On Saturday 31 October 2009 01:24:16 troy d. straszheim wrote:
> I take it that you have a use-case where it is difficult to specify
>
> as<R(A1,A2)>(thing)
>
> and easy to specify
>
> as<mpl::vector<R, A1, A2> >(thing)
>
> Could you elaborate?
I have some code that takes member function pointers of the form
R Obj::*( A1, A2, A3, ..., An )
and converts them to a function object Q with the following signature:
Rnew Q( Obj&, B1, B2, ..., Bn )
where
Rnew = result_conversion_metafunction<R>::type
Bi = arg_conversion_metafunction<Ai>::type
and Q is exposed via boost.python as a member of Obj.
For example, Ai could be a fixed-point number, with Bi being a double so that
the python side does not know anything about fixed-point numbers. An instance
of Q would convert eh floating point numbers passed from the python side into
fixed-point numbers, call the member function, and convert the returned value
to a double for use on the python side.
For these "converters", metafunctions from boost.function_types are used to
obtain the mpl::vector specifying result type and argument types. Extending
the technique above to function objects which take Obj* as their first
argument, I have a protocol which relies on the presence of a typedef called
'components' in the function object so that I can use the converter when
exposing via boost.python:
struct my_functor {
typedef mpl::vector<R, Obj*, A1, A2, A3, A4> components;
R operator()( Obj*, A1, A2, A3, A4 ) { ... }
};
// my_converter uses the components typedef
typedef typename my_converter<my_functor>::type Q;
Q q;
class_<Obj,...>( "Obj" )
.def( "my_func", q )
;
Regards,
Ravi
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