[C++-sig] Re: Explaining the Magic
David Abrahams
dave at boost-consulting.com
Sat Jul 12 19:46:16 CEST 2003
"Brett Calcott" <brett.calcott at paradise.net.nz> writes:
> Hi David,
>
> I am in Aussie now, and though still homeless
In winter!! Brrrrr... <wink>
> I at least have an office. I thought we could make a start on our
> plans to document some of the boost python implementation.
Great!
> I brushed up on reStructuredText so, as discussed I'll try and
> construct a document from our conversations.
>
> As suggested I have had a look at the luabind conversation, and
> though I understand bits here and there I need something to tie it
> all too. Here's two starting questions:
>
> 1) What happens when you do the following:
>
> struct boring {};
> ...etc...
> class_<boring>("boring")
> ;
>
> There seems to be a fair bit going on.
>
> - Python needs a new ClassType to be registered.
> - We need to construct a new type that can hold our boring struct.
> - Inward and outward converters need to be registered for the type.
>
> Can you gesture in the general direction where these things are done.
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.
OK:
A new (Python) subclass of Boost.Python.Instance (see
libs/python/src/object/class.cpp) is created by invoking
Boost.Python.class, the metatype:
>>> boring = Boost.Python.class(
... 'boring'
... , bases_tuple # in this case, just ()
... , {
... '__module__' : module_name
... , '__doc__' : doc_string # optional
... }
... )
A handle to this object is stuck in the m_class_object field
of the registration associated with typeid(boring). 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).
Because you didn't specify class<boring, non_copyable, ...>,
a to-python converter for boring is registered which copies
its argument into a value_holder held by the the Python
boring object.
Because you didn't specify class<boring ...>(no_init), an
__init__ 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.
register_class_from_python is used to register a from-python
converter for shared_ptr<boring>. boost::shared_ptrs 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.
If there were any bases<>, we'd also be registering the
relationship between these base classes and boring in the
up/down cast graph (inheritance.[hpp/cpp]).
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.
Hmm, that from-python converter probably ought to be handled
the way class converters are, with no explicit conversions
registered.
> 2) Can you give a brief overview of the data structures that are
> present in the registry
The registry is simple: it's just a map from typeid -> registration
(see boost/python/converter/registrations.hpp). lvalue_chain and
rvalue_chain are simple endogenous linked lists.
If you want to know more, just ask.
If you want to know about the cast graph, ask me something specific in
a separate message.
> and an overview of the process that happens as a type makes its
> way from c++ to python and back again.
Big subject. I suggest some background reading: look for relevant
info in the LLNL progress reports and the messages they link to.
Also,
http://mail.python.org/pipermail/c++-sig/2002-May/001023.html
http://mail.python.org/pipermail/c++-sig/2002-December/003115.html
http://aspn.activestate.com/ASPN/Mail/Message/1280898
http://mail.python.org/pipermail/c++-sig/2002-July/001755.html
from c++ to python:
It depends on the type and the call policies in use or, for
call<>(...), call_method<>(...), or object(...), if ref or ptr 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
object() 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.
The "default" case is by-value (copying) conversion, which uses
to_python_value as a to-python converter.
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.
Otherwise, to_python_value uses the m_to_python function in
the registration for the C++ type.
Other conversions, like by-reference conversions, are only
available for wrapped classes, and are requested explicitly by
using ref(...), ptr(...), 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.
from python to C++:
Once again I think there is a distinction between "return value"
and "argument" conversions, and I forget exactly what that is.
What happens depends on whether an lvalue conversion is needed
(see http://mail.python.org/pipermail/c++-sig/2002-May/001023.html)
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.
An lvalue conversion can be done in one step (just get me the
pointer to the object - it can be NULL 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.
> 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).
I hope this helped.
> ps. Take your time to answer these
Too late!
> -- I am trying to write a paper, and get some foundation
> boost_python simulation framework written, so I am happy if we
> progress slowly...
Well, this should give you plenty to chew on for a while.
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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