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Oracle® XML Developer's Kit Programmer's Guide 10g Release 2 (10.1.2) Part No. B14033-01 |
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# 23 XML Class Generator for C++

This chapter contains these topics:

- Accessing XML C++ Class Generator

- Using XML C++ Class Generator

- XML C++ Class Generator Usage

- Using the XML C++ Class Generator Examples

## Accessing XML C++ Class Generator

The XML C++ Class Generator is provided with the database and is also available for download from the OTN site `http://www.oracle.com/technology/tech/xml`.

## Using XML C++ Class Generator

The XML C++ Class Generator creates source files from an XML DTD or XML Schema. The Class Generator takes the Document Type Definition (DTD) or the XML Schema, and generates classes for each defined element. Those classes are then used in a C++ program to construct XML documents conforming to the DTD.

This is useful when an application wants to send an XML message to another application based on an agreed-upon DTD or XML Schema, or as the back end of a Web form to construct an XML document. Using these classes, C++ applications can construct, validate, and print XML documents that comply with the input.

The Class Generator works in conjunction with the Oracle XML Parser for C++, which parses the input and passes the parsed document to the class generator.

### External DTD Parsing

The XML C++ Class Generator can also parse an external DTD directly without requiring a complete (dummy) document by using the Oracle XML Parser for C++ routine `xmlparsedtd()`.

The provided command-line program `xmlcg` has a '-d' option that is used to parse external DTDs.

### Error Message Files

Error message files are provided in the mesg/ subdirectory. The messages files also exist in the `$ORACLE_HOME/xdk/mesg` directory. You may set the environment variable `ORA_XML_MESG` to point to the absolute path of the `mesg` subdirectory although this not required.

## XML C++ Class Generator Usage

The standalone class generator can be called as an executable by invoking `bin/xmlcg`.

1. The C++ command line class generator, `bin/xmlcg` is invoked in the following way:

       xmlcg [options] input_file

   where the options are described in Table 23-1:

   Table 23-1 Class Generator Options

   Option | Meaning
   --- | ---
   `-d` `name` | Input is an external DTD or a DTD file. Generates `name.cpp` and `name.h.`
   `-o` `directory` | Output directory for generated files (the default is the current directory).
   `-e` `encoding` | Default input file encoding.
   `-h` | Show this usage help.
   -v | Show the class generator version validator options.
   -s name | Input is an XML Schema file with the given name. Generates `name.cpp` and `name.h`.

   `input_file` name is the name of the parsed XML document with <!DOCTYPE> definitions, or parsed DTD, or an XML Schema document. The XML document must have an associated DTD.

   The DTD input to the XML C++ Class Generator is an XML document containing a DTD, or an external DTD. The document body itself is ignored; only the DTD is relevant, though the document must conform to the DTD.

2. If invalid options, or no input is provided, a usage message with the above information is output.

3. Two source files are output, a name.h header file and a C++ file, name.cpp. These are named after the DTD file.

4. The output files are typically used to generate XML documents.

Constructors are provided for each class (element) that allow an object to be created in the following two ways:

- Initially empty, then adding the children or data after the initial creation

- Created with the initial full set of children or initial data

A method is provided for `#PCDATA` (and Mixed) elements to set the data and, when appropriate, set an element's attributes.

### Input to the XML C++ Class Generator

The input is an XML document containing a DTD. The document body itself is ignored; only the DTD is relevant, though the dummy document must conform to the DTD. The underlying XML parser only accepts file names for the document and associated external entities.

## Using the XML C++ Class Generator Examples

Table 23-2 lists the demo XML C++ Class Generator files:

Table 23-2 XML C++ Class Generator Files

File Name | Description
--- | ---
`CG.cpp` | Sample program
`CG.xml` | XML file contains DTD and dummy document
`CG.dtd` | DTD file referenced by CG.xml
`Make.bat` on Windows NT `Makefile` on UNIX | Batch file (on Windows) or Make file (on UNIX) to generate classes and build the sample programs.
README | A readme file with these instructions

The `make.bat` batch file (on Windows NT) or `Makefile` (on UNIX) do the following:

- Generate classes based on `CG.xml` into Sample.h and Sample.cpp

- Compile the program `CG.cpp` (using `Sample.h`), and link this with the Sample object into an executable named `CG.exe` in the...`\bin` (or ...`/bin`) directory.

### XML C++ Class Generator Example 1: XML — Input File to Class Generator, CG.xml

This XML file, `CG.xml`, inputs XML C++ Class Generator. It references the DTD file, `CG.dtd`.

    <?xml version="1.0"?>
    <!DOCTYPE Sample SYSTEM "CG.dtd">
      <Sample>
        <B>Be!</B>
        <D attr="value"></D>
        <E>
          <F>Formula1</F>
          <F>Formula2</F>
        </E>
      </Sample>

### XML C++ Class Generator Example 2: DTD — Input File to Class Generator, CG.dtd

This DTD file, `CG.dtd` is referenced by the XML file `CG.xml`. `CG.xml` inputs XML C++ Class Generator.

    <!ELEMENT Sample (A | (B, (C | (D, E))) | F)>
    <!ELEMENT A (#PCDATA)>
    <!ELEMENT B (#PCDATA | F)*>
    <!ELEMENT C (#PCDATA)>
    <!ELEMENT D (#PCDATA)>
    <!ATTLIST D attr CDATA #REQUIRED>
    <!ELEMENT E (F, F)>
    <!ELEMENT F (#PCDATA)>

### XML C++ Class Generator Example 3: CG Sample Program

The CG sample program, `CG.cpp`, does the following:

1. Initializes the XML parser.

2. Loads the DTD (by parsing the DTD-containing file-- the dummy document part is ignored).

3. Creates some objects using the generated classes.

4. Invokes the validation function which verifies that the constructed classes match the DTD.

5. Writes the constructed document to `Sample.xml`.

    //////////////////////////////////////////////////////////////////////////////
    // NAME        CG.cpp
    // DESCRIPTION Demonstration program for C++ Class Generator usage
    //////////////////////////////////////////////////////////////////////////////

    #ifndef ORAXMLDOM_ORACLE
    # include <oraxmldom.h>
    #endif

    #include <fstream.h>

    #include "Sample.h"

    #define DTD_DOCUMENT "CG.xml"
    #define OUT_DOCUMENT Sample.xml"

    int main()
    {
        XMLParser parser;
        Document *doc;
        Sample   *samp;
        B        *b;
        D        *d;
        E        *e;
        F        *f1, *f2;
        fstream  *out;
        ub4       flags = XML_FLAG_VALIDATE;
        uword     ecode;

        // Initialize XML parser
        cout << "Initializing XML parser...\n";
        if (ecode = parser.xmlinit())
        {
            cout << "Failed to initialize parser, code " << ecode << "\n";
            return 1;
        }

        // Parse the document containing a DTD; parsing just a DTD is not
        // possible yet, so the file must contain a valid document (which
        // is parsed but we're ignoring).
        cout << "Loading DTD from " << DTD_DOCUMENT << "...\n";
        if (ecode = parser.xmlparse((oratext *) DTD_DOCUMENT, (oratext *)0, flags))
        {
            cout << "Failed to parse DTD document " << DTD_DOCUMENT <<
            ", code " << ecode << "\n";
            return 2;
        }

        // Fetch dummy document
        cout << "Fetching dummy document...\n";
        doc = parser.getDocument();

        // Create the constituent parts of a Sample
        cout << "Creating components...\n";
        b = new B(doc, (String) "Be there or be square");
        d = new D(doc, (String) "Dit dah");
        d->setattr((String) "attribute value");
        f1 = new F(doc, (String) "Formula1");
        f2 = new F(doc, (String) "Formula2");
        e = new E(doc, f1, f2);

        // Create the Sample
        cout << "Creating top-level element...\n";
        samp = new Sample(doc, b, d, e);

        // Validate the construct
        cout << "Validating...\n";
        if (ecode = parser.validate(samp))
        {
         cout << "Validation failed, code " << ecode << "\n";
         return 3;
        }

        // Write out doc
        cout << "Writing document to " << OUT_DOCUMENT << "\n";
        if (!(out = new fstream(OUT_DOCUMENT, ios::out)))
        {
          cout << "Failed to open output stream\n";
          return 4;
        }
        samp->print(out, 0);
        out->close();

        // Everything's OK
        cout << "Success.\n";

        // Shut down
        parser.xmlterm();
        return 0;
    }

    // end of CG.cpp