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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<h1 class="c1"><a href="../index.html">Boost.Python</a></h1>
<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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