Re: [boost] [function_types] is there an equivalent for functors?

Subject: Re: [boost] [function_types] is there an equivalent for functors?
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-07-05 13:59:58

On 7/5/2010 10:46 AM, Edd Dawson wrote:
> Hi all,
>
> I'm looking for code that does what
> boost::function_types::parameter_types does but for functors. Support
> for things such as boost::bind/lambda/function/signals(1&2)/etc and the
> standard binders and adaptors would be nice.
>
> I specifically want to be able to do something like:
>
> template<typename Functor>
> void f(const Functor &f)
> {
> typedef typename functor_params<Functor>::type params; // an mpl sequence
> const std::size_t arity = functor_arity<Functor>::value;
> // ...
> }
>
> I started writing code for this but I've come to the point where I can't
> see a way to support libraries such as boost::bind without relying on
> their implementation details.
>
> But perhaps something like this already exists?
>
> Thanks,
>
> Edd

At the outset that looks ill-defined, since function objects may 
generally be polymorphic (accept a variety of parameter types and 
arities, each combination potentially returning a different result 
type).  For example,

struct identity
{
     template< class T >
     T operator()(T x) const { return x; }
};

If you want restrict yourself to monomorphic function objects, then it 
might be possible, but I don't know of a generic way to obtain the 
information you're seeking.

You can, however, get away with querying whether a function object can 
be called with a specific signature, though the machinery to do this is 
relatively complex.  I think something working is found in Boost.Proto 
as "can_be_called".

- Jeff