Programming Guide

This page has sections on the following topics:

• SAX Programming Guide
• Constructing a parser
• Using the SAX API
• SAX2 Programming Guide
• Constructing an XML Reader
• Using the SAX2 API
• Supported Features
• DOM Programming Guide
• Comparision of Java and C++ DOM's
• Accessing the API from application code
• Class Names
• Objects and Memory Management
• DOMString
• Equality Testing
• Downcasting
• Subclassing

SAX1 Programming Guide

Constructing a parser

In order to use Xerces-C to parse XML files, you will need to create an instance of the SAXParser class. The example below shows the code you need in order to create an instance of SAXParser. The DocumentHandler and ErrorHandler instances required by the SAX API are provided using the HandlerBase class supplied with Xerces-C.

int main (int argc, char* args[]) {

    try {
        XMLPlatformUtils::Initialize();
    }
    catch (const XMLException& toCatch) {
        cout << "Error during initialization! :\n"
             << toCatch.getMessage() << "\n";
        return 1;
    }

    char* xmlFile = "x1.xml";
    SAXParser* parser = new SAXParser();
    parser->setDoValidation(true);    // optional.
	parser->setDoNamespaces(true);    // optional

    DocumentHandler* docHandler = new HandlerBase();
    ErrorHandler* errHandler = (ErrorHandler*) docHandler;
    parser->setDocumentHandler(docHandler);
    parser->setErrorHandler(errHandler);

    try {
        parser->parse(xmlFile);
    }
    catch (const XMLException& toCatch) {
        cout << "\nFile not found: '" << xmlFile << "'\n"
             << "Exception message is: \n"
             << toCatch.getMessage() << "\n" ;
        return -1;
    }
}

Using the SAX API

The SAX API for XML parsers was originally developed for Java. Please be aware that there is no standard SAX API for C++, and that use of the Xerces-C SAX API does not guarantee client code compatibility with other C++ XML parsers.

The SAX API presents a callback based API to the parser. An application that uses SAX 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 DocumentHandler which is called when XML constructs are recognized, and ErrorHandler which is called when an error occurs. The header files for the various SAX handler classes are in '<xerces-c1_4_0>/include/sax'

As a convenience, Xerces-C provides the class HandlerBase, which is a single class which is publicly derived from all the Handler classes. HandlerBase'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 HandlerBase 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 MySAXHandler.hpp:

#include <sax/HandlerBase.hpp>

class MySAXHandler : public HandlerBase {
public:
    void startElement(const XMLCh* const, AttributeList&);
    void fatalError(const SAXParseException&);
};

This is the implementation file MySAXHandler.cpp:

#include "MySAXHandler.hpp"
#include <iostream.h>

MySAXHandler::MySAXHandler()
{
}

MySAXHandler::startElement(const XMLCh* const name,
                           AttributeList& attributes)
{
    // transcode() is an user application defined function which
    // converts unicode strings to usual 'char *'. Look at
    // the sample program SAXCount for an example implementation.
    cout << "I saw element: " << transcode(name) << endl;
}

MySAXHandler::fatalError(const SAXParseException& exception)
{
    cout << "Fatal Error: " << transcode(exception.getMessage())
         << " at line: " << exception.getLineNumber()
         << endl;
}

The XMLCh and AttributeList 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.

SAX2 Programming Guide

Constructing an XML Reader

In order to use Xerces-C to parse XML files, you will need to create an instance of the SAX2XMLReader class. The example below shows the code you need in order to create an instance of SAX2XMLReader. The ContentHandler and ErrorHandler instances required by the SAX API are provided using the DefaultHandler class supplied with Xerces-C.

int main (int argc, char* args[]) {

    try {
        XMLPlatformUtils::Initialize();
    }
    catch (const XMLException& toCatch) {
        cout << "Error during initialization! :\n"
             << toCatch.getMessage() << "\n";
        return 1;
    }

char* xmlFile = "x1.xml";
SAX2XMLReader* parser = XMLReaderFactory::createXMLReader();
parser->setFeature(XMLString::transcode("http://xml.org/sax/
  features/validation", true)
 // optional
parser->setFeature(XMLString::transcode("http://xml.org/sax/
  features/namespaces", true)
 // optional

ContentHandler* contentHandler = new DefaultHandler();
ErrorHandler* errHandler = (ErrorHandler*) contentHandler;
parser->setContentHandler(docHandler);
parser->setErrorHandler(errHandler);

try {
        parser->parse(xmlFile);
    }
    catch (const XMLException& toCatch) {
        cout << "\nFile not found: '" << xmlFile << "'\n"
             << "Exception message is: \n"
             << toCatch.getMessage() << "\n" ;
        return -1;
    }
}

Using the SAX2 API

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:

#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:

#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.

Xerces SAX2 Supported Features

The behavior of the SAX2XMLReader is dependant on the values of the following features. All of the features below can be set using the SAX2XMLReader::setFeature(XMLCh*,bool) function. None of these features can be modified in the middle of a parse, or an exception will be thrown.

DOM Programming Guide

Java and C++ DOM comparisons

The C++ DOM API is very similar in design and use, to the Java DOM API bindings. As a consequence, conversion of existing Java code that makes use of the DOM to C++ is a straight forward process.

This section outlines the differences between Java and C++ bindings.

Accessing the API from application code

// C++
#include <dom/DOM.hpp>

// Java
import org.w3c.dom.*

The header file <dom/DOM.hpp> includes all the individual headers for the DOM API classes.

Class Names

The C++ class names are prefixed with "DOM_". The intent is to prevent conflicts between DOM class names and other names that may already be in use by an application or other libraries that a DOM based application must link with.

The use of C++ namespaces would also have solved this conflict problem, but for the fact that many compilers do not yet support them.

DOM_Document   myDocument;   // C++
DOM_Node       aNode;
DOM_Text       someText;

Document       myDocument;   // Java
Node           aNode;
Text           someText;

If you wish to use the Java class names in C++, then you need to typedef them in C++. This is not advisable for the general case - conflicts really do occur - but can be very useful when converting a body of existing Java code to C++.

typedef DOM_Document  Document;
typedef DOM_Node      Node;

Document   myDocument;        // Now C++ usage is
                              // indistinguishable from Java
Node       aNode;

Objects and Memory Management

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

// This is C++
DOM_Node       aNode;
aNode = someDocument.createElement("ElementName");
DOM_Node docRootNode = someDoc.getDocumentElement();
docRootNode.AppendChild(aNode);

// 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.

DOMString

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.

Equality Testing

The DOMString equality operators (and all of the rest of the DOM class conventions) are modeled after the Java equivalents. The equals() method compares the content of the string, while the == operator checks whether the string reference variables (the application program variables) refer to the same underlying string in memory. This is also true of DOM_Node, DOM_Element, etc., in that operator == tells whether the variables in the application are referring to the same actual node or not. It's all very Java-like

• bool operator == () is true if the DOMString variables refer to the same underlying storage.
• bool equals() is true if the strings contain the same characters.

Here is an example of how the equality operators work:

DOMString a = "Hello";
DOMString b = a;
DOMString c = a.clone();
if (b == a)           //  This is true
if (a == c)           //  This is false
if (a.equals(c))       //  This is true
b = b + " World";
if (b == a)           // Still true, and the string's
                      //    value is "Hello World"
if (a.equals(c))      // false.  a is "Hello World";
                      //    c is still "Hello".

Downcasting

Application code sometimes must cast an object reference from DOM_Node to one of the classes deriving from DOM_Node, DOM_Element, for example. The syntax for doing this in C++ is different from that in Java.

// This is C++
DOM_Node       aNode = someFunctionReturningNode();
DOM_Element    el = (Element &) aNode;

// This is Java
Node       aNode = someFunctionReturningNode();
Element    el = (Element) aNode;

The C++ cast is not type-safe; the Java cast is checked for compatible types at runtime. If necessary, a type-check can be made in C++ using the node type information:

// This is C++

DOM_Node       aNode = someFunctionReturningNode();
DOM_Element    el;    // by default, el will == null.

if (anode.getNodeType() == DOM_Node::ELEMENT_NODE)
   el = (Element &) aNode;
else
   // aNode does not refer to an element.
   // Do something to recover here.

Subclassing

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.

Copyright © 2000 The Apache Software Foundation. All Rights Reserved.