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std::result_of

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Revision as of 02:00, 9 September 2013 by 109.12.67.237 (Talk)

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

Deduces the return type of a function call 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)
F and all types in ArgTypes can be any complete type, array of unknown bound, or (cv-qualified) void (since C++14)

Contents

Member types

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

Helper types

template< class T >
using result_of_t = typename result_of<T>::type;
(since C++14)

Possible implementation

template<class>
struct result_of;
 
// C++11 implementation, does not satisfy C++14 requirements
template<class F, class... ArgTypes>
struct result_of<F(ArgTypes...)>
{
    typedef decltype(
                     std::declval<F>()(std::declval<ArgTypes>()...)
                    ) type;
};

Notes

As formulated in C++11, std::result_of would fail to compile when F(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(Args...)> simply doesn't have the type member).

Examples

std::result_of can be used to determine the result of invoking a functor, in particular if the result type is different for different sets of arguments:

#include <type_traits>
 
struct S {
    double operator()(char, int&);
    float operator()(int);
};
 
struct C {
    double Func(char, int&);
};
 
int main()
{
    // the result of invoking S with char and int& arguments is double
    std::result_of<S(char, int&)>::type f = 3.14; // f has type double
    static_assert(std::is_same<decltype(f), double>::value, "");
 
    // the result of invoking S with int argument is float
    std::result_of<S(int)>::type d = 3.14; // f has type float
    static_assert(std::is_same<decltype(d), float>::value, "");
 
    // result_of can be used with a pointer to member function as follows
    std::result_of<decltype(&C::Func)(C, char, int&)>::type g = 3.14;
    static_assert(std::is_same<decltype(g), double>::value, "");
}


demonstrates the C++14 changes to result_of requirements

#include <type_traits>
#include <iostream>
 
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;
}
 
struct S {};
 
int main() {
  f<S>(1); // fails to compile in C++11, calls the non-callable overload in C++14
}
output=
overload of f for non-callable T


See also

(C++11)
obtains the type of expression in unevaluated context
(function template) [edit]