Comment by charcircuit
8 hours ago
What the callee does it out of scope. We are talking about a single assignment or construction of a variable. This has nothing to do with tracing execution. It happens at one place, and you can look at the place to see if it is using a copy or move contructor.
When talking C++ move semantics it's easy to talk past each other. So I'm not sure what your claim is. Another commenter said that one can tell if something is moved or not without looking at the body of the callee. Is that what you're saying? Because you can't.
I apologize if you're making a different claim, but I'm not clear on what that is.
Anyway, for my point, here's an example where neither copy nor move happens, which one can only know by looking at the body of the callee: https://godbolt.org/z/d7f6MWcb5
By changing only the callee we can cause a move: https://godbolt.org/z/b8M495Exq
Equally we can remove the use of `std::move` in the callee, and now it's instead a copy. (of course, in this example with `unique_ptr`, it means a build failure as `unique_ptr` is not copyable)
> [assignment or construction of a variable] happens at one place
Not sure what you mean by that. The callee taking an rvalue reference could first copy, then move, if it wants to. Or do neither (per my example above). Unlike in Rust, the copy/move doesn't get decided at the call point.
You can, at one point, statically determine if the (usually const) single ampersand reference function is called, or the rvalue reference function, via standard polymorphism. But that's not the point where the move cons/assign happens, so for that one has to look in the callee.