| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
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| 2 | |
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| 3 | <!-- Copyright David Abrahams 2006. Distributed under the Boost --> |
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| 4 | <!-- Software License, Version 1.0. (See accompanying --> |
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| 5 | <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> |
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| 6 | <html> |
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| 7 | <head> |
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| 8 | <meta name="generator" content= |
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| 9 | "HTML Tidy for Windows (vers 1st August 2002), see www.w3.org"> |
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| 10 | <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> |
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| 11 | <link rel="stylesheet" type="text/css" href="../boost.css"> |
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| 12 | |
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| 13 | <title>Boost.Python - ObjectWrapper Concept</title> |
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| 14 | </head> |
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| 15 | |
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| 16 | <body link="#0000ff" vlink="#800080"> |
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| 17 | <table border="0" cellpadding="7" cellspacing="0" width="100%" summary= |
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| 18 | "header"> |
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| 19 | <tr> |
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| 20 | <td valign="top" width="300"> |
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| 21 | <h3><a href="../../../../index.htm"><img height="86" width="277" |
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| 22 | alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3> |
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| 23 | </td> |
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| 24 | |
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| 25 | <td valign="top"> |
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| 26 | <h1 align="center"><a href="../index.html">Boost.Python</a></h1> |
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| 27 | |
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| 28 | <h2 align="center">ObjectWrapper and TypeWrapper Concepts</h2> |
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| 29 | </td> |
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| 30 | </tr> |
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| 31 | </table> |
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| 32 | <hr> |
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| 33 | |
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| 34 | <dl class="page-index"> |
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| 35 | <dt><a href="#introduction">Introduction</a></dt> |
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| 36 | |
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| 37 | <dt><a href="#concept-requirements">Concept Requirements</a></dt> |
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| 38 | |
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| 39 | <dd> |
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| 40 | <dl class="page-index"> |
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| 41 | <dt><a href="#ObjectWrapper-concept">ObjectWrapper Concept</a></dt> |
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| 42 | |
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| 43 | <dt><a href="#TypeWrapper-concept">TypeWrapper Concept</a></dt> |
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| 44 | </dl> |
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| 45 | </dd> |
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| 46 | |
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| 47 | <dt><a href="#caveat">Caveat</a></dt> |
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| 48 | </dl> |
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| 49 | |
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| 50 | <h2><a name="introduction"></a>Introduction</h2> |
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| 51 | |
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| 52 | <p>This page defines two concepts used to describe classes which manage a |
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| 53 | Python objects, and which are intended to support usage with a |
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| 54 | Python-like syntax.</p> |
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| 55 | |
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| 56 | <h2><a name="concept-requirements"></a>Concept Requirements</h2> |
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| 57 | |
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| 58 | <h3><a name="ObjectWrapper-concept"></a>ObjectWrapper Concept</h3> |
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| 59 | Models of the ObjectWrapper concept have <a href= |
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| 60 | "object.html#object-spec">object</a> as a publicly-accessible base class, |
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| 61 | and are used to supply special construction behavior and/or additional |
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| 62 | convenient functionality through (often templated) member functions. |
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| 63 | Except when the return type <code>R</code> is itself an <a href= |
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| 64 | "#TypeWrapper-concept">TypeWrapper</a>, a member function invocation of |
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| 65 | the form |
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| 66 | <pre> |
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| 67 | x.<i>some_function</i>(<i>a<small>1</small>, a<small>2</small>,...a<small>n</small></i>) |
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| 68 | </pre> |
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| 69 | always has semantics equivalent to: |
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| 70 | <pre> |
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| 71 | <a href= |
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| 72 | "extract.html#extract-spec">extract</a><R>(x.attr("<i>some_function</i>")(<a |
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| 73 | href= |
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| 74 | "object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a |
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| 75 | href= |
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| 76 | "object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a |
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| 77 | href="object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>)))() |
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| 78 | </pre> |
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| 79 | When the <code>R</code> is an <a href= |
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| 80 | "#TypeWrapper-concept">TypeWrapper</a>, the result type may be |
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| 81 | constructed by taking direct posession of: |
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| 82 | <pre> |
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| 83 | x.attr("<i>some_function</i>")(<a href= |
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| 84 | "object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a |
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| 85 | href= |
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| 86 | "object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a |
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| 87 | href= |
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| 88 | "object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>)).ptr() |
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| 89 | </pre> |
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| 90 | [see <a href="#caveat">caveat</a> below] |
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| 91 | |
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| 92 | <h3><a name="TypeWrapper-concept"></a>TypeWrapper Concept</h3> |
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| 93 | TypeWrapper is a refinement of ObjectWrapper which is associated with a |
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| 94 | particular Python type <code>X</code>. For a given TypeWrapper |
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| 95 | <code>T</code>, a valid constructor expression |
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| 96 | <pre> |
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| 97 | T(<i>a<small>1</small>, a<small>2</small>,...a<small>n</small></i>) |
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| 98 | </pre> |
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| 99 | builds a new <code>T</code> object managing the result of invoking |
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| 100 | <code>X</code> with arguments corresponding to |
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| 101 | <pre> |
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| 102 | <a href= |
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| 103 | "object.html#object-spec-ctors">object</a>(<i>a<small>1</small></i>), <a |
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| 104 | href= |
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| 105 | "object.html#object-spec-ctors">object</a>(<i>a<small>2</small></i>),...<a |
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| 106 | href= |
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| 107 | "object.html#object-spec-ctors">object</a>(<i>a<small>n</small></i>) |
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| 108 | </pre> |
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| 109 | |
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| 110 | When used as arguments to wrapped C++ functions, or as the template |
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| 111 | parameter to <code><a |
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| 112 | href="extract.html#extract-spec">extract</a><></code>, only |
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| 113 | instances of the associated Python type will be considered a match. |
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| 114 | |
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| 115 | <h3><a name="caveat">Caveat</a></h3> |
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| 116 | The upshot of the special member function invocation rules when the |
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| 117 | return type is a TypeWrapper is that it is possible for the returned |
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| 118 | object to manage a Python object of an inappropriate type. This is not |
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| 119 | usually a serious problem; the worst-case result is that errors will be |
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| 120 | detected at runtime a little later than they might otherwise be. For an |
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| 121 | example of how this can occur, note that the <code><a href= |
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| 122 | "dict.html#dict-spec">dict</a></code> member function <code>items</code> |
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| 123 | returns an object of type <code><a href= |
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| 124 | "list.html#list-spec">list</a></code>. Now suppose the user defines this |
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| 125 | <code>dict</code> subclass in Python: |
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| 126 | <pre> |
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| 127 | >>> class mydict(dict): |
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| 128 | ... def items(self): |
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| 129 | ... return tuple(dict.items(self)) # return a tuple |
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| 130 | </pre> |
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| 131 | Since an instance of <code>mydict</code> is also an instance of |
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| 132 | <code>dict</code>, when used as an argument to a wrapped C++ function, |
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| 133 | <code><a href="dict.html#dict-spec">boost::python::dict</a></code> can |
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| 134 | accept objects of Python type <code>mydict</code>. Invoking |
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| 135 | <code>items()</code> on this object can result in an instance of <code><a |
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| 136 | href="list.html#list-spec">boost::python::list</a></code> which actually |
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| 137 | holds a Python tuple. Subsequent attempts to use list methods (e.g. |
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| 138 | <code>append</code>, or any other mutating operation) on this object will |
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| 139 | raise the same exception that would occur if you tried to do it from |
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| 140 | Python. |
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| 141 | <hr> |
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| 142 | |
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| 143 | <p>Revised |
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| 144 | <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan --> |
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| 145 | 13 November, 2002 |
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| 146 | <!--webbot bot="Timestamp" endspan i-checksum="39359" --> |
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| 147 | </p> |
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| 148 | |
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| 149 | <p><i>© Copyright <a href= |
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| 150 | "../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p> |
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| 151 | </body> |
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| 152 | </html> |
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| 153 | |
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