C++ meta function that determines if a type is callable for supplied arguments

Question

I am trying to implement a C++ template meta function that determines if a type is callable from the method input arguments.

i.e. for a function void foo(double, double) the meta function would return true for callable_t<foo, double, double>, true for callable_t<foo, int, int> (due to compiler doing implicit cast) and false for anything else such as wrong number of arguments callable_t<foo, double>.

My attempt is as follows, however it fails for any function that returns anything other than void and I can't seem to fix it.

I am new to template reprogramming so any help would be appreciated.

#include <iostream>
#include <type_traits>
#include <utility>
#include <functional>

namespace impl
{

template <typename...>
struct callable_args
{
};

template <class F, class Args, class = void>
struct callable : std::false_type
{
};

template <class F, class... Args>
struct callable<F, callable_args<Args...>, std::result_of_t<F(Args...)>> : std::true_type
{
};

}

template <class F, class... Args>
struct callable : impl::callable<F, impl::callable_args<Args...>>
{
};

template <class F, class... Args>
constexpr auto callable_v = callable<F, Args...>::value;


int main()
{
    {
        using Func = std::function<void()>;
        auto result = callable_v<Func>;
        std::cout << "test 1 (should be 1) = " << result << std::endl;
    }

    {
        using Func = std::function<void(int)>;
        auto result = callable_v<Func, int>;
        std::cout << "test 2 (should be 1) = " << result << std::endl;
    }

    {
        using Func = std::function<int(int)>;
        auto result = callable_v<Func, int>;
        std::cout << "test 3 (should be 1) = " << result << std::endl;
    }

    std::getchar();

    return EXIT_SUCCESS;
}

I am using a compiler that supports C++ 14.

Answer 1

The shorten use of std::result_of to do what you want could look as follows:

template <class T, class, class... Args>
struct callable: std::false_type {
};

template <class T, class... Args>
struct callable<T, decltype(std::result_of_t<T(Args...)>(), void()), Args...>:std::true_type {
};

template <class F, class... Args>
constexpr auto callable_v = callable<F, void, Args...>::value;

[live demo]

you need to remember that type returned by result_of is always the result type of a function you pass to this trait by type. To let your sfinae work you need a method to change this type to void in every possible situation. You can accomplish it by using the trick with decltype (decltype(std::result_of_t<T(Args...)>(), void())).

Edit:

To elaborate the thread from comments about the possible drawbacks of the solution. The std::result_of_t<T(Args...)> type don't need to be equipped with a default non-parametric constructor and as such the sfinae may cause false negative result of callable_v for function that result in this kind of types. In comments I proposed a workaround for the issue that does not really solve the problem or actually generate a new one:

decltype(std::declval<std::result_of_t<T(Args...)>*>(), void())

Intention of this code was to make sfinae work as in previously proposed solution but in case of non-constructable types to create an easy to construct (I thought) object of pointer to given type... In this reasoning I didn't take into consideration the types that one cannot create a pointer to e.g. references. This one again can be workaround by using some additional wrapper class:

decltype(std::declval<std::tuple<std::result_of_t<T(Args...)>>*>(), void())

or by decaying the result type:

decltype(std::declval<std::decay_t<std::result_of_t<T(Args...)>>*>(), void())

but I think it might not be worth it and maybe use of void_t is actually a more straightforward solution:

template <class...>
struct voider {
    using type = void;
};

template <class... Args>
using void_t = typename voider<Args...>::type;

template <class T, class, class... Args>
struct callable: std::false_type {
};

template <class T, class... Args>
struct callable<T, void_t<std::result_of_t<T(Args...)>>, Args...>:std::true_type {
};

template <class F, class... Args>
constexpr auto callable_v = callable<F, void, Args...>::value;

[live demo]

Answer 2

Here's how I'd approach this:

namespace detail {

template<typename Func, typename...Params> static auto helper(int) ->
    decltype((void)std::declval<Func>()(std::declval<Params>()...), std::true_type{});

template<typename Func, typename...Params> static std::false_type helper(...);

}

template<typename Func, typename... Params> struct callable:
    decltype(detail::helper<Func, Params...>(0)){};

template <class F, class... Args> constexpr auto callable_v =
    callable<F, Args...>::value;

demo

It's a poor man's version of C++1z's is_callable, but it doesn't handle pointers to members. Other than that, I think it's fine.

Answer 3

The problem with your original code is that you're using the parameter pack in a non deducible context

namespace impl
{

  template <class F, class... Args>
  struct callable : std::false_type
  {
  };

  template <class F, class... Args>
  struct callable<F, std::result_of_t<F(Args...)>> : std::true_type
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  {
  };

}

At this point in the source code parsing there might be no way that std::result_of_t<Func, int> can yield another return value, but there might be another specialization later in the file as in the following (very perverted) snippet

namespace std {

    template <>
    struct result_of<Func(int)> {
        using type = double;
    };
}

therefore your compiler should check all of them at the same time before being able to pick up the correct one.

That is also the reason why workarounds like

template< class... > using void_t = void;

namespace impl
{

  template <typename...>
  struct callable_args
  {
  };

  template <class F, class Args, class = void>
  struct callable : std::false_type
  {
  };

  template <class F, class... Args>
  struct callable<F, callable_args<Args...>, void_t<std::result_of_t<F(Args...)>>> : std::true_type
  {
  };

}

work in your case: they help the compiler solve the dependent type as something that always resolves to void. Remember that the code above is a workaround and you should rather use is_callable (C++17) or study how is_callable is implemented and get an insight into its technical challenges.