Storing function return values ​​in a tuple

Question

Consider

#include <tuple>

template <typename... F>
auto execute (F... f) {
    return std::make_tuple(f(0)...);
}

int foo(int) { return 5; }
int bar(int) { return 3; }

int main() {
    auto tuple = execute(foo, bar);
}

What is a good workaround so that a bar can return a void?

I tried this, but it will not compile:

#include <tuple>

struct Void { };

template <typename T>
T check(T n) { return n; }

Void check(void) { return Void{}; }

template <typename... F>
auto execute (F... f) {
    return std::make_tuple(check(f(0))...);
}

int foo(int) { return 5; }
void bar(int) { }

int main() {
    auto tuple = execute(foo, bar);
}

Update. I have a preliminary solution, but it only works if we know that the arguments passed are always int 0. I will try to get it to work in any general settings. Or maybe use std :: optional as Steve suggested.

#include <tuple>
#include <type_traits>

struct Void { };

template <typename F>
auto execute_h (F f, std::enable_if_t<!std::is_void_v<std::result_of_t<F(int)>>>* = nullptr) {
    const auto result = f(0);
    return result;  
}

template <typename F>
auto execute_h (F f, std::enable_if_t<std::is_void_v<std::result_of_t<F(int)>>>* = nullptr) {
    f(0);
    return Void{};  
}

template <typename... F>
auto execute (F... f) {
    return std::make_tuple(execute_h(f)...);
}

int foo(int) { return 5; }
void bar(int) { }

int main() {
    auto tuple = execute(foo, bar);
}

Answer 1

You can use std::enable_ifa wrapper function to return an object Voidfor functors that return Void:

#include <tuple>
#include <type_traits>

struct Void { };

template <typename Func, typename... Args>
auto check(Func func, Args&&... args)
    -> std::enable_if_t<!std::is_void<decltype(func(std::forward<Args>(args)...))>::value, decltype(func(std::forward<Args>(args)...))>
{
    return func(std::forward<Args>(args)...);
}

template <typename Func, typename... Args>
auto check(Func func, Args&&... args)
    -> std::enable_if_t<std::is_void<decltype(func(std::forward<Args>(args)...))>::value, Void>
{
    func(std::forward<Args>(args)...); return Void{};
}

template <typename... F>
auto execute (F... f) {
    return std::make_tuple(check(f, 0)...);
}

int foo(int) { return 5; }
void bar(int) { }

int main() {
    auto tuple = execute(foo, bar);
}

Live demo

, , .

, , Void , , , - .

Answer 2

operator , ( void(), T operator,):

#include <tuple>

struct Void {};

template <typename T>
T&& operator , (T&& t, Void) { return std::forward<T>(t); }

template <typename... Fs>
auto execute (Fs... fs) {
    return std::make_tuple((f(0), Void{})...);
}

int foo(int) { return 5; }
void bar(int) { }

int main() {
    auto tuple = execute(foo, bar); // tuple<int, Void>
}