1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>boost/python/errors.hpp</title> 5<link rel="stylesheet" href="../../boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Boost.Python Reference Manual"> 8<link rel="up" href="../high_level_components.html" title="Chapter 2. High Level Components"> 9<link rel="prev" href="boost_python_enum_hpp.html" title="boost/python/enum.hpp"> 10<link rel="next" href="boost_python_exception_translato.html" title="boost/python/exception_translator.hpp"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="" width="" height="" src="../../images/boost.png"></td></tr></table> 14<hr> 15<div class="spirit-nav"> 16<a accesskey="p" href="boost_python_enum_hpp.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../high_level_components.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="boost_python_exception_translato.html"><img src="../../images/next.png" alt="Next"></a> 17</div> 18<div class="section"> 19<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 20<a name="high_level_components.boost_python_errors_hpp"></a><a class="link" href="boost_python_errors_hpp.html" title="boost/python/errors.hpp">boost/python/errors.hpp</a> 21</h2></div></div></div> 22<div class="toc"><dl class="toc"> 23<dt><span class="section"><a href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.introduction">Introduction</a></span></dt> 24<dt><span class="section"><a href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.class_error_already_set">Class 25 <code class="computeroutput"><span class="identifier">error_already_set</span></code></a></span></dt> 26<dt><span class="section"><a href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.functions">Functions</a></span></dt> 27<dt><span class="section"><a href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.example">Example</a></span></dt> 28</dl></div> 29<div class="section"> 30<div class="titlepage"><div><div><h3 class="title"> 31<a name="high_level_components.boost_python_errors_hpp.introduction"></a><a class="link" href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.introduction" title="Introduction">Introduction</a> 32</h3></div></div></div> 33<p> 34 <boost/python/errors.hpp> provides types and functions for managing 35 and translating between Python and C++ exceptions. This is relatively low-level 36 functionality that is mostly used internally by Boost.Python. Users should 37 seldom need it. 38 </p> 39</div> 40<div class="section"> 41<div class="titlepage"><div><div><h3 class="title"> 42<a name="high_level_components.boost_python_errors_hpp.class_error_already_set"></a><a class="link" href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.class_error_already_set" title="Class error_already_set">Class 43 <code class="computeroutput"><span class="identifier">error_already_set</span></code></a> 44</h3></div></div></div> 45<p> 46 error_already_set is an exception type which can be thrown to indicate 47 that a Python error has occurred. If thrown, the precondition is that 48 <a href="http://www.python.org/doc/2.2/api/exceptionHandling.html#l2h-71" target="_top">PyErr_Occurred()</a> 49 returns a value convertible to <code class="computeroutput"><span class="keyword">true</span></code>. 50 Portable code shouldn't throw this exception type directly, but should 51 instead use <a class="link" href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.functions" title="Functions">throw_error_already_set()</a>, 52 below. 53 </p> 54<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">python</span> 55<span class="special">{</span> 56 <span class="keyword">class</span> <span class="identifier">error_already_set</span> <span class="special">{};</span> 57<span class="special">}}</span> 58</pre> 59</div> 60<div class="section"> 61<div class="titlepage"><div><div><h3 class="title"> 62<a name="high_level_components.boost_python_errors_hpp.functions"></a><a class="link" href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.functions" title="Functions">Functions</a> 63</h3></div></div></div> 64<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">bool</span> <span class="identifier">handle_exception</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">f</span><span class="special">)</span> <span class="keyword">throw</span><span class="special">();</span> 65<span class="keyword">void</span> <span class="identifier">handle_exception</span><span class="special">()</span> <span class="keyword">throw</span><span class="special">();</span> 66</pre> 67<div class="variablelist"> 68<p class="title"><b></b></p> 69<dl class="variablelist"> 70<dt><span class="term">Requires</span></dt> 71<dd><p> 72 The first form requires that the expression function0<void>(f) 73 is valid. The second form requires that a C++ exception is currently 74 being handled (see section 15.1 in the C++ standard). 75 </p></dd> 76<dt><span class="term">Effects</span></dt> 77<dd><p> 78 The first form calls f() inside a try block which first attempts 79 to use all registered <a class="link" href="boost_python_exception_translato.html" title="boost/python/exception_translator.hpp">exception 80 translators</a>. If none of those translates the exception, the 81 catch clauses then set an appropriate Python exception for the C++ 82 exception caught, returning true if an exception was thrown, false 83 otherwise. The second form passes a function which rethrows the exception 84 currently being handled to the first form. 85 </p></dd> 86<dt><span class="term">Postconditions</span></dt> 87<dd><p> 88 No exception is being handled 89 </p></dd> 90<dt><span class="term">Throws</span></dt> 91<dd><p> 92 nothing 93 </p></dd> 94<dt><span class="term">Rationale</span></dt> 95<dd><p> 96 At inter-language boundaries it is important to ensure that no C++ 97 exceptions escape, since the calling language usually doesn't have 98 the equipment necessary to properly unwind the stack. Use handle_exception 99 to manage exception translation whenever your C++ code is called 100 directly from the Python API. This is done for you automatically 101 by the usual function wrapping facilities: <a class="link" href="../function_invocation_and_creation/boost_python_make_function_hpp.html#function_invocation_and_creation.boost_python_make_function_hpp.functions" title="Functions">make_function()</a>, 102 <a class="link" href="../function_invocation_and_creation/boost_python_make_function_hpp.html#function_invocation_and_creation.boost_python_make_function_hpp.functions" title="Functions">make_constructor()</a>, 103 <a class="link" href="boost_python_def_hpp.html#high_level_components.boost_python_def_hpp.functions" title="Functions">def()</a> 104 and <a class="link" href="../high_level_components.html#high_level_components.boost_python_class_hpp.class_template_class_t_bases_hel.class_template_class_modifier_fu" title="Class template class_ modifier functions">class_::def()</a>. 105 The second form can be more convenient to use (see the example below), 106 but various compilers have problems when exceptions are rethrown 107 from within an enclosing try block. 108 </p></dd> 109</dl> 110</div> 111<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">expect_non_null</span><span class="special">(</span><span class="identifier">T</span><span class="special">*</span> <span class="identifier">x</span><span class="special">);</span></pre> 112<div class="variablelist"> 113<p class="title"><b></b></p> 114<dl class="variablelist"> 115<dt><span class="term">Returns</span></dt> 116<dd><p> 117 x 118 </p></dd> 119<dt><span class="term">Throws</span></dt> 120<dd><p> 121 error_already_set() iff x == 0. 122 </p></dd> 123<dt><span class="term">Rationale</span></dt> 124<dd><p> 125 Simplifies error-handling when calling functions in the Python/C 126 API which return 0 on error. 127 </p></dd> 128</dl> 129</div> 130<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">throw_error_already_set</span><span class="special">();</span></pre> 131<div class="variablelist"> 132<p class="title"><b></b></p> 133<dl class="variablelist"> 134<dt><span class="term">Effects</span></dt> 135<dd><p> 136 throw error_already_set(); 137 </p></dd> 138<dt><span class="term">Rationale</span></dt> 139<dd><p> 140 Simplifies error-handling when calling functions in the Python/C 141 API which return 0 on error. 142 </p></dd> 143</dl> 144</div> 145<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">throw_error_already_set</span><span class="special">();</span></pre> 146<div class="variablelist"> 147<p class="title"><b></b></p> 148<dl class="variablelist"> 149<dt><span class="term">Effects</span></dt> 150<dd><p> 151 throw error_already_set(); 152 </p></dd> 153<dt><span class="term">Rationale</span></dt> 154<dd><p> 155 Many platforms and compilers are not able to consistently catch exceptions 156 thrown across shared library boundaries. Using this function from 157 the Boost.Python library ensures that the appropriate catch block 158 in handle_exception() can catch the exception. 159 </p></dd> 160</dl> 161</div> 162</div> 163<div class="section"> 164<div class="titlepage"><div><div><h3 class="title"> 165<a name="high_level_components.boost_python_errors_hpp.example"></a><a class="link" href="boost_python_errors_hpp.html#high_level_components.boost_python_errors_hpp.example" title="Example">Example</a> 166</h3></div></div></div> 167<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> 168<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">/</span><span class="identifier">errors</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 169<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">/</span><span class="identifier">object</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 170<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">/</span><span class="identifier">handle</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 171 172<span class="comment">// Returns a std::string which has the same value as obj's "__name__"</span> 173<span class="comment">// attribute.</span> 174<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">get_name</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">object</span> <span class="identifier">obj</span><span class="special">)</span> 175<span class="special">{</span> 176 <span class="comment">// throws if there's no __name__ attribute</span> 177 <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">expect_non_null</span><span class="special">(</span> 178 <span class="identifier">PyObject_GetAttrString</span><span class="special">(</span><span class="identifier">obj</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">(),</span> <span class="string">"__name__"</span><span class="special">));</span> 179 180 <span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">s</span> <span class="special">=</span> <span class="identifier">PyString_AsString</span><span class="special">(</span><span class="identifier">p</span><span class="special">);</span> 181 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">s</span> <span class="special">!=</span> <span class="number">0</span><span class="special">)</span> 182 <span class="identifier">Py_DECREF</span><span class="special">(</span><span class="identifier">p</span><span class="special">);</span> 183 184 <span class="comment">// throws if it's not a Python string</span> 185 <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">result</span><span class="special">(</span> 186 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">expect_non_null</span><span class="special">(</span> 187 <span class="identifier">PyString_AsString</span><span class="special">(</span><span class="identifier">p</span><span class="special">)));</span> 188 189 <span class="identifier">Py_DECREF</span><span class="special">(</span><span class="identifier">p</span><span class="special">);</span> <span class="comment">// Done with p</span> 190 191 <span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span> 192<span class="special">}</span> 193 194<span class="comment">//</span> 195<span class="comment">// Demonstrate form 1 of handle_exception</span> 196<span class="comment">//</span> 197 198<span class="comment">// Place into result a Python Int object whose value is 1 if a and b have</span> 199<span class="comment">// identical "__name__" attributes, 0 otherwise.</span> 200<span class="keyword">void</span> <span class="identifier">same_name_impl</span><span class="special">(</span><span class="identifier">PyObject</span><span class="special">*&</span> <span class="identifier">result</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">object</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">object</span> <span class="identifier">b</span><span class="special">)</span> 201<span class="special">{</span> 202 <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">PyInt_FromLong</span><span class="special">(</span> 203 <span class="identifier">get_name</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">get_name</span><span class="special">(</span><span class="identifier">a2</span><span class="special">));</span> 204<span class="special">}</span> 205 206<span class="identifier">object</span> <span class="identifier">borrowed_object</span><span class="special">(</span><span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">p</span><span class="special">)</span> 207<span class="special">{</span> 208 <span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">object</span><span class="special">(</span> 209 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">handle</span><span class="special"><>(</span> 210 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">borrowed</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)));</span> 211<span class="special">}</span> 212 213<span class="comment">// This is an example Python 'C' API interface function</span> 214<span class="keyword">extern</span> <span class="string">"C"</span> <span class="identifier">PyObject</span><span class="special">*</span> 215<span class="identifier">same_name</span><span class="special">(</span><span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">args</span><span class="special">,</span> <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">keywords</span><span class="special">)</span> 216<span class="special">{</span> 217 <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">a1</span><span class="special">;</span> 218 <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">a2</span><span class="special">;</span> 219 <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">result</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 220 221 <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">PyArg_ParseTuple</span><span class="special">(</span><span class="identifier">args</span><span class="special">,</span> <span class="keyword">const_cast</span><span class="special"><</span><span class="keyword">char</span><span class="special">*>(</span><span class="string">"OO"</span><span class="special">),</span> <span class="special">&</span><span class="identifier">a1</span><span class="special">,</span> <span class="special">&</span><span class="identifier">a2</span><span class="special">))</span> 222 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 223 224 <span class="comment">// Use boost::bind to make an object compatible with</span> 225 <span class="comment">// boost::Function0<void></span> 226 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">handle_exception</span><span class="special">(</span> 227 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special"><</span><span class="keyword">void</span><span class="special">>(</span><span class="identifier">same_name_impl</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">result</span><span class="special">),</span> <span class="identifier">borrowed_object</span><span class="special">(</span><span class="identifier">a1</span><span class="special">),</span> <span class="identifier">borrowed_object</span><span class="special">(</span><span class="identifier">a2</span><span class="special">))))</span> 228 <span class="special">{</span> 229 <span class="comment">// an exception was thrown; the Python error was set by</span> 230 <span class="comment">// handle_exception()</span> 231 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 232 <span class="special">}</span> 233 234 <span class="keyword">return</span> <span class="identifier">result</span><span class="special">;</span> 235<span class="special">}</span> 236 237<span class="comment">//</span> 238<span class="comment">// Demonstrate form 2 of handle_exception. Not well-supported by all</span> 239<span class="comment">// compilers.</span> 240<span class="comment">//</span> 241<span class="keyword">extern</span> <span class="string">"C"</span> <span class="identifier">PyObject</span><span class="special">*</span> 242<span class="identifier">same_name2</span><span class="special">(</span><span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">args</span><span class="special">,</span> <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">keywords</span><span class="special">)</span> 243<span class="special">{</span> 244 <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">a1</span><span class="special">;</span> 245 <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">a2</span><span class="special">;</span> 246 <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">result</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 247 248 <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">PyArg_ParseTuple</span><span class="special">(</span><span class="identifier">args</span><span class="special">,</span> <span class="keyword">const_cast</span><span class="special"><</span><span class="keyword">char</span><span class="special">*>(</span><span class="string">"OO"</span><span class="special">),</span> <span class="special">&</span><span class="identifier">a1</span><span class="special">,</span> <span class="special">&</span><span class="identifier">a2</span><span class="special">))</span> 249 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 250 251 <span class="keyword">try</span> <span class="special">{</span> 252 <span class="keyword">return</span> <span class="identifier">PyInt_FromLong</span><span class="special">(</span> 253 <span class="identifier">get_name</span><span class="special">(</span><span class="identifier">borrowed_object</span><span class="special">(</span><span class="identifier">a1</span><span class="special">))</span> <span class="special">==</span> <span class="identifier">get_name</span><span class="special">(</span><span class="identifier">borrowed_object</span><span class="special">(</span><span class="identifier">a2</span><span class="special">)));</span> 254 <span class="special">}</span> 255 <span class="keyword">catch</span><span class="special">(...)</span> 256 <span class="special">{</span> 257 <span class="comment">// If an exception was thrown, translate it to Python</span> 258 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">handle_exception</span><span class="special">();</span> 259 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 260 <span class="special">}</span> 261<span class="special">}</span> 262</pre> 263</div> 264</div> 265<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 266<td align="left"></td> 267<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2015 David Abrahams, Stefan Seefeld<p> 268 Distributed under the Boost Software License, Version 1.0. (See accompanying 269 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> 270 </p> 271</div></td> 272</tr></table> 273<hr> 274<div class="spirit-nav"> 275<a accesskey="p" href="boost_python_enum_hpp.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../high_level_components.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="boost_python_exception_translato.html"><img src="../../images/next.png" alt="Next"></a> 276</div> 277</body> 278</html> 279
