<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">


<!-- Copyright David Abrahams 2006. 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) -->
<html>
  <head>
    <meta name="generator" content="HTML Tidy, see www.w3.org">
    <meta http-equiv="Content-Type" content=
    "text/html; charset=iso-8859-1">
    <link rel="stylesheet" type="text/css" href="../boost.css">

    <title>Boost.Python - &lt;boost/python/instance_holder.hpp&gt;</title>
  </head>

<style type="text/css">
 p.c4 {font-style: italic}
 span.c3 {color: #ff0000}
 h2.c2 {text-align: center}
 h1.c1 {text-align: center}
</style>

  <body link="#0000ff" vlink="#800080">
    <table border="0" cellpadding="7" cellspacing="0" width="100%"
    summary="header">
      <tr>
        <td valign="top" width="300">
          <h3><a href="../../../../index.htm"><img height="86" width= 
          "277" alt="C++ Boost" src="../../../../boost.png" border=
          "0"></a></h3>

        <td valign="top">
          <h1 class="c1"><a href="../index.html">Boost.Python</a></h1>

          <h2 class="c2">Header &lt;boost/python/instance_holder.hpp&gt;</h2>
    </table>
    <hr>

    <h2>Contents</h2>

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

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

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

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

              <dt><a href="#instance_holder-spec-ctors">Class
              <code>instance_holder</code> destructor</a>

              <dt><a href="#instance_holder-spec-modifiers">Class
              <code>instance_holder</code> modifier functions</a>

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

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

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

    <p><code>&lt;boost/python/instance_holder.hpp&gt;</code> provides
    <code>class&nbsp;instance_holder</code>, the base class for types
    which hold C++ instances of wrapped classes.

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

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

    <p><code>instance_holder</code> is an abstract base class whose
    concrete derived classes hold C++ class instances within their
    Python object wrappers. To allow multiple inheritance in Python
    from C++ class wrappers, each such Python object contains a chain
    of <code>instance_holder</code>s. When an <code>__init__</code>
    function for a wrapped C++ class is invoked, a new
    <code>instance_holder</code> instance is created and installed in
    the Python object using its <code><a
    href="#instance_holder-spec-modifiers">install</a></code>()
    function. Each concrete class derived from
    <code>instance_holder</code> must provide a <code><a
    href="#instance_holder-spec-observers">holds</a>()</code>
    implementation which allows Boost.Python to query it for the
    type(s) it is holding. In order to support the held type's wrapped
    constructor(s), the class must also provide constructors that can
    accept an initial <code>PyObject*</code> argument referring to the
    owning Python object, and which forward the rest of their
    arguments to the constructor of the held type. The initial
    argument is needed to enable virtual function overriding in
    Python, and may be ignored, depending on the specific
    <code>instance_holder</code> subclass.

    <h4><a name="instance_holder-spec-synopsis"></a>Class instance_holder
    synopsis</h4>
<pre>
namespace boost { namespace python
{
  class instance_holder : <a href="../../../utility/utility.htm#Class_noncopyable">noncopyable</a>
  {
   public:
      // destructor
      virtual ~instance_holder();

      // instance_holder modifiers
      void install(PyObject* inst) throw();

      // instance_holder observers
      virtual void* holds(type_info) = 0;
  };
}}
</pre>

    <h4><a name="instance_holder-spec-ctors">Class <code>instance_holder</code>
    destructor</a></h4>
<pre>
virtual ~instance_holder();
</pre>

    <dl class="function-semantics">
      <dt><b>Effects:</b> destroys the object
    </dl>

    <h4><a name="instance_holder-spec-modifiers">Class
    <code>instance_holder</code> modifiers</a></h4>
<pre>
void install(PyObject* inst) throw();
</pre>

    <dl class="function-semantics">
      <dt><b>Requires:</b> <code>inst</code> is a Python instance of a
      wrapped C++ class type, or is a type derived from a wrapped C++
      class type.
      <dt><b>Effects:</b> installs the new instance at the head of the
      Python object's chain of held instances.
      <dt><b>Throws:</b> nothing
    </dl>

    <h4><a name="instance_holder-spec-observers">Class <code>instance_holder</code>
    observers</a></h4>
<pre>
virtual void* holds(type_info x) = 0;
</pre>

    <dl class="function-semantics">
      <dt><b>Returns:</b> A pointer to an object of the type described
      by <code>x</code> if <code>*this</code> contains such an object,
      0 otherwise.
    </dl>

    <h2><a name="examples"></a>Example</h2> 

The following is a simplified version of the instance holder template
used by Boost.Python to wrap classes held by smart pointers:
<pre>
template &lt;class SmartPtr, class Value&gt;
struct pointer_holder : instance_holder
{
   // construct from the SmartPtr type
   pointer_holder(SmartPtr p)
       :m_p(p)

   // Forwarding constructors for the held type
   pointer_holder(PyObject*)
       :m_p(new Value())
   {
   }

   template&lt;class A0&gt;
   pointer_holder(PyObject*,A0 a0)
       :m_p(new Value(a0))
   {
   }

   template&lt;class A0,class A1&gt;
   pointer_holder(PyObject*,A0 a0,A1 a1)
       :m_p(new Value(a0,a1))
   {
   }
   ...

 private: // required holder implementation
   void* holds(type_info dst_t)
   {
       // holds an instance of the SmartPtr type...
       if (dst_t == python::type_id&lt;SmartPtr&gt;())
           return &amp;this-&gt;m_p;

       // ...and an instance of the SmartPtr's element_type, if the
       // pointer is non-null
       return python::type_id&lt;Value&gt;() == dst_t ? &amp;*this-&gt;m_p : 0;
   }

 private: // data members
   SmartPtr m_p;
};
</pre>

    <p>Revised 
    <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
  13 November, 2002
  <!--webbot bot="Timestamp" endspan i-checksum="39359" -->


    <p class="c4">&copy; Copyright <a href= 
    "http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002. 

  </body>
</html>