How do I determine if a type is callable with only const references?

Question

I want to write a C++ metafunction is_callable that defines value to be true, if and only if the type F has the function cal

Answer 1

Something like this maybe? It's a bit round about to make it work on VS2010.

template
struct function_traits_impl;

template
struct function_traits_impl
{
    typedef A1 arg1_type;
};

template
struct function_traits_impl
{
    typedef A1 arg1_type;
};

template
struct function_traits_impl
{
    typedef A1 arg1_type;
};

template
typename function_traits_impl::arg1_type arg1_type_helper(T);

template
struct function_traits
{
    typedef decltype(arg1_type_helper(&F::operator())) arg1_type;
};

template
struct is_callable : public std::is_same::arg1_type, const Arg&>
{
}