This example shows how to perform more secure XML parsing by disabling
the XML External Entity (XXE) Processing. If XML Schema validation is
used, then it would also make sense to pre-load the known schemas and
to disable loading of any external schemas, for example, via the
schemaLocation attribute found in the XML documents. See the comment
in driver.cxx for more information on how to achieve this.

The example consists of the following files:

library.xsd
  XML Schema which describes a library of books.

library.xml
  Sample XML instance document. It includes (commented out) DOCTYPE
  declarations with internal and external subsets that the parser
  will refuse to process.

library.hxx
library.cxx
  C++ types that represent the given vocabulary and a set of parsing
  functions that convert XML instance documents to a tree-like in-memory
  object model.

  These files are generated by the XSD compiler from library.xsd using the
  following command line:

  xsd cxx-tree library.xsd

secure-dom-parser.hxx
secure-dom-parser.cxx
  A secure Xerces-C++ DOM parser implementation that disables processing
  of internal/external DTD subsets.

driver.cxx
  Driver for the example. It first sets up the secure DOM parser. It then
  parses the input file to a DOM document using the secure DOM parser and
  calls one of the parsing functions that constructs the object model from
  this DOM document. Finally, the driver prints a number of books in the
  object model to STDERR.

To compile and link the example manually from the command line we can use
the following commands (replace 'c++' with your C++ compiler name):

c++ -DXSD_CXX11 -c library.cxx
c++ -DXSD_CXX11 -c secure-dom-parser.cxx
c++ -DXSD_CXX11 -c driver.cxx
c++ -o driver driver.o library.o secure-dom-parser.o -lxerces-c

Note that we need to define the XSD_CXX11 preprocessor macro since the
source code includes libxsd headers directly.

To run the example on the sample XML instance document execute:

./driver library.xml

To verify that DTD processing is disabled, uncomment a different DOCTYPE
version in the sample document.