std::function vs function pointer [duplicate]

Question

• Is there any differences?

• Which is the best way to "save/transfer" function?

  function<void(int)> fcn = 
                      [](int par) {std::cout<<"fcn: "<<par<<std::endl; };
  void(*fcn_a)(int) = 
                      [](int par) {std::cout<<"fcn_a: "<<par<<std::endl; };
  
  
  fcn(12);
  fcn_a(12);
  

Answer 1

std::function is more generic - you can store in it any callable object with correct signature (function pointer, method pointer, object with operator()) and you can construct std::function using std::bind.

Function pointer can only accept functions with correct signature but might be slightly faster and might generate slightly smaller code.

Answer 2

In the case of a non-capturing lambda, using a function pointer would be faster than using std::function. This is because std::function is a far more general beast and uses type-erasure to store the function object passed to it. It achieves this through type-erasure, implying that you end up with a call to operator() being virtual.

OTOH, non-capturing lambdas are implicitly convertible to the corresponding function pointer. If you need a full-fledged closure however, you will have to assign the lambda to std::function, or rely on type deduction through templates, whenever possible.