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<title>Boost.Python - <boost/python/extract.hpp></title>
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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 <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>
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