Enable if to check whether the passed type is a compliant function?

Question

Consider the following code:

template <class Function, class = typename std::enable_if</*SOMETHING*/>::type>
apply(Function&& f);

which execute the provided function.

To use SFINAE, I would like to check whether the function satisfy the following conditions:

• it takes one parameter convertible to const unsigned int
• it returns something convertible to a std::string

What do I have to write instead of /*SOMETHING*/ ?

Answer 1

You haven't specified a return value for apply, I'll choose int:

#include <string>
#include <type_traits>

template <class Function,
  class = typename std::enable_if<
    std::is_convertible<
      decltype(std::declval<Function>()(std::declval<const unsigned int>())),std::string>::value>::type>
int apply(Function&& f);

Breaking it down: this ensures that a Function:

std::declval<Function>()

called with a const unsigned int parameter:

std::declval<Function()(std::declval<const unsigned int>())

has a return type:

decltype(std::declval<Function()(std::declval<const unsigned int>()))

that is convertible to std::string:

std::is_convertible<decltype(std::declval<Function()(std::declval<const unsigned int>())), std::string>

whew.