doc/program.xml

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<s1 title="Programming Guide">

  <p>This page has sections on the following topics:</p>
  <ul>
    <li><link anchor="SAXProgGuide">SAX Programming Guide</link></li>
    <ul>
      <li><link anchor="ConstructParser">Constructing a parser</link></li>
      <li><link anchor="UsingSAXAPI">Using the SAX API</link></li>
    </ul>
    <li><link anchor="DOMProgGuide">DOM Programming Guide</link></li>
    <ul>
      <li><link anchor="JAVAandCPP">Comparision of Java and C++ DOM's</link></li>
      <ul>
        <li><link anchor="AccessAPI">Accessing the API from application code</link></li>
        <li><link anchor="ClassNames">Class Names</link></li>
        <li><link anchor="ObjMemMgmt">Objects and Memory Management</link></li>
      </ul>
      <li><link anchor="DOMString">DOMString</link></li>
      <ul>
        <li><link anchor="EqualityTesting">Equality Testing</link></li>
      </ul>
      <li><link anchor="Downcasting">Downcasting</link></li>
      <li><link anchor="Subclassing">Subclassing</link></li>
    </ul>
  </ul>


  <anchor name="SAXProgGuide"/>
  <s2 title="SAX Programming Guide">

    <anchor name="ConstructParser"/>
    <s3 title="Constructing a parser">
      <p>In order to use &XercesCName; 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 &XercesCName;.</p>

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

    try {
        XMLPlatformUtils::Initialize();
    }
    catch (const XMLException&amp; toCatch) {
        cout &lt;&lt; "Error during initialization! :\n"
             &lt;&lt; toCatch.getMessage() &lt;&lt; "\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&amp; toCatch) {
        cout &lt;&lt; "\nFile not found: '" &lt;&lt; xmlFile &lt;&lt; "'\n"
             &lt;&lt; "Exception message is: \n"
             &lt;&lt; toCatch.getMessage() &lt;&lt; "\n" ;
        return -1;
    }
}</source>
    </s3>

    <anchor name="UsingSAXAPI"/>
    <s3 title="Using the SAX API">
      <p>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 &XercesCName; SAX API does not
        guarantee client code compatibility with other C++ XML
        parsers.</p>

      <p>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
        '&lt;&XercesCInstallDir;>/include/sax'</p>

      <p>As a convenience, &XercesCName; 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 &XercesCName; 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.</p>

      <p>This is the header file MySAXHandler.hpp:</p>
      <source>
#include &lt;sax/HandlerBase.hpp>

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

      <p>This is the implementation file MySAXHandler.cpp:</p>

      <source>
#include "MySAXHandler.hpp"
#include &lt;iostream.h>

MySAXHandler::MySAXHandler()
{
}

MySAXHandler::startElement(const XMLCh* const name,
                           AttributeList&amp; 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 &lt;&lt; "I saw element: " &lt;&lt; transcode(name) &lt;&lt; endl;
}

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

      <p>The XMLCh and AttributeList types are supplied by
        &XercesCName; and are documented in the include
        files. Examples of their usage appear in the source code to
        the sample applications.</p>
    </s3>
  </s2>

  <anchor name="DOMProgGuide"/>
  <s2 title="DOM Programming Guide">

    <anchor name="JAVAandCPP"/>
    <s3 title="Java and C++ DOM comparisons">
      <p>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.
      </p>
      <p>
        This section outlines the differences between Java and C++ bindings.
      </p>
    </s3>

    <anchor name="AccessAPI"/>
    <s3 title="Accessing the API from application code">

<source>
// C++
#include &lt;dom/DOM.hpp></source>

<source>
// Java
import org.w3c.dom.*</source>

      <p>The header file &lt;dom/DOM.hpp&gt; includes all the
        individual headers for the DOM API classes. </p>

    </s3>

    <anchor name="ClassNames"/>
    <s3 title="Class Names">
      <p>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.</p>

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

<source>
DOM_Document   myDocument;   // C++
DOM_Node       aNode;
DOM_Text       someText;</source>

<source>
Document       myDocument;   // Java
Node           aNode;
Text           someText;</source>

      <p>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++.</p>

<source>
typedef DOM_Document  Document;
typedef DOM_Node      Node;

Document   myDocument;        // Now C++ usage is
                              // indistinguishable from Java
Node       aNode;</source>
    </s3>


    <anchor name="ObjMemMgmt"/>
    <s3 title="Objects and Memory Management">
      <p>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.</p>

      <p>Consider the following code snippets </p>

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

<source>
// This is Java
Node       aNode;
aNode = someDocument.createElement("ElementName");
Node docRootNode = someDoc.getDocumentElement();
docRootNode.AppendChild(aNode);</source>

      <p>The Java and the C++ are identical on the surface, except for
        the class names, and this similarity remains true for most DOM
        code. </p>

      <p>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.</p>

      <p>In Java, the variable <code>aNode</code> 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.</p>

      <p>In C++ the variable <code>aNode</code> 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. </p>

      <p>Key points to remember when using the C++ DOM classes:</p>

      <ul>
        <li>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. </li>

        <li>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.</li>

        <li>The visible DOM classes may be freely copied (assigned),
        passed as parameters to functions, or returned by value from
        functions.</li>

        <li>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. </li>

        <li>There are restrictions on the ability to subclass the DOM
        classes. </li>

      </ul>

    </s3>

    <anchor name="DOMString"/>
    <s3 title="DOMString">
      <p>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.</p>

      <p>The design derives from two primary sources: from the DOM's
        CharacterData interface and from class java.lang.string</p>

      <p>Main features are:</p>

      <ul>
        <li>It stores Unicode text.</li>

        <li>Automatic memory management, using reference counting.</li>

        <li>DOMStrings are mutable - characters can be inserted,
        deleted or appended.</li>

      </ul>
      <p></p>

      <p>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.</p>

      <note>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 <jump
        href="http://www.software.ibm.com/developerworks/opensource/icu/index.html">IBM
        DeveloperWorks website</jump>.</note>

    </s3>

    <anchor name="EqualityTesting"/>
    <s3 title="Equality Testing">
      <p>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 </p>

      <ul>
        <li>bool operator == () is true if the DOMString variables
        refer to the same underlying storage. </li>

        <li>bool equals() is true if the strings contain the same
        characters. </li>

      </ul>
      <p>Here is an example of how the equality operators work: </p>
      <source>
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".</source>
    </s3>

    <anchor name="Downcasting"/>
    <s3 title="Downcasting">
      <p>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.</p>

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

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

      <p>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: </p>

<source>
// This is C++

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

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

    </s3>

    <anchor name="Subclassing"/>
    <s3 title="Subclassing">
      <p>The C++ DOM classes, DOM_Node, DOM_Attr, DOM_Document, etc.,
        are not designed to be subclassed by an application
        program. </p>

      <p>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.</p>
    </s3>

  </s2>

</s1>