Simplest way to determine return type of function

Question

Given a very simple, but lengthy function, such as:

int foo(int a, int b, int c, int d) {
    return 1;
}

// using ReturnTypeOfFoo = ???

What is the most simple and concise way to determine the function's return type (ReturnTypeOfFoo, in this example: int) at compile time without repeating the function's parameter types (by name only, since it is known that the function does not have any additional overloads)?

Answer 1

You can leverage std::function here which will give you an alias for the functions return type. This does require C++17 support, since it relies on class template argument deduction, but it will work with any callable type:

using ReturnTypeOfFoo = decltype(std::function{foo})::result_type;

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We can make this a little more generic like

template<typename Callable>
using return_type_of_t = 
    typename decltype(std::function{std::declval<Callable>()})::result_type;

which then lets you use it like

int foo(int a, int b, int c, int d) {
    return 1;
}

auto bar = [](){ return 1; };

struct baz_ 
{ 
    double operator()(){ return 0; } 
} baz;

using ReturnTypeOfFoo = return_type_of_t<decltype(foo)>;
using ReturnTypeOfBar = return_type_of_t<decltype(bar)>;
using ReturnTypeOfBaz = return_type_of_t<decltype(baz)>;

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This technique only works if the function is not overloaded or the function do not have multiple operator()'s defined.