Finding out the return type of a function, lambda or function

Question

This seems to be solved for the case of lambdas in this question. But that one is a 2011 answer and I\'m looking for a general case: lambdas, regular functions, and functors

Answer 1

Assuming that:

1. You want only the return type.
2. You don't know what are/will be the types of arguments (so neither decltype() nor std::result_of<> is an option.
3. The functor object passed as argument will not have an overloaded or generic operator().

then you can use the below trait that infers the return type of any functor object:

template 
struct return_type_impl;

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type_impl { using type = R; };

template 
struct return_type
    : return_type_impl {};

template 
struct return_type
    : return_type_impl {};

template 
using return_type_t = typename return_type::type;

Tests:

#include 

template 
void test(F h)
{
    static_assert(std::is_same, int>{}, "!");

    return_type_t i = 1;
}

int function(int i) { return 2*i; }

int c_varargs_function(...) { return 1; }

struct A
{
    int mem_function(double, float) { return 1; } 
};

int main()
{
    // Function
    test(function);

    // C-style variadic function
    test(c_varargs_function);

    // Non-generic lambda
    test([](int i) { return 2*i; });

    // Member function
    test(&A::mem_function);
}

DEMO