- Introduction
- Implementing an extension function
- Installing an extension function
- Using an extension function

| Introduction |

At times, you may want to call your own custom C functions from a stylesheet. For these situations, Xalan-C++ supports the creation and use of extension functions. Xalan-C++ also provides a library of extension functions for your use.

You can think of extension functions as extending the core library of functions that XPath provides. Like the XPath functions, an extension function returns an XObject, which may contain a value of any of the five XSLT data types: node-set, result-tree-fragment, string, boolean, or number.

You can send arguments to an extension function in the form of XPath expressions, literals (for string, boolean, and number), the values returned by other functions, and XSL variables or parameters set to any of the preceding.

For an example that implements, installs, and uses three extension functions, see the External Functions sample.

| Xalan-C++ does not support extension elements.

| Implementing an extension function |

Like the standard XPath functions, the functions you create derive from the Function base class. Set up your extension function class as follows:

1. The body of a function is the execute() method. Use the appropriate XObjectFactory method -- createNumber() in the example below -- to create an XObject corresponding to the XSLT data type the function returns.
2. Implement a clone() method to enable Xalan to create and maintain a copy of the extension function.
3. (Optional) As Xalan does for the XPath functions, you may want to prevent the compiler from generating an assignment or equality operator for this function.

These features all appear in the following example.

    // Base header file.  Must be first.
    #include <xalanc/Include/PlatformDefinitions.hpp>

    #include <cmath>
    #include <ctime>

    #include <xercesc/util/PlatformUtils.hpp>
    #include <xalanc/XalanTransformer/XalanTransformer.hpp>
    #include <xalanc/XPath/XObjectFactory.hpp>

    XALAN_CPP_NAMESPACE_USE

    // This class defines a function that will return the square root
    // of its argument.
    class FunctionSquareRoot : public Function
    {
    public:

      /**
       * Execute an XPath function object.  The function must return a valid
       * XObject.
       *
       * @param executionContext executing context
       * @param context          current context node
       * @param opPos            current op position
       * @param args             vector of pointers to XObject arguments
       * @return                 pointer to the result XObject
       */
      virtual XObjectPtr
      execute(
          XPathExecutionContext& executionContext,
          XalanNode* /* context */,
          const XObjectPtr arg,
         	const Locator*	/* locator */) const
      {
        if (args.size() != 1)
        {
          executionContext.error("The square-root() function takes one argument!",
                                  context);
        }
        assert(args[0] != 0);
        // Use the XObjectFactory createNumber() method to create an XObject
        // corresponding to the XSLT number data type.
        return executionContext.getXObjectFactory().createNumber(
                                                            sqrt(args[0]->num()));
      }

      /**
       * Implement clone() so Xalan can copy the square-root function into
       * its own function table.
       *
       * @return pointer to the new object
       */
    // For compilers that do not support covariant return types,
    // clone() must be declared to return the base type.
    #if defined(XALAN_NO_COVARIANT_RETURN_TYPE)
      virtual Function*
    #else
      virtual FunctionSquareRoot*
    #endif
      clone() const
      {
        return new FunctionSquareRoot(*this);
      }

    private:
      // The assignment and equality operators are not implemented...
      FunctionSquareRoot&
      operator=(const FunctionSquareRoot&);
      bool
      operator==(const FunctionSquareRoot&) const;
    }

| Installing an extension function |

XalanTransformer provides methods for installing and uninstalling external functions:

- installExternalFunction() makes the function available in the current instance of XalanTransformer. Use uninstallExternalFunction() to remove the function.
- installExternalFunctionGlobal() makes the function available globally. Use uninstallExternalFunctionGlobal() to remove the function. The global install and uninstall operations are not thread-safe. However, all global functions should be thread-safe, because multiple threads could call a particular function instance at the same time.

These methods include arguments for the namespace, the function name, and the function implementation.

When you install an extension function, the function inhabits the namespace you designate. For information about XML namespaces, see Namespaces in XML.

The following code fragment installs locally the square root function defined above, and binds it to the extension-function name "square-root" in the namespace "http://MyExternalFunction.mycompany.org" so it can be accessed from stylesheets. Keep in mind that the function name does not need to be the same as the name of the function class, and that a function name may be used more than once provided that each function with that name is installed in a different namespace.

    #include <xalanc/Include/PlatformDefinitions.hpp>
    #include <xercesc/util/PlatformUtils.hpp>
    #include <xalanc/XalanTransformer/XalanTransformer.hpp>
    // You have created a header file for FunctionSquareRoot.
    #include <MyFunctions/FunctionSquareRoot.hpp>
    // The namespace...
    const XalanDOMString
        theNamespace("http://MyExternalFunction.mycompany.org");

    theXalanTransformer.installExternalFunction(theNamespace,
                                                XalanDOMString("square-root"),
                                                FunctionSquareRoot());

For an example that installs three functions, see the External Functions sample.

| Using an extension function |

To use the extension function in a stylesheet, you must do the following:

1. Declare the extension function namespace.
   `xmlns:prefix=URI`
   The prefix identifies the namespace, and URI matches the namespace specified when the function is installed.
   By default, namespace declarations are included in the transformation output. To exclude namespaces from the output, use
   `exclude-result-prefixes="prefix-1 prefix-2 ..."`
   in the stylesheet element or
   `xsl:exclude-result-prefixes="prefix-1 prefix-2 ..."`
   in a literal result element or extension element.
2. Call the extension function in the same manner you would call an XPath function. The function name you use in the stylesheet is a Qualified Name (QName) made up of the prefix you declared in step 1 and the function name you specified when you installed the function.
   You can use XPath expressions, literals (for string, boolean, and number), and values returned by other functions to specify function arguments.

Suppose, for example, you are working with XML documents containing area elements like `<area value="397"/>`, where the value attribute identifies the area of a square.

The following stylesheet declares the square-root function namespace (the prefix is up to you), instructs the processor not to copy the namespace declaration to the result tree, and uses the square-root function to return the square root of //area/@value:

    <?xml version="1.0"?>
    <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
      version="1.0"
    	xmlns:external="http://ExternalFunction.xalan-c.xml.apache.org"
      exclude-result-prefixes="external">

        <xsl:template match="//area">
        <out>
          The area of the square is
          <xsl:value-of select="@value"/> square units.
          The length of each side is
          <xsl:value-of select="external:square-root(@value)"/> units
        </out>

This stylesheet converts `<area value="397"/>` into the following output:

    <out>
      The area of the square is
      397 square units.
      The length of each side is
      19.9249 units.
    </out>

For a slightly more complex variation on this example, see the External Functions sample.

| Passing Nodes to a function |

Please keep in mind that all LocationPath expressions return a node-set, even if the expression only returns a single attribute or a text node (node-sets with one member). You can use the XSLT string() function to convert a node-set value to string, and the number() function to convert a node-set value to number (a double).

If you pass a node-set to an extension function, be sure to set up the function to process a node-set.

Suppose, for example, you have a ProcessNodes function class that uses
`const NodeRefListBase& theNodeList = args[0]->nodeset();`
in the execute() method to get a reference to the node-set.

Assuming you install the function as "ProcessNodes" and use the "node-ext" prefix in a stylesheet to refer to the ProcessNodes function namespace, any of the following function calls are syntactically possible:

`<!--Process the current node-->`
`<xsl:variable name="success" select="node-ext:ProcessNodes(.)"/>`

`<!--Process all nodes in current context-->`
`<xsl:variable name="success" select="node-ext:ProcessNodes(*)"/>`

`<!-- Process all nodes -->`
`<xsl:variable name="success" select="node-ext:ProcessNodes(/*)"/>`

`<!--Process the foo/baz nodes in current context -->`
`<xsl:variable name="success" select="node-ext:ProcessNodes(foo/baz)"/>`

`<!--Process the/foo/baz and /bar/saz nodes -->`
`<xsl:variable name="success" select="node-ext:ProcessNodes(/foo/baz | /bar/saz)"/>`

The NodeRefListBase is in fact a list of references into the XML document, so keep in mind that getNextSibling(), for example, gets you the next sibling in the document, which may not be the next Node in the node-set.

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