What is the most efficient way to pass a non generic function?

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I am learning functional programming in C++. My intention is to pass a non generic function as argument. I know about the template method, however I would like to restrict the function signature as part of the API design. I worked out 4 different methods example on cpp.sh:

// Example program #include <iostream> #include <string> #include <functional>  typedef int(functor_type)(int);   int by_forwarding(functor_type &&x)  {     return x(1); }  int functor_by_value(functor_type x) {     return x(1); }  int std_func_by_value(std::function<functor_type> x) {     return x(1); }  int std_func_by_forwarding(std::function<functor_type> &&x) {     return x(1); }  int main() {     std::cout << functor_by_value([](int a){return a;}); // works     std::cout << std_func_by_value([](int a){return a;}); // works     std::cout << std_func_by_forwarding(std::move([](int a){return a;})); // works      //std::cout << by_forwarding([](int a){return a;}); // how to move lambda with forwarding ? } 

Is any of the above attempts correct? If not, how do i achieve my goal?

 


(based on clarification from comments)

Signature can be restricted by using std::is_invocable:

template<typename x_Action> auto functor_by_value(x_Action && action) {     static_assert(std::is_invocable_r_v<int, x_Action, int>);     return action(1); } 

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