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<div class="chapter">
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<div class="titlepage"><div><div><h1 class="title">
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<a name="concepts"></a>Chapter 1. Concepts</h1></div></div></div>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl class="toc">
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<dt><span class="section"><a href="concepts.html#concepts.callpolicies">CallPolicies</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="concepts.html#concepts.callpolicies.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="concepts.html#concepts.callpolicies.callpolicies_composition">CallPolicies
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Composition</a></span></dt>
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<dt><span class="section"><a href="concepts.html#concepts.callpolicies.concept_requirements">Concept
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Requirements</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="concepts/dereferenceable.html">Dereferenceable</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="concepts/dereferenceable.html#concepts.dereferenceable.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="concepts/dereferenceable.html#concepts.dereferenceable.concept_requirements">Concept
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Requirements</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="concepts/extractor.html">Extractor</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="concepts/extractor.html#concepts.extractor.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="concepts/extractor.html#concepts.extractor.concept_requirements">Concept Requirements</a></span></dt>
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<dt><span class="section"><a href="concepts/extractor.html#concepts.extractor.notes">Notes</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="concepts/holdergenerator.html">HolderGenerator</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="concepts/holdergenerator.html#concepts.holdergenerator.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="concepts/holdergenerator.html#concepts.holdergenerator.concept_requirements">Concept
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Requirements</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="concepts/resultconverter.html">ResultConverter</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="concepts/resultconverter.html#concepts.resultconverter.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="concepts/resultconverter.html#concepts.resultconverter.resultconverter_concept_requirem">ResultConverter
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Concept Requirements</a></span></dt>
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<dt><span class="section"><a href="concepts/resultconverter.html#concepts.resultconverter.resultconvertergenerator_concept">ResultConverterGenerator
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Concept Requirements</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="concepts/objectwrapper.html">ObjectWrapper</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="concepts/objectwrapper.html#concepts.objectwrapper.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="concepts/objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen">ObjectWrapper
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Concept Requirements</a></span></dt>
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<dt><span class="section"><a href="concepts/objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements">TypeWrapper
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Concept Requirements</a></span></dt>
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<dt><span class="section"><a href="concepts/objectwrapper.html#concepts.objectwrapper.caveat">Caveat</a></span></dt>
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</dl></dd>
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</dl>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="concepts.callpolicies"></a><a class="link" href="concepts.html#concepts.callpolicies" title="CallPolicies">CallPolicies</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="concepts.html#concepts.callpolicies.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="concepts.html#concepts.callpolicies.callpolicies_composition">CallPolicies
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Composition</a></span></dt>
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<dt><span class="section"><a href="concepts.html#concepts.callpolicies.concept_requirements">Concept
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Requirements</a></span></dt>
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="concepts.callpolicies.introduction"></a><a class="link" href="concepts.html#concepts.callpolicies.introduction" title="Introduction">Introduction</a>
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</h3></div></div></div>
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<p>
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Models of the CallPolicies concept are used to specialize the behavior
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of Python callable objects generated by Boost.Python to wrapped C++ objects
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like function and member function pointers, providing three behaviors:
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</p>
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<div class="orderedlist"><ol class="orderedlist" type="1">
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">precall</span></code> - Python argument
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tuple management before the wrapped object is invoked
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">result_converter</span></code> -
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C++ return value handling
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">postcall</span></code> - Python argument
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tuple and result management after the wrapped object is invoked
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">extract_return_type</span></code>
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- metafunction for extracting the return type from a given signature
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type sequence
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</li>
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</ol></div>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="concepts.callpolicies.callpolicies_composition"></a><a class="link" href="concepts.html#concepts.callpolicies.callpolicies_composition" title="CallPolicies Composition">CallPolicies
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Composition</a>
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</h3></div></div></div>
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<p>
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In order to allow the use of multiple models of CallPolicies in the same
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callable object, Boost.Python's CallPolicies class templates provide a
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chaining interface which allows them to be recursively composed. This interface
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takes the form of an optional template parameter, <code class="computeroutput"><span class="identifier">Base</span></code>,
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which defaults to <code class="computeroutput"><span class="identifier">default_call_policies</span></code>.
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By convention, the <code class="computeroutput"><span class="identifier">precall</span></code>
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function of the <code class="computeroutput"><span class="identifier">Base</span></code> is
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invoked after the <code class="computeroutput"><span class="identifier">precall</span></code>
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function supplied by the <code class="computeroutput"><span class="identifier">outer</span></code>
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template, and the <code class="computeroutput"><span class="identifier">postcall</span></code>
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function of the <code class="computeroutput"><span class="identifier">Base</span></code> is
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invoked before the <code class="computeroutput"><span class="identifier">postcall</span></code>
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function of the <code class="computeroutput"><span class="identifier">outer</span></code> template.
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If a <code class="computeroutput"><span class="identifier">result_converter</span></code> is
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supplied by the <code class="computeroutput"><span class="identifier">outer</span></code> template,
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it replaces any <code class="computeroutput"><span class="identifier">result_converter</span></code>
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supplied by the <code class="computeroutput"><span class="identifier">Base</span></code>. For
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an example, see <code class="computeroutput"><span class="identifier">return_internal_reference</span></code>.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="concepts.callpolicies.concept_requirements"></a><a class="link" href="concepts.html#concepts.callpolicies.concept_requirements" title="Concept Requirements">Concept
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Requirements</a>
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</h3></div></div></div>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col>
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<col>
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<col>
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</colgroup>
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<thead><tr>
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<th>
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<p>
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Expression
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</p>
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</th>
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<th>
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<p>
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Type
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</p>
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</th>
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<th>
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<p>
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Result/Semantics
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</p>
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</th>
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</tr></thead>
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<tbody>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">precall</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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convertible to <code class="computeroutput"><span class="keyword">bool</span></code>
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</p>
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</td>
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<td>
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<p>
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returns <code class="computeroutput"><span class="keyword">false</span></code> and
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<code class="computeroutput"><span class="identifier">PyErr_Occurred</span><span class="special">()</span> <span class="special">!=</span>
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<span class="number">0</span></code> upon failure, <code class="computeroutput"><span class="keyword">true</span></code> otherwise.
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">P</span><span class="special">::</span><span class="identifier">result_converter</span></code>
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</p>
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</td>
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<td>
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<p>
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A model of <code class="computeroutput"><span class="identifier">ResultConverterGenerator</span></code>.
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</p>
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</td>
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<td>
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<p>
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An MPL unary Metafunction Class used produce the "preliminary"
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result object.
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">postcall</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span>
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<span class="identifier">r</span><span class="special">)</span></code>
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</p>
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</td>
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<td>
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<p>
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convertible to <code class="computeroutput"><span class="identifier">PyObject</span><span class="special">*</span></code>
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</p>
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</td>
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<td>
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<p>
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<code class="computeroutput"><span class="number">0</span></code> and <code class="computeroutput"><span class="identifier">PyErr_Occurred</span><span class="special">()</span>
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<span class="special">!=</span> <span class="number">0</span></code>
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upon failure. Must "conserve references" even in the
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event of an exception. In other words, if <code class="computeroutput"><span class="identifier">r</span></code>
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is not returned, its reference count must be decremented; if
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another existing object is returned, its reference count must
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be incremented.
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</p>
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</td>
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</tr>
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<tr>
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<td>
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<p>
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<code class="computeroutput"><span class="identifier">P</span><span class="special">::</span><span class="identifier">extract_return_type</span></code>
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</p>
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</td>
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<td>
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<p>
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A model of Metafunction.
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</p>
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</td>
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<td>
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<p>
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An MPL unary Metafunction used extract the return type from a
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given signature. By default it is derived from <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">front</span></code>.
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</p>
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</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2015 David Abrahams, Stefan Seefeld<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>
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</p>
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</div></td>
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</tr></table>
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<hr>
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