This page has sections on the following topics:
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The SAX2 API for XML parsers was originally developed for
Java. Please be aware that there is no standard SAX2 API for
C++, and that use of the Xerces-C SAX2 API does not
guarantee client code compatibility with other C++ XML
parsers.
The SAX2 API presents a callback based API to the parser. An
application that uses SAX2 provides an instance of a handler
class to the parser. When the parser detects XML constructs,
it calls the methods of the handler class, passing them
information about the construct that was detected. The most
commonly used handler classes are ContentHandler which is
called when XML constructs are recognized, and ErrorHandler
which is called when an error occurs. The header files for the
various SAX2 handler classes are in
'<xerces-c1_4_0>/include/sax2'
As a convenience, Xerces-C provides the class
DefaultHandler, which is a single class which is publicly derived
from all the Handler classes. DefaultHandler's default
implementation of the handler callback methods is to do
nothing. A convenient way to get started with Xerces-C is
to derive your own handler class from DefaultHandler and override
just those methods in HandlerBase which you are interested in
customizing. This simple example shows how to create a handler
which will print element names, and print fatal error
messages. The source code for the sample applications show
additional examples of how to write handler classes.
This is the header file MySAX2Handler.hpp:
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#include <sax2/DefaultHandler.hpp>
class MySAX2Handler : public DefaultHandler {
public:
void startElement(
const XMLCh* const uri,
const XMLCh* const localname,
const XMLCh* const qname,
const Attributes& attrs
);
void fatalError(const SAXParseException&);
}; |  |  |  |  |
This is the implementation file MySAX2Handler.cpp:
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#include "MySAX2Handler.hpp"
#include <iostream.h>
MySAX2Handler::MySAX2Handler()
{
}
MySAX2Handler::startElement(const XMLCh* const uri,
const XMLCh* const localname,
const XMLCh* const qname,
const Attributes& attrs)
{
// transcode() is an user application defined function which
// converts unicode strings to usual 'char *'. Look at
// the sample program SAX2Count for an example implementation.
cout << "I saw element: " << transcode(qname) << endl;
}
MySAX2Handler::fatalError(const SAXParseException& exception)
{
cout << "Fatal Error: " << transcode(exception.getMessage())
<< " at line: " << exception.getLineNumber()
<< endl;
} |  |  |  |  |
The XMLCh and Attributes types are supplied by
Xerces-C and are documented in the include
files. Examples of their usage appear in the source code to
the sample applications.
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 |  |  |  | Objects and Memory Management |  |  |  |  |
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The C++ DOM implementation uses automatic memory management,
implemented using reference counting. As a result, the C++
code for most DOM operations is very similar to the equivalent
Java code, right down to the use of factory methods in the DOM
document class for nearly all object creation, and the lack of
any explicit object deletion.
Consider the following code snippets
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// This is C++
DOM_Node aNode;
aNode = someDocument.createElement("ElementName");
DOM_Node docRootNode = someDoc.getDocumentElement();
docRootNode.AppendChild(aNode); |  |  |  |  |
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// This is Java
Node aNode;
aNode = someDocument.createElement("ElementName");
Node docRootNode = someDoc.getDocumentElement();
docRootNode.AppendChild(aNode); |  |  |  |  |
The Java and the C++ are identical on the surface, except for
the class names, and this similarity remains true for most DOM
code.
However, Java and C++ handle objects in somewhat different
ways, making it important to understand a little bit of what
is going on beneath the surface.
In Java, the variable aNode is an object reference ,
essentially a pointer. It is initially == null, and references
an object only after the assignment statement in the second
line of the code.
In C++ the variable aNode is, from the C++ language's
perspective, an actual live object. It is constructed when the
first line of the code executes, and DOM_Node::operator = ()
executes at the second line. The C++ class DOM_Node
essentially a form of a smart-pointer; it implements much of
the behavior of a Java Object Reference variable, and
delegates the DOM behaviors to an implementation class that
lives behind the scenes.
Key points to remember when using the C++ DOM classes:
- Create them as local variables, or as member variables of
some other class. Never "new" a DOM object into the heap or
make an ordinary C pointer variable to one, as this will
greatly confuse the automatic memory management.
- The "real" DOM objects - nodes, attributes, CData
sections, whatever, do live on the heap, are created with the
create... methods on class DOM_Document. DOM_Node and the
other DOM classes serve as reference variables to the
underlying heap objects.
- The visible DOM classes may be freely copied (assigned),
passed as parameters to functions, or returned by value from
functions.
- Memory management of the underlying DOM heap objects is
automatic, implemented by means of reference counting. So long
as some part of a document can be reached, directly or
indirectly, via reference variables that are still alive in
the application program, the corresponding document data will
stay alive in the heap. When all possible paths of access have
been closed off (all of the application's DOM objects have
gone out of scope) the heap data itself will be automatically
deleted.
- There are restrictions on the ability to subclass the DOM
classes.
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Class DOMString provides the mechanism for passing string
data to and from the DOM API. DOMString is not intended to be
a completely general string class, but rather to meet the
specific needs of the DOM API.
The design derives from two primary sources: from the DOM's
CharacterData interface and from class java.lang.string .
Main features are:
- It stores Unicode text.
- Automatic memory management, using reference counting.
- DOMStrings are mutable - characters can be inserted,
deleted or appended.
When a string is passed into a method of the DOM, when
setting the value of a Node, for example, the string is cloned
so that any subsequent alteration or reuse of the string by
the application will not alter the document contents.
Similarly, when strings from the document are returned to an
application via the DOM API, the string is cloned so that the
document can not be inadvertently altered by subsequent edits
to the string.
 | The ICU classes are a more general solution to UNICODE
character handling for C++ applications. ICU is an Open
Source Unicode library, available at the IBM
DeveloperWorks website. |
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The C++ DOM classes, DOM_Node, DOM_Attr, DOM_Document, etc.,
are not designed to be subclassed by an application
program.
As an alternative, the DOM_Node class provides a User Data
field for use by applications as a hook for extending nodes by
referencing additional data or objects. See the API
description for DOM_Node for details.
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