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156 lines
6.1 KiB
HTML
156 lines
6.1 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta name="generator" content=
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"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python - ObjectWrapper Concept</title>
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</head>
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<body link="#0000ff" vlink="#800080">
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
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"header">
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<tr>
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<td valign="top" width="300">
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<h3><a href="../../../../index.htm"><img height="86" width="277"
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alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
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</td>
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<td valign="top">
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<h1 align="center">Boost.Python</h1>
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<h2 align="center">ObjectWrapper and TypeWrapper Concepts</h2>
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</td>
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</tr>
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</table>
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<hr>
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<dl class="page-index">
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<dt><a href="#introduction">Introduction</a></dt>
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<dt><a href="#concept-requirements">Concept Requirements</a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#ObjectWrapper-concept">ObjectWrapper Concept</a></dt>
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<dt><a href="#TypeWrapper-concept">TypeWrapper Concept</a></dt>
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</dl>
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</dd>
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<dt><a href="#caveat">Caveat</a></dt>
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</dl>
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<h2><a name="introduction"></a>Introduction</h2>
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<p>This page defines two concepts used to describe classes which manage a
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Python objects, and which are intended to support usage with a
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Python-like syntax.</p>
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<h2><a name="concept-requirements"></a>Concept Requirements</h2>
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<h3><a name="ObjectWrapper-concept"></a>ObjectWrapper Concept</h3>
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Models of the ObjectWrapper concept have <a href=
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"object.html#object-spec">object</a> as a publicly-accessible base class,
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and are used to supply special construction behavior and/or additional
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convenient functionality through (often templated) member functions.
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Except when the return type <code>R</code> is itself an <a href=
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"#TypeWrapper-concept">TypeWrapper</a>, a member function invocation of
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the form
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<pre>
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x.<i>some_function</i>(<i>a<small>1</small>, a<small>2</small>,...a<small>n</small></i>)
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</pre>
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always has semantics equivalent to:
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<pre>
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<a href=
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"extract.html#extract-spec">extract</a><R>(x.attr("<i>some_function</i>")(<a
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href=
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"object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a
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href=
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"object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a
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href="object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>)))()
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</pre>
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When the <code>R</code> is an <a href=
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"#TypeWrapper-concept">TypeWrapper</a>, the result type may be
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constructed by taking direct posession of:
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<pre>
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x.attr("<i>some_function</i>")(<a href=
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"object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a
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href=
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"object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a
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href=
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"object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>)).ptr()
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</pre>
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[see <a href="#caveat">caveat</a> below]
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<h3><a name="TypeWrapper-concept"></a>TypeWrapper Concept</h3>
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TypeWrapper is a refinement of ObjectWrapper which is associated with a
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particular Python type <code>X</code>. For a given TypeWrapper
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<code>T</code>, a valid constructor expression
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<pre>
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T(<i>a<small>1</small>, a<small>2</small>,...a<small>n</small></i>)
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</pre>
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builds a new <code>T</code> object managing the result of invoking
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<code>X</code> with arguments corresponding to
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<pre>
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<a href=
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"object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a
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href=
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"object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a
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href=
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"object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>)
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</pre>
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When used as arguments to wrapped C++ functions, or as the template
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parameter to <code><a
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href="extract.html#extract-spec">extract</a><></code>, only
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instances of the associated Python type will be considered a match.
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<h3><a name="caveat">Caveat</a></h3>
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The upshot of the special member function invocation rules when the
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return type is a TypeWrapper is that it is possible for the returned
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object to manage a Python object of an inappropriate type. This is not
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usually a serious problem; the worst-case result is that errors will be
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detected at runtime a little later than they might otherwise be. For an
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example of how this can occur, note that the <code><a href=
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"dict.html#dict-spec">dict</a></code> member function <code>items</code>
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returns an object of type <code><a href=
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"list.html#list-spec">list</a></code>. Now suppose the user defines this
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<code>dict</code> subclass in Python:
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<pre>
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>>> class mydict(dict):
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... def items(self):
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... return tuple(dict.items(self)) # return a tuple
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</pre>
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Since an instance of <code>mydict</code> is also an instance of
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<code>dict</code>, when used as an argument to a wrapped C++ function,
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<code><a href="dict.html#dict-spec">boost::python::dict</a></code> can
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accept objects of Python type <code>mydict</code>. Invoking
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<code>items()</code> on this object can result in an instance of <code><a
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href="list.html#list-spec">boost::python::list</a></code> which actually
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holds a Python tuple. Subsequent attempts to use list methods (e.g.
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<code>append</code>, or any other mutating operation) on this object will
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raise the same exception that would occur if you tried to do it from
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Python.
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<hr>
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
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30 Sept, 2002 <!--webbot bot="Timestamp" endspan i-checksum="39359" -->
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</p>
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<p><i>© Copyright <a href=
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"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002. All Rights
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Reserved.</i></p>
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<p>Permission to copy, use, modify, sell and distribute this software is
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granted provided this copyright notice appears in all copies. This
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software is provided "as is" without express or implied warranty, and
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with no claim as to its suitability for any purpose.</p>
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</body>
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</html>
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