[C++-sig] Re: Is it possible to expose functions with variable argument lists?

[David Abrahams]

GameGuy <gameguy at eastlink.ca> writes:

> Hello.
> I have been using PYSTE and Boost.Python to wrap a C++ library for use in Python.  Very nice stuff.
>
> I have reached a stumbling block though:  There are several methods in
> this library that have an ellipsis in their method declarations.  Eg:
>
> class Foo
> {
>      virtual void method(const char* bar, ... ) const = 0;
> };
>
> Which I think can be wrapped with Boost.Python in the following way:
>
> struct Foo_Wrapper : Foo
> {
>      // ...
>
>      virtual void method(const char* p0, ... ) const {
>          va_list args;
>          va_start(args, p0);
>          call_method< void >(self, "method", p0, args);
>      }
> };
>
> // ...
>
>      class_< Foo, Foo_Wrapper >("Foo", init<  >())
>          .def("method", &Foo::method)
>          ;
>
>
> I saw it done this way in another library.  I haven't verified that it
> works or not yet as I can't compile it yet (running into MSVC7
> issues).
>
> Has anyone else had experience wrapping functions with variable argument lists?
> Will this work in Boost.Python?


Short answer: no, this doesn't work.

Longer answer: you can't really get this to work in Boost.Python
because it relies on being able to deduce the expected C++ types of
arguments.

Longer answer still: you might be able use
BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS to wrap this method for
specific sequences of arguments.

http://tinyurl.com/2wptk

But now that I look I think it'll fail for the same reason.  I think
the need to use call_method internally might give you problems, too.

I think your best bet might be to use the facilities of
http://www.boost.org/libs/python/doc/v2/raw_function.html

> Is it worth extending PYSTE to handle these cases?  (It doesn't currently)
>
>
> Thank you for any assistance,
> ~Shaun

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com