I already know automatic move is not woking with the function which return value from Rvalue Reference input. But why?

Below is example code which automatic move is not working with.

Widget makeWidget(Widget&& w) {
  ....
  return w; // Compiler copies w. not move it.
}

If the function input is from copy by value, automatic move works.

Widget makeWidget(Widget w) {
  ....
  return w; // Compiler moves w. not copy it.
}

CodePudding user response：

The code should work as you expected since C 20; automatic move operation is performed when returning local variables and parameters, and it works for rvalue reference too (since C 20).

If expression is a (possibly parenthesized) id-expression that names a variable whose type is either

• a non-volatile object type or
• a non-volatile rvalue reference to object type (since C 20)

LIVE

CodePudding user response：

Relevant part of the C 17 standard [class.copy.elision/3]:

In the following copy-initialization contexts, a move operation might be used instead of a copy operation:

• If the expression in a return statement is a (possibly parenthesized) id-expression that names an object with automatic storage duration declared in the body or parameter-declaration-clause of the innermost enclosing function...

This does not hold for:

Widget makeWidget(Widget&& w)
{
  return w;
}

since w does not name an object with automatic storage duration (within the function).

In C 20, the situation is different:

An implicitly movable entity is a variable of automatic storage duration that is either a non-volatile object or an rvalue reference to a non-volatile object type. In the following copy-initialization contexts, a move operation might be used instead of a copy operation:

• If the expression in a return or co_­return statement is a (possibly parenthesized) id-expression that names an implicitly movable entity declared in the body or parameter-declaration-clause of the innermost enclosing function...

Here, the reference w satisfies those requirements since it is an rvalue reference declared in the parameter-declaration-clause.

Why the same was not until C 20? I cannot say; likely nobody proposed such a change before.