[Boost-users] [function_traits] How to detect function arity for functors with member template?

Subject: [Boost-users] [function_traits] How to detect function arity for functors with member template?
From: jesseperla (jesseperla_at_[hidden])
Date: 2009-07-27 09:23:06

I am interested in tag dispatching in a generic function depending on
whether the passed in argument is arity 1 or 2.

My problem is that I want to have a generic expectation operator that
is passed in a function to take the expectation over, and a
distribution for the integration.  The expectation needs to support
both real and mixed-integer measures and functions.  Consider the
following functions it should work with:

struct my_real_functor
{
  template<std::size_t N>
  double operator()(ublas::bounded_vector<double,N>& x) {...}
};

template<std::size_t N>
double my_real_function(ublas::bounded_vector<double,N>& x) {...}

struct my_mixed_functor
{
  template<std::size_t RealN, std::size_t IntN>
  double operator()(ublas::bounded_vector<double,RealN>& real_x,
                  ublas::bounded_vector<int,RealN>& int_x) {...}
};

.... naturally, part of the problem is that the functor could have
both specializations,
so I recognize this might be tricky.  If both are on a functor, then I
can default to
one of the implementations(probably the unary) for the matching.

---------------------------------------------------
For the operator itself, the signature would be something like.

<class F, class Distribution>
double expectation(F f, const Distribution& dist)
{
  .....// tag dispatch depending on F arity, or something similar.
}
..... or even better:

<class F, class Distribution>
double expectation(mpl::enable_if<has_unary,F>::type f, const
Distribution& dist)
{
  .....//unary implementation
}

<class F, class Distribution>
double expectation(mpl::enable_if<has_binary,F>::type f, const
Distribution& dist)
{
  .....// binary implementation.
}

.... or even better if I could fully match the signatures of the
passed in F for conformance.

------------------------------------------------------
I looked at boost function_traits but it doesn't work with member
functors....
http://www.boost.org/doc/libs/1_39_0/libs/type_traits/doc/html/boost_typetraits/reference/function_traits.html

Any ideas on how best to implement this?

Thanks as always,
Jesse