Should I use std::move or std::forward in move ctors/assignment operators?

Question

Unless I\'m wrong it seems like either works just fine - is there a best practice reason to prefer one over the other?

Example:

struct A
{
    A(){}
         


        

Answer 1

The question is: Are those really the move constructor / assignment operator for the class? Or do they only look like that from the corner of your eye?

struct X{
  X(X&&); // move ctor #1

  template<class T>
  X(T&&); // perfect forwarding ctor #2

  X& operator=(X&&); // move assignment operator #3

  template<class T>
  X& operator=(T&&); // perfect forwarding ass. operator #4
};

In a real move ctor (#1) and move assignment operator (#3), you will never use std::forward, since, as you correctly assessed, you will always move.

Note that std::forward never makes sense without a perfect forwarding template (T&&). That is exactly the case for #2 and #4. Here, you will never use std::move, since you don't know if you actually got an rvalue (A-OK) or an lvalue (not so much).

See this answer of mine for an explanation of how std::forward actually works.