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| <h1 class="title"><a class="reference" href="index.html">Boost.Python</a> Internals <a class="reference" href="../../../index.htm"><img alt="Boost" src="../../../boost.png" /></a></h1> |
| <div class="section" id="a-conversation-between-brett-calcott-and-david-abrahams"> |
| <h1><a name="a-conversation-between-brett-calcott-and-david-abrahams">A conversation between Brett Calcott and David Abrahams</a></h1> |
| <table class="field-list" frame="void" rules="none"> |
| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams and Brett Calcott 2003. See |
| accompanying <a class="reference" href="../../../LICENSE_1_0.txt">license</a> for terms of use.</td> |
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| <p>In both of these cases, I'm quite capable of reading code - but the |
| thing I don't get from scanning the source is a sense of the |
| architecture, both structurally, and temporally (er, I mean in what |
| order things go on).</p> |
| <ol class="arabic"> |
| <li><p class="first">What happens when you do the following:</p> |
| <pre class="literal-block"> |
| struct boring {}; |
| ...etc... |
| class_<boring>("boring") |
| ; |
| </pre> |
| </li> |
| </ol> |
| <p>There seems to be a fair bit going on.</p> |
| <blockquote> |
| <ul class="simple"> |
| <li>Python needs a new ClassType to be registered.</li> |
| <li>We need to construct a new type that can hold our boring struct.</li> |
| <li>Inward and outward converters need to be registered for the type.</li> |
| </ul> |
| </blockquote> |
| <p>Can you gesture in the general direction where these things are done?</p> |
| <blockquote> |
| <p>I only have time for a "off-the-top-of-my-head" answer at the moment; |
| I suggest you step through the code with a debugger after reading this |
| to see how it works, fill in details, and make sure I didn't forget |
| anything.</p> |
| <blockquote> |
| <p>A new (Python) subclass of Boost.Python.Instance (see |
| libs/python/src/object/class.cpp) is created by invoking |
| Boost.Python.class, the metatype:</p> |
| <pre class="literal-block"> |
| >>> boring = Boost.Python.class( |
| ... 'boring' |
| ... , bases_tuple # in this case, just () |
| ... , { |
| ... '__module__' : module_name |
| ... , '__doc__' : doc_string # optional |
| ... } |
| ... ) |
| </pre> |
| <p>A handle to this object is stuck in the m_class_object field |
| of the registration associated with <tt class="literal"><span class="pre">typeid(boring)</span></tt>. The |
| registry will keep that object alive forever, even if you |
| wipe out the 'boring' attribute of the extension module |
| (probably not a good thing).</p> |
| <p>Because you didn't specify <tt class="literal"><span class="pre">class<boring,</span> <span class="pre">non_copyable,</span> |
| <span class="pre">...></span></tt>, a to-python converter for boring is registered which |
| copies its argument into a value_holder held by the the |
| Python boring object.</p> |
| <p>Because you didn't specify <tt class="literal"><span class="pre">class<boring</span> <span class="pre">...>(no_init)</span></tt>, |
| an <tt class="literal"><span class="pre">__init__</span></tt> function object is added to the class |
| dictionary which default-constructs a boring in a |
| value_holder (because you didn't specify some smart pointer |
| or derived wrapper class as a holder) held by the Python |
| boring object.</p> |
| <p><tt class="literal"><span class="pre">register_class_from_python</span></tt> is used to register a |
| from-python converter for <tt class="literal"><span class="pre">shared_ptr<boring></span></tt>. |
| <tt class="literal"><span class="pre">boost::shared_ptr</span></tt>s are special among smart pointers |
| because their Deleter argument can be made to manage the |
| whole Python object, not just the C++ object it contains, no |
| matter how the C++ object is held.</p> |
| <p>If there were any <tt class="literal"><span class="pre">bases<></span></tt>, we'd also be registering the |
| relationship between these base classes and boring in the |
| up/down cast graph (<tt class="literal"><span class="pre">inheritance.[hpp/cpp]</span></tt>).</p> |
| <p>In earlier versions of the code, we'd be registering lvalue |
| from-python converters for the class here, but now |
| from-python conversion for wrapped classes is handled as a |
| special case, before consulting the registry, if the source |
| Python object's metaclass is the Boost.Python metaclass.</p> |
| <p>Hmm, that from-python converter probably ought to be handled |
| the way class converters are, with no explicit conversions |
| registered.</p> |
| </blockquote> |
| </blockquote> |
| <ol class="arabic" start="2"> |
| <li><p class="first">Can you give a brief overview of the data structures that are |
| present in the registry</p> |
| <blockquote> |
| <p>The registry is simple: it's just a map from typeid -> |
| registration (see boost/python/converter/registrations.hpp). |
| <tt class="literal"><span class="pre">lvalue_chain</span></tt> and <tt class="literal"><span class="pre">rvalue_chain</span></tt> are simple endogenous |
| linked lists.</p> |
| <p>If you want to know more, just ask.</p> |
| <p>If you want to know about the cast graph, ask me something specific in |
| a separate message.</p> |
| </blockquote> |
| <p>and an overview of the process that happens as a type makes its |
| way from c++ to python and back again.</p> |
| </li> |
| </ol> |
| <blockquote> |
| <p>Big subject. I suggest some background reading: look for relevant |
| info in the LLNL progress reports and the messages they link to. |
| Also,</p> |
| <blockquote> |
| <p><a class="reference" href="http://mail.python.org/pipermail/c++-sig/2002-May/001023.html">http://mail.python.org/pipermail/c++-sig/2002-May/001023.html</a></p> |
| <p><a class="reference" href="http://mail.python.org/pipermail/c++-sig/2002-December/003115.html">http://mail.python.org/pipermail/c++-sig/2002-December/003115.html</a></p> |
| <p><a class="reference" href="http://aspn.activestate.com/ASPN/Mail/Message/1280898">http://aspn.activestate.com/ASPN/Mail/Message/1280898</a></p> |
| <p><a class="reference" href="http://mail.python.org/pipermail/c++-sig/2002-July/001755.html">http://mail.python.org/pipermail/c++-sig/2002-July/001755.html</a></p> |
| </blockquote> |
| <p>from c++ to python:</p> |
| <blockquote> |
| <p>It depends on the type and the call policies in use or, for |
| <tt class="literal"><span class="pre">call<>(...)</span></tt>, <tt class="literal"><span class="pre">call_method<>(...)</span></tt>, or <tt class="literal"><span class="pre">object(...)</span></tt>, if |
| <tt class="literal"><span class="pre">ref</span></tt> or <tt class="literal"><span class="pre">ptr</span></tt> is used. There are also two basic |
| categories to to-python conversion, "return value" conversion |
| (for Python->C++ calls) and "argument" conversion (for |
| C++->Python calls and explicit <tt class="literal"><span class="pre">object()</span></tt> conversions). The |
| behavior of these two categories differs subtly in various ways |
| whose details I forget at the moment. You can probably find |
| the answers in the above references, and certainly in the code.</p> |
| <p>The "default" case is by-value (copying) conversion, which uses |
| to_python_value as a to-python converter.</p> |
| <blockquote> |
| <p>Since there can sensibly be only one way to convert any type |
| to python (disregarding the idea of scoped registries for the |
| moment), it makes sense that to-python conversions can be |
| handled by specializing a template. If the type is one of |
| the types handled by a built-in conversion |
| (builtin_converters.hpp), the corresponding template |
| specialization of to_python_value gets used.</p> |
| <p>Otherwise, to_python_value uses the <tt class="literal"><span class="pre">m_to_python</span></tt> |
| function in the registration for the C++ type.</p> |
| </blockquote> |
| <p>Other conversions, like by-reference conversions, are only |
| available for wrapped classes, and are requested explicitly by |
| using <tt class="literal"><span class="pre">ref(...)</span></tt>, <tt class="literal"><span class="pre">ptr(...)</span></tt>, or by specifying different |
| CallPolicies for a call, which can cause a different to-python |
| converter to be used. These conversions are never registered |
| anywhere, though they do need to use the registration to find |
| the Python class corresponding to the C++ type being referred |
| to. They just build a new Python instance and stick the |
| appropriate Holder instance in it.</p> |
| </blockquote> |
| <p>from python to C++:</p> |
| <blockquote> |
| <p>Once again I think there is a distinction between "return value" |
| and "argument" conversions, and I forget exactly what that is.</p> |
| <p>What happens depends on whether an lvalue conversion is needed |
| (see <a class="reference" href="http://mail.python.org/pipermail/c++-sig/2002-May/001023.html">http://mail.python.org/pipermail/c++-sig/2002-May/001023.html</a>) |
| All lvalue conversions are also registered in a type's rvalue |
| conversion chain, since when an rvalue will do, an lvalue is |
| certainly good enough.</p> |
| <p>An lvalue conversion can be done in one step (just get me the |
| pointer to the object - it can be <tt class="literal"><span class="pre">NULL</span></tt> if no conversion is |
| possible) while an rvalue conversion requires two steps to |
| support wrapped function overloading and multiple converters for |
| a given C++ target type: first tell me if a conversion is |
| possible, then construct the converted object as a second step.</p> |
| </blockquote> |
| </blockquote> |
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