Why does copy elision make an exception for formal parameters?

Question

Here is a complete program:

#include 
using std::cout;
using std::endl;
using std::move;

int count {0};  // global for monitoring


class Triple         


        

Answer 1

If copy elision is disabled, both cases consist of 1 copy followed by 3 moves. (Any output of 2 for your code would indicate a compiler bug).

The copy is:

• initialization of left

and the moves are:

• initialization of return value from left
• initialization of temporary object denoted by a+b from return value
• initialization of result from a+b

It would be more illuminating to replace count with an output message saying "in copy constructor" and "in move constructor" . Currently you are not tracking moves at all.

In the pass-by-reference case , all 3 moves can be elided. In the pass-by-value case, 2 of the moves can be elided. The move which cannot be elided is the move from left to the return value.

I don't know the rationale for why this move can't be elided. Maybe it'd be difficult for a compiler to do something like A a = foo( A() ); if A() were elidable all the way up to a.