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

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	<h2 class="c2">Header <boost/python/instance_holder.hpp></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><boost/python/instance_holder.hpp></code> provides
	<code>class 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 <class SmartPtr, class Value>
	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<class A0>
	pointer_holder(PyObject*,A0 a0)
	:m_p(new Value(a0))
	{
	}

	template<class A0,class A1>
	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<SmartPtr>())
	return &this->m_p;

	// ...and an instance of the SmartPtr's element_type, if the
	// pointer is non-null
	return python::type_id<Value>() == dst_t ? &*this->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">© Copyright <a href=
	"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.

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