std::result_of,std::invoke_result (3) - Linux Manuals

std::result_of,std::invoke_result: std::result_of,std::invoke_result

NAME

std::result_of,std::invoke_result - std::result_of,std::invoke_result

Synopsis

Defined in header <type_traits>
template< class >
class result_of; // not defined (since C++11)
                                       (1) (deprecated in C++17)
template< class F, class... ArgTypes > (removed in C++20)
class result_of<F(ArgTypes...)>;
template< class F, class... ArgTypes> (2) (since C++17)
class invoke_result;

Deduces the return type of an INVOKE expression at compile time.

F must be a callable type, reference to function, or reference to callable type. Invoking F with ArgTypes... must be a well-formed expression. (since C++11)
                                                                                                                                               (until C++14)
F and all types in ArgTypes can be any complete type, array of unknown bound, or (possibly cv-qualified) void. (since C++14)

Member types

Member type Definition
            the return type of the Callable type F if invoked with the arguments ArgTypes....
type Only defined if F can be called with the arguments ArgTypes... in unevaluated context.
            (since C++14)

Helper types

template< class T > (since C++14)
using result_of_t = typename result_of<T>::type; (1) (deprecated in C++17)
                                                                          (removed in C++20)
template< class F, class... ArgTypes> (2) (since C++17)
using invoke_result_t = typename invoke_result<F, ArgTypes...>::type;

Possible implementation

  namespace detail {
  template <class T>
  struct is_reference_wrapper : std::false_type {};
  template <class U>
  struct is_reference_wrapper<std::reference_wrapper<U>> : std::true_type {};

  template<class T>
  struct invoke_impl {
      template<class F, class... Args>
      static auto call(F&& f, Args&&... args)
          -> decltype(std::forward<F>(f)(std::forward<Args>(args)...));
  };

  template<class B, class MT>
  struct invoke_impl<MT B::*> {
      template<class T, class Td = typename std::decay<T>::type,
          class = typename std::enable_if<std::is_base_of<B, Td>::value>::type
      >
      static auto get(T&& t) -> T&&;

      template<class T, class Td = typename std::decay<T>::type,
          class = typename std::enable_if<is_reference_wrapper<Td>::value>::type
      >
      static auto get(T&& t) -> decltype(t.get());

      template<class T, class Td = typename std::decay<T>::type,
          class = typename std::enable_if<!std::is_base_of<B, Td>::value>::type,
          class = typename std::enable_if<!is_reference_wrapper<Td>::value>::type
      >
      static auto get(T&& t) -> decltype(*std::forward<T>(t));

      template<class T, class... Args, class MT1,
          class = typename std::enable_if<std::is_function<MT1>::value>::type
      >
      static auto call(MT1 B::*pmf, T&& t, Args&&... args)
          -> decltype((invoke_impl::get(std::forward<T>(t)).*pmf)(std::forward<Args>(args)...));

      template<class T>
      static auto call(MT B::*pmd, T&& t)
          -> decltype(invoke_impl::get(std::forward<T>(t)).*pmd);
  };

  template<class F, class... Args, class Fd = typename std::decay<F>::type>
  auto INVOKE(F&& f, Args&&... args)
      -> decltype(invoke_impl<Fd>::call(std::forward<F>(f), std::forward<Args>(args)...));

  } // namespace detail

  // Minimal C++11 implementation:
  template <class> struct result_of;
  template <class F, class... ArgTypes>
  struct result_of<F(ArgTypes...)> {
      using type = decltype(detail::INVOKE(std::declval<F>(), std::declval<ArgTypes>()...));
  };

  // Conforming C++14 implementation (is also a valid C++11 implementation):
  namespace detail {
  template <typename AlwaysVoid, typename, typename...>
  struct invoke_result { };
  template <typename F, typename...Args>
  struct invoke_result<decltype(void(detail::INVOKE(std::declval<F>(), std::declval<Args>()...))),
                   F, Args...> {
      using type = decltype(detail::INVOKE(std::declval<F>(), std::declval<Args>()...));
  };
  } // namespace detail

  template <class> struct result_of;
  template <class F, class... ArgTypes>
  struct result_of<F(ArgTypes...)> : detail::invoke_result<void, F, ArgTypes...> {};

  template <class F, class... ArgTypes>
  struct invoke_result : detail::invoke_result<void, F, ArgTypes...> {};

Notes

As formulated in C++11, the behavior of std::result_of is undefined when INVOKE(std::declval<F>(), std::declval<ArgTypes>()...) is ill-formed (e.g. when F is not a callable type at all). C++14 changes that to a SFINAE (when F is not callable, std::result_of<F(ArgTypes...)> simply doesn't have the type member).
The motivation behind std::result_of is to determine the result of invoking a Callable, in particular if that result type is different for different sets of arguments.
F(Args...) is a function type with Args... being the argument types and F being the return type. As such, std::result_of suffers from several quirks that lead to its deprecation in favor of std::invoke_result in C++17:

* F cannot be a function type or an array type (but can be a reference to them);
* if any of the Args has type "array of T" or a function type T, it is automatically adjusted to T*;
* neither F nor any of Args... can be an abstract class type;
* if any of Args... has a top-level cv-qualifier, it is discarded;
* none of Args... may be of type void.

To avoid these quirks, result_of is often used with reference types as F and Args.... For example:

  template<class F, class... Args>
  std::result_of_t<F&&(Args&&...)> // instead of std::result_of_t<F(Args...)>, which is wrong
    my_invoke(F&& f, Args&&... args) {
      /* implementation */
  }

Examples

// Run this code

  #include <type_traits>
  #include <iostream>

  struct S {
      double operator()(char, int&);
      float operator()(int) { return 1.0;}
  };

  template<class T>
  typename std::result_of<T(int)>::type f(T& t)
  {
      std::cout << "overload of f for callable T\n";
      return t(0);
  }

  template<class T, class U>
  int f(U u)
  {
      std::cout << "overload of f for non-callable T\n";
      return u;
  }

  int main()
  {
      // the result of invoking S with char and int& arguments is double
      std::result_of<S(char, int&)>::type d = 3.14; // d has type double
      static_assert(std::is_same<decltype(d), double>::value, "");

      // the result of invoking S with int argument is float
      std::result_of<S(int)>::type x = 3.14; // x has type float
      static_assert(std::is_same<decltype(x), float>::value, "");

      // result_of can be used with a pointer to member function as follows
      struct C { double Func(char, int&); };
      std::result_of<decltype(&C::Func)(C, char, int&)>::type g = 3.14;
      static_assert(std::is_same<decltype(g), double>::value, "");

      f<C>(1); // may fail to compile in C++11; calls the non-callable overload in C++14
  }

Output:

  overload of f for non-callable T