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343 lines
11 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<meta name="generator" content=
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"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python - FAQ</title>
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</head>
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<body link="#0000ff" vlink="#800080">
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
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"header">
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<tr>
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<td valign="top" width="300">
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<h3><a href="../../../../index.htm"><img height="86" width="277"
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alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
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</td>
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<td valign="top">
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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
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<h2 align="center">Frequently Asked Questions (FAQs)</h2>
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</td>
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</tr>
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</table>
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<hr>
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<dl class="page-index">
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<dt><a href="#question1">Is return_internal reference
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efficient?</a></dt>
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<dt><a href="#question2">How can I wrap containers which take C++
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containers as arguments?</a></dt>
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<dt><a href="#c1204">fatal error C1204:Compiler limit:internal
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structure overflow</a></dt>
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<dt><a href="#debugging">How do I debug my Python extensions?</a></dt>
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<dt><a href="#imul">Why doesn't my <code>*=</code> operator work?</a></dt>
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</dl>
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<h2><a name="question1"></a>Is return_internal reference efficient?</h2>
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<blockquote>
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<b>Q:</b> <i>I have an object composed of 12 doubles. A const& to
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this object is returned by a member function of another class. From the
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viewpoint of using the returned object in Python I do not care if I get
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a copy or a reference to the returned object. In Boost.Python Version 2
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I have the choice of using copy_const_reference or
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return_internal_reference. Are there considerations that would lead me
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to prefer one over the other, such as size of generated code or memory
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overhead?</i>
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<p><b>A:</b> copy_const_reference will make an instance with storage
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for one of your objects, size = base_size + 12 * sizeof(double).
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return_internal_reference will make an instance with storage for a
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pointer to one of your objects, size = base_size + sizeof(void*).
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However, it will also create a weak reference object which goes in the
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source object's weakreflist and a special callback object to manage the
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lifetime of the internally-referenced object. My guess?
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copy_const_reference is your friend here, resulting in less overall
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memory use and less fragmentation, also probably fewer total
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cycles.</p>
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</blockquote>
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<h2><a name="question2"></a>How can I wrap functions which take C++
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containers as arguments?</h2>
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<p>Ralf W. Grosse-Kunstleve provides these notes:</p>
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<ol>
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<li>
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Using the regular <code>class_<></code> wrapper:
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<pre>
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class_<std::vector<double> >("std_vector_double")
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.def(...)
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...
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;
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</pre>
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This can be moved to a template so that several types (double, int,
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long, etc.) can be wrapped with the same code. This technique is used
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in the file
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<blockquote>
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scitbx/include/scitbx/array_family/boost_python/flex_wrapper.h
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</blockquote>
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in the "scitbx" package. The file could easily be modified for
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wrapping std::vector<> instantiations.
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<p>This type of C++/Python binding is most suitable for containers
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that may contain a large number of elements (>10000).</p>
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</li>
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<li>
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Using custom rvalue converters. Boost.Python "rvalue converters"
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match function signatures such as:
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<pre>
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void foo(std::vector<double> const& array); // pass by const-reference
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void foo(std::vector<double> array); // pass by value
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</pre>
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Some custom rvalue converters are implemented in the file
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<blockquote>
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scitbx/include/scitbx/boost_python/container_conversions.h
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</blockquote>
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This code can be used to convert from C++ container types such as
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std::vector<> or std::list<> to Python tuples and vice
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versa. A few simple examples can be found in the file
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<blockquote>
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scitbx/array_family/boost_python/regression_test_module.cpp
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</blockquote>
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Automatic C++ container <-> Python tuple conversions are most
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suitable for containers of moderate size. These converters generate
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significantly less object code compared to alternative 1 above.
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</li>
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</ol>
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A disadvantage of using alternative 2 is that operators such as
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arithmetic +,-,*,/,% are not available. It would be useful to have custom
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rvalue converters that convert to a "math_array" type instead of tuples.
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This is currently not implemented but is possible within the framework of
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Boost.Python V2 as it will be released in the next couple of weeks. [ed.:
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this was posted on 2002/03/10]
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<p>It would also be useful to also have "custom lvalue converters" such
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as std::vector<> <-> Python list. These converters would
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support the modification of the Python list from C++. For example:</p>
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<p>C++:</p>
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<pre>
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void foo(std::vector<double>& array)
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{
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for(std::size_t i=0;i<array.size();i++) {
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array[i] *= 2;
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}
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}
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</pre>
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Python:
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<pre>
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>>> l = [1, 2, 3]
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>>> foo(l)
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>>> print l
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[2, 4, 6]
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</pre>
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Custom lvalue converters require changes to the Boost.Python core library
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and are currently not available.
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<p>P.S.:</p>
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<p>The "scitbx" files referenced above are available via anonymous
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CVS:</p>
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<pre>
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cvs -d:pserver:anonymous@cvs.cctbx.sourceforge.net:/cvsroot/cctbx login
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cvs -d:pserver:anonymous@cvs.cctbx.sourceforge.net:/cvsroot/cctbx co scitbx
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</pre>
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<h2><a name="c1204"></a>fatal error C1204:Compiler limit:internal
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structure overflow</h2>
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<blockquote>
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<b>Q:</b> <i>I get this error message when compiling a large source
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file. What can I do?</i>
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<p><b>A:</b> You have two choices:</p>
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<ol>
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<li>Upgrade your compiler (preferred)</li>
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<li>
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Break your source file up into multiple translation units.
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<p><code><b>my_module.cpp</b></code>:</p>
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<pre>
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...
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void more_of_my_module();
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BOOST_PYTHON_MODULE(my_module)
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{
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def("foo", foo);
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def("bar", bar);
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...
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more_of_my_module();
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}
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</pre>
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<code><b>more_of_my_module.cpp</b></code>:
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<pre>
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void more_of_my_module()
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{
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def("baz", baz);
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...
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}
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</pre>
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If you find that a <code><a href=
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"class.html#class_-spec">class_</a><...></code> declaration
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can't fit in a single source file without triggering the error, you
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can always pass a reference to the <code>class_</code> object to a
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function in another source file, and call some of its member
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functions (e.g. <code>.def(...)</code>) in the auxilliary source
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file: <code>
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<p><b>more_of_my_class.cpp</b></code>:
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<pre>
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void more_of_my_class(class<my_class>& x)
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{
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x
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.def("baz", baz)
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.add_property("xx", &my_class::get_xx, &my_class::set_xx)
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;
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...
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}
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</pre>
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</li>
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</ol>
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</blockquote>
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<h2><a name="debugging"></a>How do I debug my Python extensions?</h2>
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<p>Greg Burley gives the following answer for Unix GCC users:
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<blockquote>
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Once you have created a boost python extension for your c++ library or
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class, you may need to debug the code. Afterall this is one of the
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reasons for wrapping the library in python. An expected side-effect or
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benefit of using BPL is that debugging should be isolated to the c++
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library that is under test, given that python code is minimal and
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boost::python either works or it doesn't. (ie. While errors can occur
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when the wrapping method is invalid, most errors are caught by the
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compiler ;-).
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<p>
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The basic steps required to initiate a gdb session to debug a c++
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library via python are shown here. Note, however that you should start
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the gdb session in the directory that contains your BPL my_ext.so
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module.
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<pre>
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(gdb) target exec python
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(gdb) run
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>>> from my_ext import *
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>>> [C-c]
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(gdb) break MyClass::MyBuggyFunction
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(gdb) cont
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>>> pyobj = MyClass()
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>>> pyobj.MyBuggyFunction()
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Breakpoint 1, MyClass::MyBuggyFunction ...
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Current language: auto; currently c++
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(gdb) do debugging stuff
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</pre>
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</blockquote>
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<p>
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Greg's approach works even better using Emacs'
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"<code>gdb</code>" command, since it will show you each line
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of source as you step through it.
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<p>
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On Windows, my favorite debugging solution is the debugger that comes
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with Microsoft Visual C++ 7. This debugger seems to work with code
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generated by all versions of Microsoft and Metrowerks toolsets; it's
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rock solid and "just works" without requiring any special
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tricks from the user.
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<p>
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Unfortunately for Cygwin and MinGW users, as of this writing gdb on
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Windows has a very hard time dealing with shared libraries, which
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could make Greg's approach next to useless for you. My best advice for
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you is to use Metrowerks C++ for compiler conformance and Microsoft
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Visual Studio as a debugger when you need one.
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<h3>Debugging extensions through Boost.Build</h3>
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If you are launching your extension module tests with <a
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href="../../../tools/build">Boost.Build</a> using the
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<code>boost-python-runtest</code> rule, you can ask it to launch your
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debugger for you by adding "-sPYTHON_LAUNCH=<i>debugger</i>"
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to your bjam command-line:
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<pre>
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bjam -sTOOLS=metrowerks "-sPYTHON_LAUNCH=devenv /debugexe" test
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bjam -sTOOLS=gcc -sPYTHON_LAUNCH=gdb test
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</pre>
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It can also be extremely useful to add the <code>-d+2</code> option
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when you run your test, because Boost.Build will then show you the
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exact commands it uses to invoke it. This will invariably involve
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setting up PYTHONPATH and other important environment variables such
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as LD_LIBRARY_PATH which may be needed by your debugger in order to
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get things to work right.
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<h2><a name="imul"></a>Why doesn't my <code>*=</code> operator work?</h2>
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<blockquote>
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<b>Q:</b> <i>I have exported my class to python, with many overloaded operators. it
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works fine for me except the </i><code>*=</code><i> operator. It always tells
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me "can't multiply sequence with non int type". If I use
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</i><code>p1.__imul__(p2)</code><i> instead of </i><code>p1 *=
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p2</code><i>, it successfully executes my code. What's wrong with
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me?</i>
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<p><b>A:</b> There's nothing wrong with you. This is a bug in Python
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2.2. You can see the same effect in Pure Python (you can learn a lot
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about what's happening in Boost.Python by playing with new-style
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classes in Pure Python).
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<pre>
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>>> class X(object):
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... def __imul__(self, x):
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... print 'imul'
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...
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>>> x = X()
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>>> x *= 1
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</pre>
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To cure this problem, all you need to do is upgrade your Python to
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version 2.2.1 or later.
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</blockquote>
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<hr>
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
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24 November, 2002
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<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
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</p>
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<p><i>© Copyright <a href=
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"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002. All Rights
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Reserved.</i></p>
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</body>
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</html>
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