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    <title>Boost.Python - &lt;boost/python/errors.hpp&gt;</title>
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          <h1 align="center"><a href="../index.html">Boost.Python</a></h1>

          <h2 align="center">Header &lt;boost/python/errors.hpp&gt;</h2>
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    <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="#error_already_set-spec">Class
          <code>error_already_set</code></a></dt>

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

      <dt><a href="#functions">Functions</a></dt>

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

          <dt><a href="#expect_non_null-spec">expect_non_null</a></dt>

          <dt><a href=
          "#throw_error_already_set-spec">throw_error_already_set</a></dt>
        </dl>
      </dd>

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

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

    <p><code>&lt;boost/python/errors.hpp&gt;</code> provides types and
    functions for managing and translating between Python and C++ exceptions.
    This is relatively low-level functionality that is mostly used internally
    by Boost.Python. Users should seldom need it.</p>

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

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

    <p><code>error_already_set</code> is an exception type which can be
    thrown to indicate that a Python error has occurred. If thrown, the
    precondition is that <a href=
    "http://www.python.org/doc/2.2/api/exceptionHandling.html#l2h-71">PyErr_Occurred()</a>
    returns a value convertible to <code>true</code>. Portable code shouldn't
    throw this exception type directly, but should instead use <code><a href=
    "#throw_error_already_set-spec">throw_error_already_set</a>()</code>,
    below.</p>

    <h4><a name="error_already_set-spec-synopsis"></a>Class error_already_set
    synopsis</h4>
<pre>
namespace boost { namespace python
{
    class error_already_set {};
}}
</pre>

    <h2><a name="functions"></a>Functions</h2>
<pre>
<a name=
"handle_exception-spec">template &lt;class T&gt; bool handle_exception</a>(T f) throw();

void handle_exception() throw();
</pre>

    <dl class="handle_exception-semantics">
      <dt><b>Requires:</b> The first form requires that the expression
      <code><a href=
      "../../../../doc/html/functionN.html">function0</a>&lt;void&gt;(f)</code>
      is valid. The second form requires that a C++ exception is currently
      being handled (see section 15.1 in the C++ standard).</dt>

      <dt><b>Effects:</b> The first form calls <code>f()</code> inside a
      <code>try</code> block which first attempts to use all registered <a
      href="exception_translator.html">exception translators</a>. If none of
      those translates the exception, the <code>catch</code> clauses then set
      an appropriate Python exception for the C++ exception caught, returning
      <code>true</code> if an exception was thrown, <code>false</code>
      otherwise. The second form passes a function which rethrows the
      exception currently being handled to the first form.</dt>

      <dt><b>Postconditions:</b> No exception is being handled</dt>

      <dt><b>Throws:</b> nothing</dt>

      <dt><b>Rationale:</b> At inter-language boundaries it is important to
      ensure that no C++ exceptions escape, since the calling language
      usually doesn't have the equipment necessary to properly unwind the
      stack. Use <code>handle_exception</code> to manage exception
      translation whenever your C++ code is called directly from the Python
      API. This is done for you automatically by the usual function wrapping
      facilities: <code><a href=
      "make_function.html#make_function-spec">make_function</a>()</code>,
      <code><a href=
      "make_function.html#make_constructor-spec">make_constructor</a>()</code>,
      <code><a href="def.html#class_-spec-modifiers">def</a>()</code> and <code><a href=
      "class.html#def-spec">class_::def</a>()</code>. The second form can be
      more convenient to use (see the <a href="#examples">example</a> below),
      but various compilers have problems when exceptions are rethrown from
      within an enclosing <code>try</code> block.</dt>
    </dl>
<pre>
<a name=
"expect_non_null-spec">template &lt;class T&gt; T* expect_non_null(T* x);</a>
</pre>

    <dl class="expect_non_null-semantics">
      <dt><b>Returns:</b> <code>x</code></dt>

      <dt><b>Throws:</b> <code><a href=
      "#error_already_set-spec">error_already_set</a>()</code> iff <code>x ==
      0</code>.</dt>

      <dt><b>Rationale:</b> Simplifies error-handling when calling functions
      in the <a href="http://www.python.org/doc/2.2/api/api.html">Python/C
      API</a> which return 0 on error.</dt>
    </dl>
<pre>
<a name="throw_error_already_set-spec">void throw_error_already_set();</a>
</pre>

    <dl class="throw_error_already_set-semantics">
      <dt><b>Effects:</b> <code>throw&nbsp;<a href=
      "#error_already_set-spec">error_already_set</a>();</code></dt>
    </dl>

    <dl>
      <dt><b>Rationale:</b> Many platforms and compilers are not able to
      consistently catch exceptions thrown across shared library boundaries.
      Using this function from the Boost.Python library ensures that the
      appropriate <code>catch</code> block in <code><a href=
      "#handle_exception-spec">handle_exception</a>()</code> can catch the
      exception.</dt>
    </dl>

    <h2><a name="examples"></a>Examples</h2>
<pre>
#include &lt;string&gt;
#include &lt;boost/python/errors.hpp&gt;
#include &lt;boost/python/object.hpp&gt;
#include &lt;boost/python/handle.hpp&gt;

// Returns a std::string which has the same value as obj's "__name__"
// attribute.
std::string get_name(boost::python::object obj)
{
   // throws if there's no __name__ attribute
   PyObject* p = boost::python::expect_non_null(
      PyObject_GetAttrString(obj.ptr(), "__name__"));

   char const* s = PyString_AsString(p);
   if (s != 0) 
        Py_DECREF(p);

   // throws if it's not a Python string
   std::string result(
      boost::python::expect_non_null(
         PyString_AsString(p)));

   Py_DECREF(p); // Done with p
   
   return result;
}

//
// Demonstrate form 1 of handle_exception
//

// Place into result a Python Int object whose value is 1 if a and b have
// identical "__name__" attributes, 0 otherwise.
void same_name_impl(PyObject*&amp; result, boost::python::object a, boost::python::object b)
{
   result = PyInt_FromLong(
      get_name(a) == get_name(a2));
}

object borrowed_object(PyObject* p)
{
   return boost::python::object(
        boost::python::handle&lt;&gt;(
             boost::python::borrowed(a1)));
}

// This is an example Python 'C' API interface function
extern "C" PyObject*
same_name(PyObject* args, PyObject* keywords)
{
   PyObject* a1;
   PyObject* a2;
   PyObject* result = 0;

   if (!PyArg_ParseTuple(args, const_cast&lt;char*&gt;("OO"), &amp;a1, &amp;a2))
      return 0;
   
   // Use boost::bind to make an object compatible with
   // boost::Function0&lt;void&gt;
   if (boost::python::handle_exception(
         boost::bind&lt;void&gt;(same_name_impl, boost::ref(result), borrowed_object(a1), borrowed_object(a2))))
   {
      // an exception was thrown; the Python error was set by
      // handle_exception()
      return 0;
   }

   return result;
}

//
// Demonstrate form 2 of handle_exception. Not well-supported by all
// compilers.
//
extern "C" PyObject*
same_name2(PyObject* args, PyObject* keywords)
{
   PyObject* a1;
   PyObject* a2;
   PyObject* result = 0;

   if (!PyArg_ParseTuple(args, const_cast&lt;char*&gt;("OO"), &amp;a1, &amp;a2))
      return 0;

   try {
      return PyInt_FromLong(
         get_name(borrowed_object(a1)) == get_name(borrowed_object(a2)));
   }
   catch(...)
   {
      // If an exception was thrown, translate it to Python
      boost::python::handle_exception();
      return 0;
   }
}
</pre>

    <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>&copy; Copyright <a href=
    "http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
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