boost_1_45_0/libs/python/doc/v2/extract.html - nest-learning-thermostat/5.0/boost - Git at Google

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           <h3><a href="../../../../index.htm"><img height="86" width="277"
           alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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         <td valign="top">
           <h1 align="center"><a href="../index.html">Boost.Python</a></h1>

           <h2 align="center">Header &lt;boost/python/extract.hpp&gt;</h2>
         </td>
       </tr>
     </table>
     <hr>

     <h2>Contents</h2>

     <dl class="page-index">
       <dt><a href="#introduction">Introduction</a></dt>

       <dt><a href="#classes">Classes</a></dt>

       <dd>
         <dl class="page-index">
           <dt><a href="#extract-spec">Class <code>extract</code></a></dt>

           <dd>
             <dl class="page-index">
               <dt><a href="#extract-spec-synopsis">Class <code>extract</code>
               synopsis</a></dt>

               <dt><a href="#extract-spec-ctors">Class <code>extract</code>
               constructors and destructor</a></dt>

               <dt><a href="#extract-spec-observers">Class
               <code>extract</code> observer functions</a></dt>
             </dl>
           </dd>
         </dl>
       </dd>


       <dt><a href="#examples">Example</a></dt>
     </dl>
     <hr>

     <h2><a name="introduction"></a>Introduction</h2>

     <p>Exposes a mechanism for extracting C++ object values from
     generalized Python objects. Note that
     <code>extract&lt;</code>...<code>&gt;</code> can also be used to
     &quot;downcast&quot; an <a
     href="object.html#object-spec">object</a> to some specific <a
     href="ObjectWrapper.html#ObjectWrapper-concept">ObjectWrapper</a>. Because
     invoking a mutable python type with an argument of the same type
     (e.g. <code>list([1,2])</code> typically makes a <em>copy</em> of
     the argument object, this may be the only way to access the <a
     href="ObjectWrapper.html#ObjectWrapper-concept">ObjectWrapper</a>'s
     interface on the original object.

     <h2><a name="classes"></a>Classes</h2>

     <h3><a name="extract-spec"></a>Class template <code>extract</code></h3>

     <p><code>extract&lt;T&gt;</code> can be used to extract a value of
     an arbitrary C++ type from an instance of <code><a
     href="object.html#object-spec">object</a></code>. Two usages are supported:
 <ol>
 <li><b><code>extract&lt;T&gt;(o)</code></b> is a temporary object
 which is implicitly convertible to <code>T</code> (explicit conversion
 is also available through the object's function-call
 operator). However, if no conversion is available which can convert
 <code>o</code> to an object of type <code>T</code>, a Python
 <code>TypeError</code> exception will be <a
 href="definitions.html#raise">raised</a>.

 <li><b><code>extract&lt;T&gt; x(o);</code></b> constructs an extractor
 whose <code>check()</code> member function can be used to ask whether
 a conversion is available without causing an exception to be thrown.
 </ol>

     <h4><a name="extract-spec-synopsis"></a>Class template <code>extract</code>
     synopsis</h4>
 <pre>
 namespace boost { namespace python
 {
   template &lt;class T&gt;
   struct extract
   {
       typedef <i>unspecified</i> result_type;

       extract(PyObject*);
       extract(object const&amp;);

       result_type operator()() const;
       operator result_type() const;

       bool check() const;
   };
 }}
 </pre>

     <h4><a name="extract-spec-ctors"></a>Class <code>extract</code>
     constructors and destructor</h4>
 <pre>
 extract(PyObject* p);
 extract(object const&amp;);
 </pre>

     <dl class="function-semantics">
       <dt><b>Requires:</b> The first form requires that <code>p</code> is non-null.</dt>

       <dt><b>Effects:</b>Stores a pointer to the Python object managed
       by its constructor argument. In particular, the reference
       count of the object is not incremented. The onus is on the user
       to be sure it is not destroyed before the extractor's conversion
       function is called.</dt>
     </dl>

     <h4><a name="extract-spec-observers"></a>Class <code>extract</code>
     observer functions</h4>
 <pre>
 result_type operator()() const;
 operator result_type() const;
 </pre>

     <dl class="function-semantics">
       <dt><b>Effects:</b> Converts the stored pointer to
       <code>result_type</code>, which is either <code>T</code> or
       <code>T const&amp;</code>.
       </dt>

       <dt><b>Returns:</b> An object of <code>result_type</code>
       corresponding to the one referenced by the stored pointer.</dt>

       <dt><b>Throws:</b> <code><a
       href="errors.html#error_already_set-spec">error_already_set</a></code>
       and sets a <code>TypeError</code> if no such conversion is
       available. May also emit other unspecified exceptions thrown by
       the converter which is actually used.</dt>
     </dl>

 <pre>
 bool check() const;
 </pre>

     <dl class="function-semantics">

       <dt><b>Postconditions:</b> None. In particular, note that a
       return value of <code>true</code> does not preclude an exception
       being thrown from <code>operator result_type()</code> or
       <code>operator()()</code>.</dt>

       <dt><b>Returns:</b> <code>false</code> <i>only</i> if no conversion from the
       stored pointer to <code>T</code> is available.</dt>

     </dl>


     <h2><a name="examples"></a>Examples</h2>

 <pre>
 #include &lt;cstdio&gt;
 using namespace boost::python;
 int Print(str s)
 {
    // extract a C string from the Python string object
    char const* c_str = extract&lt;char const*&gt;(s);

    // Print it using printf
    std::printf(&quot;%s\n&quot;, c_str);

    // Get the Python string's length and convert it to an int
    return extract&lt;int&gt;(s.attr(&quot;__len__&quot;)())
 }
 </pre>

 The following example shows how extract can be used along with
 <code><a
 href="class.html#class_-spec">class_</a>&lt;</code>...<code>&gt;</code>
 to create and access an instance of a wrapped C++ class.

 <pre>
 struct X
 {
    X(int x) : v(x) {}
    int value() { return v; }
  private:
    int v;
 };

 BOOST_PYTHON_MODULE(extract_ext)
 {
     object x_class(
        class_&lt;X&gt;(&quot;X&quot;, init&lt;int&gt;())
           .def(&quot;value&quot;, &amp;X::value))
           ;

     // Instantiate an X object through the Python interface.
     // Its lifetime is now managed by x_obj.
     object x_obj = x_class(3);

     // Get a reference to the C++ object out of the Python object
     X&amp; x = extract&lt;X&amp;&gt;(x_obj);
     assert(x.value() == 3);
 }
 </pre>
     <p>Revised 15 November, 2002</p>

     <p><i>&copy; Copyright <a href=
     "http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
   </body>
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	<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

	<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
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	<title>Boost.Python - <boost/python/extract.hpp></title>
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	<body>
	<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
	"header">
	<tr>
	<td valign="top" width="300">
	<h3><a href="../../../../index.htm"><img height="86" width="277"
	alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
	</td>

	<td valign="top">
	<h1 align="center"><a href="../index.html">Boost.Python</a></h1>

	<h2 align="center">Header <boost/python/extract.hpp></h2>
	</td>
	</tr>
	</table>
	<hr>

	<h2>Contents</h2>

	<dl class="page-index">
	<dt><a href="#introduction">Introduction</a></dt>

	<dt><a href="#classes">Classes</a></dt>

	<dd>
	<dl class="page-index">
	<dt><a href="#extract-spec">Class <code>extract</code></a></dt>

	<dd>
	<dl class="page-index">
	<dt><a href="#extract-spec-synopsis">Class <code>extract</code>
	synopsis</a></dt>

	<dt><a href="#extract-spec-ctors">Class <code>extract</code>
	constructors and destructor</a></dt>

	<dt><a href="#extract-spec-observers">Class
	<code>extract</code> observer functions</a></dt>
	</dl>
	</dd>
	</dl>
	</dd>


	<dt><a href="#examples">Example</a></dt>
	</dl>
	<hr>

	<h2><a name="introduction"></a>Introduction</h2>

	<p>Exposes a mechanism for extracting C++ object values from
	generalized Python objects. Note that
	<code>extract<</code>...<code>></code> can also be used to
	"downcast" an <a
	href="object.html#object-spec">object</a> to some specific <a
	href="ObjectWrapper.html#ObjectWrapper-concept">ObjectWrapper</a>. Because
	invoking a mutable python type with an argument of the same type
	(e.g. <code>list([1,2])</code> typically makes a <em>copy</em> of
	the argument object, this may be the only way to access the <a
	href="ObjectWrapper.html#ObjectWrapper-concept">ObjectWrapper</a>'s
	interface on the original object.

	<h2><a name="classes"></a>Classes</h2>

	<h3><a name="extract-spec"></a>Class template <code>extract</code></h3>

	<p><code>extract<T></code> can be used to extract a value of
	an arbitrary C++ type from an instance of <code><a
	href="object.html#object-spec">object</a></code>. Two usages are supported:
	<ol>
	<li><b><code>extract<T>(o)</code></b> is a temporary object
	which is implicitly convertible to <code>T</code> (explicit conversion
	is also available through the object's function-call
	operator). However, if no conversion is available which can convert
	<code>o</code> to an object of type <code>T</code>, a Python
	<code>TypeError</code> exception will be <a
	href="definitions.html#raise">raised</a>.

	<li><b><code>extract<T> x(o);</code></b> constructs an extractor
	whose <code>check()</code> member function can be used to ask whether
	a conversion is available without causing an exception to be thrown.
	</ol>

	<h4><a name="extract-spec-synopsis"></a>Class template <code>extract</code>
	synopsis</h4>
	<pre>
	namespace boost { namespace python
	{
	template <class T>
	struct extract
	{
	typedef <i>unspecified</i> result_type;

	extract(PyObject*);
	extract(object const&);

	result_type operator()() const;
	operator result_type() const;

	bool check() const;
	};
	}}
	</pre>

	<h4><a name="extract-spec-ctors"></a>Class <code>extract</code>
	constructors and destructor</h4>
	<pre>
	extract(PyObject* p);
	extract(object const&);
	</pre>

	<dl class="function-semantics">
	<dt><b>Requires:</b> The first form requires that <code>p</code> is non-null.</dt>

	<dt><b>Effects:</b>Stores a pointer to the Python object managed
	by its constructor argument. In particular, the reference
	count of the object is not incremented. The onus is on the user
	to be sure it is not destroyed before the extractor's conversion
	function is called.</dt>
	</dl>

	<h4><a name="extract-spec-observers"></a>Class <code>extract</code>
	observer functions</h4>
	<pre>
	result_type operator()() const;
	operator result_type() const;
	</pre>

	<dl class="function-semantics">
	<dt><b>Effects:</b> Converts the stored pointer to
	<code>result_type</code>, which is either <code>T</code> or
	<code>T const&</code>.
	</dt>

	<dt><b>Returns:</b> An object of <code>result_type</code>
	corresponding to the one referenced by the stored pointer.</dt>

	<dt><b>Throws:</b> <code><a
	href="errors.html#error_already_set-spec">error_already_set</a></code>
	and sets a <code>TypeError</code> if no such conversion is
	available. May also emit other unspecified exceptions thrown by
	the converter which is actually used.</dt>
	</dl>

	<pre>
	bool check() const;
	</pre>

	<dl class="function-semantics">

	<dt><b>Postconditions:</b> None. In particular, note that a
	return value of <code>true</code> does not preclude an exception
	being thrown from <code>operator result_type()</code> or
	<code>operator()()</code>.</dt>

	<dt><b>Returns:</b> <code>false</code> <i>only</i> if no conversion from the
	stored pointer to <code>T</code> is available.</dt>

	</dl>


	<h2><a name="examples"></a>Examples</h2>

	<pre>
	#include <cstdio>
	using namespace boost::python;
	int Print(str s)
	{
	// extract a C string from the Python string object
	char const* c_str = extract<char const*>(s);

	// Print it using printf
	std::printf("%s\n", c_str);

	// Get the Python string's length and convert it to an int
	return extract<int>(s.attr("__len__")())
	}
	</pre>

	The following example shows how extract can be used along with
	<code><a
	href="class.html#class_-spec">class_</a><</code>...<code>></code>
	to create and access an instance of a wrapped C++ class.

	<pre>
	struct X
	{
	X(int x) : v(x) {}
	int value() { return v; }
	private:
	int v;
	};

	BOOST_PYTHON_MODULE(extract_ext)
	{
	object x_class(
	class_<X>("X", init<int>())
	.def("value", &X::value))
	;

	// Instantiate an X object through the Python interface.
	// Its lifetime is now managed by x_obj.
	object x_obj = x_class(3);

	// Get a reference to the C++ object out of the Python object
	X& x = extract<X&>(x_obj);
	assert(x.value() == 3);
	}
	</pre>
	<p>Revised 15 November, 2002</p>

	<p><i>© Copyright <a href=
	"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
	</body>
	</html>