check if type can be explicitly converted

Question

How to determine (in the <type_traits> spirit) whether or not one type explicitly convertible into another type? For example, I want to check the presence of F::explicit operator double const & () const; for some class/struct F, but, at the same time, F should not been explicitly convertible to float or long double (something like pred< double const & >::value && !pred< float >::value && !pred< long double >::value).

Note, that std::is_convertible< From, To >::value checks "if From can be converted to To using implicit conversion". But I wish to determine whether there is the explicit conversion operator.

And, if it possible, the "how to determine, type From is convertible into a namely reference to type To?"?

Answer 1

You need to define your own:

template <class U, class T>
struct is_explicitly_convertible
{    
  enum {value = std::is_constructible<T, U>::value && !std::is_convertible<U, T>::value};
};