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<title>Chapter 4. Frequently Asked Questions (FAQs)</title>
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<div class="titlepage"><div><div><h1 class="title">
<a name="faq"></a>Chapter 4. Frequently Asked Questions (FAQs)</h1></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="faq.html#faq.how_can_i_wrap_a_function_which_">How can I wrap
a function which takes a function pointer as an argument?</a></span></dt>
<dt><span class="section"><a href="faq/i_m_getting_the_attempt_to_retur.html">I'm getting the
"attempt to return dangling reference" error. What am I doing wrong?</a></span></dt>
<dt><span class="section"><a href="faq/is_return_internal_reference_eff.html">Is <code class="computeroutput"><span class="identifier">return_internal_reference</span></code> efficient?</a></span></dt>
<dt><span class="section"><a href="faq/how_can_i_wrap_functions_which_t.html">How can I wrap
functions which take C++ containers as arguments?</a></span></dt>
<dt><span class="section"><a href="faq/fatal_error_c1204_compiler_limit.html">fatal error C1204:Compiler
limit:internal structure overflow</a></span></dt>
<dt><span class="section"><a href="faq/how_do_i_debug_my_python_extensi.html">How do I debug
my Python extensions?</a></span></dt>
<dt><span class="section"><a href="faq/why_doesn_t_my_operator_work.html">Why doesn't my <code class="computeroutput"><span class="special">*=</span></code> operator work?</a></span></dt>
<dt><span class="section"><a href="faq/does_boost_python_work_with_mac_.html">Does Boost.Python
work with Mac OS X?</a></span></dt>
<dt><span class="section"><a href="faq/how_can_i_find_the_existing_pyob.html">How can I find
the existing PyObject that holds a C++ object?</a></span></dt>
<dt><span class="section"><a href="faq/how_can_i_wrap_a_function_which0.html">How can I wrap
a function which needs to take ownership of a raw pointer?</a></span></dt>
<dt><span class="section"><a href="faq/compilation_takes_too_much_time_.html">Compilation takes
too much time and eats too much memory! What can I do to make it faster?</a></span></dt>
<dt><span class="section"><a href="faq/how_do_i_create_sub_packages_usi.html">How do I create
sub-packages using Boost.Python?</a></span></dt>
<dt><span class="section"><a href="faq/error_c2064_term_does_not_evalua.html">error C2064: term
does not evaluate to a function taking 2 arguments</a></span></dt>
<dt><span class="section"><a href="faq/how_can_i_automatically_convert_.html">How can I automatically
convert my custom string type to and from a Python string?</a></span></dt>
<dt><span class="section"><a href="faq/why_is_my_automatic_to_python_co.html">Why is my automatic
to-python conversion not being found?</a></span></dt>
<dt><span class="section"><a href="faq/is_boost_python_thread_aware_com.html">Is Boost.Python
thread-aware/compatible with multiple interpreters?</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="faq.how_can_i_wrap_a_function_which_"></a><a class="link" href="faq.html#faq.how_can_i_wrap_a_function_which_" title="How can I wrap a function which takes a function pointer as an argument?">How can I wrap
a function which takes a function pointer as an argument?</a>
</h3></div></div></div>
<p>
If what you're trying to do is something like this:
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="keyword">void</span> <span class="special">(</span><span class="identifier">string</span> <span class="identifier">s</span><span class="special">)</span> <span class="special">></span> <span class="identifier">funcptr</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="identifier">funcptr</span> <span class="identifier">fp</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">fp</span><span class="special">(</span><span class="string">"hello,world!"</span><span class="special">);</span>
<span class="special">}</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">test</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">def</span><span class="special">(</span><span class="string">"foo"</span><span class="special">,</span><span class="identifier">foo</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
And then:
</p>
<pre class="programlisting"><span class="special">>>></span> <span class="identifier">def</span> <span class="identifier">hello</span><span class="special">(</span><span class="identifier">s</span><span class="special">):</span>
<span class="special">...</span> <span class="identifier">print</span> <span class="identifier">s</span>
<span class="special">...</span>
<span class="special">>>></span> <span class="identifier">foo</span><span class="special">(</span><span class="identifier">hello</span><span class="special">)</span>
<span class="identifier">hello</span><span class="special">,</span> <span class="identifier">world</span><span class="special">!</span>
</pre>
<p>
The short answer is: "you can't". This is not a Boost.Python limitation
so much as a limitation of C++. The problem is that a Python function is
actually data, and the only way of associating data with a C++ function pointer
is to store it in a static variable of the function. The problem with that
is that you can only associate one piece of data with every C++ function,
and we have no way of compiling a new C++ function on-the-fly for every Python
function you decide to pass to <code class="computeroutput"><span class="identifier">foo</span></code>.
In other words, this could work if the C++ function is always going to invoke
the <span class="emphasis"><em>same</em></span> Python function, but you probably don't want
that.
</p>
<p>
If you have the luxury of changing the C++ code you're wrapping, pass it
an <code class="computeroutput"><span class="identifier">object</span></code> instead and call
that; the overloaded function call operator will invoke the Python function
you pass it behind the <code class="computeroutput"><span class="identifier">object</span></code>.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2015 David
Abrahams, Stefan Seefeld<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>)
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