1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>ObjectWrapper</title> 5<link rel="stylesheet" href="../../boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Boost.Python Reference Manual"> 8<link rel="up" href="../concepts.html" title="Chapter 1. Concepts"> 9<link rel="prev" href="resultconverter.html" title="ResultConverter"> 10<link rel="next" href="../high_level_components.html" title="Chapter 2. High Level Components"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="" width="" height="" src="../../images/boost.png"></td></tr></table> 14<hr> 15<div class="spirit-nav"> 16<a accesskey="p" href="resultconverter.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../concepts.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../high_level_components.html"><img src="../../images/next.png" alt="Next"></a> 17</div> 18<div class="section"> 19<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 20<a name="concepts.objectwrapper"></a><a class="link" href="objectwrapper.html" title="ObjectWrapper">ObjectWrapper</a> 21</h2></div></div></div> 22<div class="toc"><dl class="toc"> 23<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.introduction">Introduction</a></span></dt> 24<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen">ObjectWrapper 25 Concept Requirements</a></span></dt> 26<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements">TypeWrapper 27 Concept Requirements</a></span></dt> 28<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.caveat">Caveat</a></span></dt> 29</dl></div> 30<div class="section"> 31<div class="titlepage"><div><div><h3 class="title"> 32<a name="concepts.objectwrapper.introduction"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.introduction" title="Introduction">Introduction</a> 33</h3></div></div></div> 34<p> 35 This page defines two concepts used to describe classes which manage a 36 Python objects, and which are intended to support usage with a Python-like 37 syntax. 38 </p> 39</div> 40<div class="section"> 41<div class="titlepage"><div><div><h3 class="title"> 42<a name="concepts.objectwrapper.objectwrapper_concept_requiremen"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen" title="ObjectWrapper Concept Requirements">ObjectWrapper 43 Concept Requirements</a> 44</h3></div></div></div> 45<p> 46 Models of the ObjectWrapper concept have <a class="link" href="../object_wrappers/boost_python_object_hpp.html#object_wrappers.boost_python_object_hpp.class_object" title="Class object">object</a> 47 as a publicly-accessible base class, and are used to supply special construction 48 behavior and/or additional convenient functionality through (often templated) 49 member functions. Except when the return type R is itself an <a class="link" href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements" title="TypeWrapper Concept Requirements">TypeWrapper</a>, 50 a member function invocation of the form 51 </p> 52<pre class="programlisting"><span class="identifier">x</span><span class="special">.</span><span class="identifier">some_function</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,...</span><span class="identifier">an</span><span class="special">)</span></pre> 53<p> 54 always has semantics equivalent to: 55 </p> 56<pre class="programlisting"><span class="identifier">extract</span><span class="special"><</span><span class="identifier">R</span><span class="special">>(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"some_function"</span><span class="special">)(</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">a1</span><span class="special">),</span> <span class="identifier">object</span><span class="special">(</span><span class="identifier">a2</span><span class="special">),...</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">an</span><span class="special">)))()</span></pre> 57<p> 58 (see <a class="link" href="objectwrapper.html#concepts.objectwrapper.caveat" title="Caveat">caveat</a> below). 59 </p> 60</div> 61<div class="section"> 62<div class="titlepage"><div><div><h3 class="title"> 63<a name="concepts.objectwrapper.typewrapper_concept_requirements"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements" title="TypeWrapper Concept Requirements">TypeWrapper 64 Concept Requirements</a> 65</h3></div></div></div> 66<p> 67 TypeWrapper is a refinement of <a class="link" href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen" title="ObjectWrapper Concept Requirements">ObjectWrapper</a> 68 which is associated with a particular Python type <code class="computeroutput"><span class="identifier">X</span></code>. 69 For a given TypeWrapper <code class="computeroutput"><span class="identifier">T</span></code>, 70 a valid constructor expression 71 </p> 72<pre class="programlisting"><span class="identifier">T</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,...</span><span class="identifier">an</span><span class="special">)</span></pre> 73<p> 74 builds a new T object managing the result of invoking X with arguments 75 corresponding to 76 </p> 77<pre class="programlisting"><span class="identifier">object</span><span class="special">(</span><span class="identifier">a1</span><span class="special">),</span> <span class="identifier">object</span><span class="special">(</span><span class="identifier">a2</span><span class="special">),...</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">an</span><span class="special">)</span></pre> 78<p> 79 . When used as arguments to wrapped C++ functions, or as the template parameter 80 to <a class="link" href="../to_from_python_type_conversion.html#to_from_python_type_conversion.boost_python_extract_hpp.class_template_extract" title="Class template extract">extract<></a>, 81 only instances of the associated Python type will be considered a match. 82 </p> 83</div> 84<div class="section"> 85<div class="titlepage"><div><div><h3 class="title"> 86<a name="concepts.objectwrapper.caveat"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.caveat" title="Caveat">Caveat</a> 87</h3></div></div></div> 88<p> 89 The upshot of the special member function invocation rules when the return 90 type is a TypeWrapper is that it is possible for the returned object to 91 manage a Python object of an inappropriate type. This is not usually a 92 serious problem; the worst-case result is that errors will be detected 93 at runtime a little later than they might otherwise be. For an example 94 of how this can occur, note that the <a class="link" href="../object_wrappers.html#object_wrappers.boost_python_dict_hpp.class_dict" title="Class dict">dict</a> 95 member function <code class="computeroutput"><span class="identifier">items</span></code> returns 96 an object of type <a class="link" href="../object_wrappers/boost_python_list_hpp.html#object_wrappers.boost_python_list_hpp.class_list" title="Class list">list</a>. 97 Now suppose the user defines this <code class="computeroutput"><span class="identifier">dict</span></code> 98 subclass in Python: 99 </p> 100<pre class="programlisting"><span class="special">>>></span> <span class="keyword">class</span> <span class="identifier">mydict</span><span class="special">(</span><span class="identifier">dict</span><span class="special">):</span> 101<span class="special">...</span> <span class="identifier">def</span> <span class="identifier">items</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span> 102<span class="special">...</span> <span class="keyword">return</span> <span class="identifier">tuple</span><span class="special">(</span><span class="identifier">dict</span><span class="special">.</span><span class="identifier">items</span><span class="special">(</span><span class="identifier">self</span><span class="special">))</span> <span class="special">#</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">tuple</span> 103</pre> 104<p> 105 Since an instance of <code class="computeroutput"><span class="identifier">mydict</span></code> 106 is also an instance of <code class="computeroutput"><span class="identifier">dict</span></code>, 107 when used as an argument to a wrapped C++ function, <a class="link" href="../object_wrappers.html#object_wrappers.boost_python_dict_hpp.class_dict" title="Class dict">boost::python::dict</a> 108 can accept objects of Python type <code class="computeroutput"><span class="identifier">mydict</span></code>. 109 Invoking <code class="computeroutput"><span class="identifier">items</span><span class="special">()</span></code> 110 on this object can result in an instance of <a class="link" href="../object_wrappers/boost_python_list_hpp.html#object_wrappers.boost_python_list_hpp.class_list" title="Class list">boost::python::list</a> 111 which actually holds a Python <code class="computeroutput"><span class="identifier">tuple</span></code>. 112 Subsequent attempts to use <code class="computeroutput"><span class="identifier">list</span></code> 113 methods (e.g. <code class="computeroutput"><span class="identifier">append</span></code>, or 114 any other mutating operation) on this object will raise the same exception 115 that would occur if you tried to do it from Python. 116 </p> 117</div> 118</div> 119<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 120<td align="left"></td> 121<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2015 David Abrahams, Stefan Seefeld<p> 122 Distributed under the Boost Software License, Version 1.0. (See accompanying 123 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a> 124 </p> 125</div></td> 126</tr></table> 127<hr> 128<div class="spirit-nav"> 129<a accesskey="p" href="resultconverter.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../concepts.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../high_level_components.html"><img src="../../images/next.png" alt="Next"></a> 130</div> 131</body> 132</html> 133
