<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <!-- Copyright David Abrahams 2006. Distributed under the Boost --> <!-- Software License, Version 1.0. (See accompanying --> <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> <html> <head> <meta name="generator" content= "HTML Tidy for Windows (vers 1st August 2002), see www.w3.org"> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <link rel="stylesheet" type="text/css" href="../boost.css"> <title>Boost.Python - <boost/python/errors.hpp></title> </head> <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/errors.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="#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><boost/python/errors.hpp></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 <class T> 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><void>(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 <class T> 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 <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 <string> #include <boost/python/errors.hpp> #include <boost/python/object.hpp> #include <boost/python/handle.hpp> // 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*& 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<>( 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<char*>("OO"), &a1, &a2)) return 0; // Use boost::bind to make an object compatible with // boost::Function0<void> if (boost::python::handle_exception( boost::bind<void>(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<char*>("OO"), &a1, &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>© Copyright <a href= "http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p> </body> </html>