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<title>Boost.Python - ObjectWrapper Concept</title>
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<h3><a href="../../../../index.htm"><img height="86" width="277"
alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h2 align="center">ObjectWrapper and TypeWrapper Concepts</h2>
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<hr>
<dl class="page-index">
<dt><a href="#introduction">Introduction</a></dt>
<dt><a href="#concept-requirements">Concept Requirements</a></dt>
<dd>
<dl class="page-index">
<dt><a href="#ObjectWrapper-concept">ObjectWrapper Concept</a></dt>
<dt><a href="#TypeWrapper-concept">TypeWrapper Concept</a></dt>
</dl>
</dd>
<dt><a href="#caveat">Caveat</a></dt>
</dl>
<h2><a name="introduction"></a>Introduction</h2>
<p>This page defines two concepts used to describe classes which manage a
Python objects, and which are intended to support usage with a
Python-like syntax.</p>
<h2><a name="concept-requirements"></a>Concept Requirements</h2>
<h3><a name="ObjectWrapper-concept"></a>ObjectWrapper Concept</h3>
Models of the ObjectWrapper concept have <a href=
"object.html#object-spec">object</a> as a publicly-accessible base class,
and are used to supply special construction behavior and/or additional
convenient functionality through (often templated) member functions.
Except when the return type <code>R</code> is itself an <a href=
"#TypeWrapper-concept">TypeWrapper</a>, a member function invocation of
the form
<pre>
x.<i>some_function</i>(<i>a<small>1</small>, a<small>2</small>,...a<small>n</small></i>)
</pre>
always has semantics equivalent to:
<pre>
<a href=
"extract.html#extract-spec">extract</a><R>(x.attr("<i>some_function</i>")(<a
href=
"object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a
href=
"object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a
href="object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>)))()
</pre>
When the <code>R</code> is an <a href=
"#TypeWrapper-concept">TypeWrapper</a>, the result type may be
constructed by taking direct posession of:
<pre>
x.attr("<i>some_function</i>")(<a href=
"object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a
href=
"object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a
href=
"object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>)).ptr()
</pre>
[see <a href="#caveat">caveat</a> below]
<h3><a name="TypeWrapper-concept"></a>TypeWrapper Concept</h3>
TypeWrapper is a refinement of ObjectWrapper which is associated with a
particular Python type <code>X</code>. For a given TypeWrapper
<code>T</code>, a valid constructor expression
<pre>
T(<i>a<small>1</small>, a<small>2</small>,...a<small>n</small></i>)
</pre>
builds a new <code>T</code> object managing the result of invoking
<code>X</code> with arguments corresponding to
<pre>
<a href=
"object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a
href=
"object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a
href=
"object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>)
</pre>
When used as arguments to wrapped C++ functions, or as the template
parameter to <code><a
href="extract.html#extract-spec">extract</a><></code>, only
instances of the associated Python type will be considered a match.
<h3><a name="caveat">Caveat</a></h3>
The upshot of the special member function invocation rules when the
return type is a TypeWrapper is that it is possible for the returned
object to manage a Python object of an inappropriate type. This is not
usually a serious problem; the worst-case result is that errors will be
detected at runtime a little later than they might otherwise be. For an
example of how this can occur, note that the <code><a href=
"dict.html#dict-spec">dict</a></code> member function <code>items</code>
returns an object of type <code><a href=
"list.html#list-spec">list</a></code>. Now suppose the user defines this
<code>dict</code> subclass in Python:
<pre>
>>> class mydict(dict):
... def items(self):
... return tuple(dict.items(self)) # return a tuple
</pre>
Since an instance of <code>mydict</code> is also an instance of
<code>dict</code>, when used as an argument to a wrapped C++ function,
<code><a href="dict.html#dict-spec">boost::python::dict</a></code> can
accept objects of Python type <code>mydict</code>. Invoking
<code>items()</code> on this object can result in an instance of <code><a
href="list.html#list-spec">boost::python::list</a></code> which actually
holds a Python tuple. Subsequent attempts to use list methods (e.g.
<code>append</code>, or any other mutating operation) on this object will
raise the same exception that would occur if you tried to do it from
Python.
<hr>
<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>
<p><i>© Copyright <a href=
"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
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