function.</p>
<p>The <code>odb::mysql::cli_exception</code> is thrown by the
command line parsing constructor of the <code>odb::mysql::database</code>
class if the MySQL option values are missing or invalid. The
<code>what()</code> function returns a human-readable description
of an error.</p>
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