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| <title>Boost.Python - ObjectWrapper Concept</title> |
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| <h3><a href="../../../../index.htm"><img height="86" width="277" |
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| <td valign="top"> |
| <h1 align="center"><a href="../index.html">Boost.Python</a></h1> |
| |
| <h2 align="center">ObjectWrapper and TypeWrapper Concepts</h2> |
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| </tr> |
| </table> |
| <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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