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          <h1 align="center"><a href="../index.html">Boost.Python</a></h1>

          <h2 align="center">Header &lt;wrapper.hpp&gt;</h2>
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    <h2>Contents</h2>

    <dl class="page-index">
      <dt><a href="#introduction">Introduction</a></dt>

      <dt><a href="#classes">Classes</a></dt>

      <dd>
        <dl class="page-index">
          <dt><a href="#override-spec">Class template
          <code>override</code></a></dt>

          <dd>
            <dl class="page-index">
              <dt><a href="#override-spec-synopsis">Class
              <code>override</code> synopsis</a></dt>

              <dt><a href="#override-spec-observers">Class
              <code>override</code> observer functions</a></dt>
            </dl>
          </dd>

          <dt><a href="#wrapper-spec">Class template
          <code>wrapper</code></a></dt>

          <dd>
            <dl class="page-index">
              <dt><a href="#wrapper-spec-synopsis">Class <code>wrapper</code>
              synopsis</a></dt>

              <dt><a href="#wrapper-spec-observers">Class
              <code>wrapper</code> observer functions</a></dt>
            </dl>
          </dd>
        </dl>
      </dd>

      <dt><a href="#examples">Example(s)</a></dt>
    </dl>
    <hr>

    <h2><a name="introduction"></a>Introduction</h2>

    <p>To wrap a class <code>T</code> such that its virtual functions can be
    "overridden in Python"&mdash;so that the corresponding method of a Python
    derived class will be called when the virtual function is invoked from
    C++&mdash;you must create a C++ wrapper class derived from ``T`` that
    overrides those virtual functions so that they call into Python. This
    header contains classes that can be used to make that job easier.</p>

    <h2><a name="classes"></a>Classes</h2>

    <h3><a name="override-spec"></a>Class <code>override</code></h3>

    <p>Encapsulates a Python override of a C++ virtual function. An
    <code>override</code> object either holds a callable Python object or
    <code>None</code>.</p>

    <h4><a name="override-spec-synopsis"></a>Class <code>override</code>
    synopsis</h4>
<pre>
namespace boost
{
  class override : object
  {
   public:
      <i>unspecified</i> operator() const;
      template &lt;class A0&gt;
      <i>unspecified</i> operator(A0) const;
      template &lt;class A0, class A1&gt;
      <i>unspecified</i> operator(A0, A1) const;
      ...
      template &lt;class A0, class A1, ...class A<i>n</i>&gt;
      <i>unspecified</i> operator(A0, A1, ...A<i>n</i>) const;
  };
};
</pre>

    <h4><a name="override-spec-observers"></a>Class <code>override</code>
    observer functions</h4>
<pre>
<i>unspecified</i> operator() const;
template &lt;class A0&gt;
<i>unspecified</i> operator(A0) const;
template &lt;class A0, class A1&gt;
<i>unspecified</i> operator(A0, A1) const;
...
template &lt;class A0, class A1, ...class A<i>n</i>&gt;
<i>unspecified</i> operator(A0, A1, ...A<i>n</i>) const;
</pre>

    <dl class="function-semantics">
      <dt><b>Effects:</b> If <code>*this</code> holds a callable Python
      object, it is invoked with the specified arguments in the manner
      specified <a href="callbacks.html">here</a>. Otherwise, throws <code><a
      href="errors.html#error_already_set-spec">error_already_set</a></code>
      .</dt>

      <dt><b>Returns:</b> An object of unspecified type that holds the Python
      result of the invocation and, when converted to a C++ type
      <code>R</code>, attempts to convert that result object to
      <code>R</code>. If that conversion fails, throws <code><a href=
      "errors.html#error_already_set-spec">error_already_set</a></code>
      .</dt>
    </dl>

    <h3><a name="wrapper-spec"></a>Class template <code>wrapper</code></h3>

    <p>Deriving your wrapper class from both ``T`` <i>and</i>
    ``wrapper&lt;T&gt; makes writing that derived class easier.</p>

    <h4><a name="wrapper-spec-synopsis"></a>Class template
    <code>wrapper</code> synopsis</h4>
<pre>
namespace boost
{
  class wrapper
  {
   protected:
      override get_override(char const* name) const;
  };
};
</pre>

    <h4><a name="wrapper-spec-observers"></a>Class <code>wrapper</code>
    observer functions</h4>
<pre>
override get_override(char const* name) const;
</pre>

    <dl class="function-semantics">
      <dt><b>Requires:</b> <code>name</code> is a <a href=
      "definitions.html#ntbs">ntbs</a>.</dt>

      <dt><b>Returns:</b> If <code>*this</code> is the C++ base class
      subobject of a Python derived class instance that overrides the named
      function, returns an <code>override</code> object that delegates to the
      Python override. Otherwise, returns an <code>override</code> object
      that holds <code>None</code>.</dt>
    </dl>

    <h2><a name="examples"></a>Example</h2>
<pre>
#include &lt;boost/python/module.hpp&gt;
#include &lt;boost/python/class.hpp&gt;
#include &lt;boost/python/wrapper.hpp&gt;
#include &lt;boost/python/call.hpp&gt;

using namespace boost::python;

// Class with one pure virtual function
struct P
{
    virtual ~P(){}
    virtual char const* f() = 0;
    char const* g() { return "P::g()"; }
};

struct PCallback : P, wrapper&lt;P&gt;
{
    char const* f()
    {
#if BOOST_WORKAROUND(BOOST_MSVC, &lt;= 1300) // Workaround for vc6/vc7
        return call&lt;char const*&gt;(this-&gt;get_override("f").ptr());
#else 
        return this-&gt;get_override("f")();
#endif 
    }
};

// Class with one non-pure virtual function
struct A
{
    virtual ~A(){}
    virtual char const* f() { return "A::f()"; }
};

struct ACallback :  A, wrapper&lt;A&gt;
{
    char const* f()
    {
        if (override f = this-&gt;get_override("f"))
#if BOOST_WORKAROUND(BOOST_MSVC, &lt;= 1300) // Workaround for vc6/vc7
            return call&lt;char const*&gt;(f.ptr());
#else 
            return f();
#endif 

        return A::f();
    }

    char const* default_f() { return this-&gt;A::f(); }
};

BOOST_PYTHON_MODULE_INIT(polymorphism)
{
    class_&lt;PCallback,boost::noncopyable&gt;("P")
        .def("f", pure_virtual(&amp;P::f))
        ;

    class_&lt;ACallback,boost::noncopyable&gt;("A")
        .def("f", &amp;A::f, &amp;ACallback::default_f)
        ;
}
</pre>

    <p>Revised 
    <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
     31 October, 2004 
    <!--webbot bot="Timestamp" endspan i-checksum="39359" -->


    <p><i>&copy; Copyright <a href="http://www.boost.org/people/dave_abrahams.htm">Dave
    Abrahams</a> 2004</i> Distributed under the Boost Software License,
  Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt)</p>