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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"
             << DOMString(toCatch.getMessage()) << "\n";
        return 1;
    }

    char* xmlFile = "x1.xml";
    SAX2XMLReader* parser = XMLReaderFactory::createXMLReader();
    parser->setFeature(XMLUni::fgSAX2CoreValidation, true)   // optional
    parser->setFeature(XMLUni::fgSAX2CoreNameSpaces, true)   // optional

    DefaultHandler* defaultHandler = new DefaultHandler();
    parser->setContentHandler(defaultHandler);
    parser->setErrorHandler(defaultHandler);

    try {
        parser->parse(xmlFile);
    }
    catch (const XMLException& toCatch) {
        cout << "Exception message is: \n"
             << DOMString(toCatch.getMessage()) << "\n" ;
        return -1;
    }
    catch (const SAXParseException& toCatch) {
        cout << "Exception message is: \n"
             << DOMString(toCatch.getMessage()) << "\n" ;
        return -1;
    }
    catch (...) {
        cout << "Unexpected Exception \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_7_0>/include/xercesc/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 <xercesc/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 function SAX2XMLReader::setFeature(cons XMLCh* const, const bool). And can be queried using the function bool SAX2XMLReader::getFeature(const XMLCh* const).

None of these features can be modified in the middle of a parse, or an exception will be thrown.

http://xml.org/sax/features/namespaces 
true:  Perform Namespace processing (default) 
false:  Optionally do not perform Namespace processing 

http://xml.org/sax/features/namespace-prefixes 
true:  Report the original prefixed names and attributes used for Namespace declarations  
false:  Do not report attributes used for Namespace declarations, and optionally do not report original prefixed names. (default) 

http://xml.org/sax/features/validation 
true:  Report all validation errors. (default) 
false:  Do not report validation errors.  

http://apache.org/xml/features/validation/dynamic 
true:  The parser will validate the document only if a grammar is specified. (http://xml.org/sax/features/validation must be true) 
false:  Validation is determined by the state of the http://xml.org/sax/features/validation feature (default) 

http://apache.org/xml/features/validation/schema 
true:  Enable the parser's schema support. (default)  
false:  Disable the parser's schema support.  

http://apache.org/xml/features/validation/schema-full-checking 
true:  Enable full schema constraint checking, including checking which may be time-consuming or memory intensive. Currently, particle unique attribution constraint checking and particle derivation restriction checking are controlled by this option.  
false:  Disable full schema constraint checking (default).  

http://apache.org/xml/features/validation/reuse-grammar 
true:  The parser will reuse grammar information from previous parses in subsequent parses.  
false:  The parser will not reuse any grammar information. (default) 

http://apache.org/xml/features/validation/reuse-validator (deprecated)
Please use http://apache.org/xml/features/validation/reuse-grammar  
true:  The parser will reuse grammar information from previous parses in subsequent parses.  
false:  The parser will not reuse any grammar information. (default) 

Xerces SAX2 Supported Properties
 

The behavior of the SAX2XMLReader is dependant on the values of the following properties. All of the properties below can be set using the function SAX2XMLReader::setProperty(const XMLCh* const, void*). It takes a void pointer as the property value. Application is required to initialize this void pointer to a correct type. Please check the column "Value Type" below to learn exactly what type of property value each property expects for processing. Passing a void pointer that was initialized with a wrong type will lead to unexpected result. If the same property is set more than once, the last one takes effect.

Property values can be queried using the function void* SAX2XMLReader::getFeature(const XMLCh* const). The parser owns the returned pointer, and the memory allocated for the returned pointer will be destroyed when the parser is deleted. To ensure accessibility of the returned information after the parser is deleted, callers need to copy and store the returned information somewhere else. Since the returned pointer is a generic void pointer, check the column "Value Type" below to learn exactly what type of object each property returns for replication.

None of these properties can be modified in the middle of a parse, or an exception will be thrown.

http://apache.org/xml/properties/schema/external-schemaLocation 
Description  The XML Schema Recommendation explicitly states that the inclusion of schemaLocation/ noNamespaceSchemaLocation attributes in the instance document is only a hint; it does not mandate that these attributes must be used to locate schemas. Similar situation happens to <import> element in schema documents. This property allows the user to specify a list of schemas to use. If the targetNamespace of a schema specified using this method matches the targetNamespace of a schema occurring in the instance document in schemaLocation attribute, or if the targetNamespace matches the namespace attribute of <import> element, the schema specified by the user using this property will be used (i.e., the schemaLocation attribute in the instance document or on the <import> element will be effectively ignored). 
Value  The syntax is the same as for schemaLocation attributes in instance documents: e.g, "http://www.example.com file_name.xsd". The user can specify more than one XML Schema in the list. 
Value Type  XMLCh*  

http://apache.org/xml/properties/schema/external-noNamespaceSchemaLocation 
Description  The XML Schema Recommendation explicitly states that the inclusion of schemaLocation/ noNamespaceSchemaLocation attributes in the instance document is only a hint; it does not mandate that these attributes must be used to locate schemas. This property allows the user to specify the no target namespace XML Schema Location externally. If specified, the instance document's noNamespaceSchemaLocation attribute will be effectively ignored. 
Value  The syntax is the same as for the noNamespaceSchemaLocation attribute that may occur in an instance document: e.g."file_name.xsd". 
Value Type  XMLCh*  


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