invoke_result

Deduces the return type of the expression passed. This deduction happens at compile time.

template < class F, class... ArgTypes >
using invoke_result_t = typename invoke_result <F, ArgTypes...>::type;

Code

#include <type_traits>

std::string intToString(int x) {
    return std::to_string(x);
}

int main() {
    // 1. Get the return type at compile-time
    using MyReturnType = std::invoke_result_t <decltype(intToString), int>

    // 2. Ask the compiler: "Is MyReturnType the same as std::string?"
    // This check happens instantly while compiling, not while running!
    if constexpr (std::is_same_v < MyReturnType, std::string>) {
        std::cout << "Return type is string!" << std::endl;
    } else {
        std::cout << "Return type is something else." << std::endl;
    }

    return 0;
}