value_or's parameter type is U&&, so no there's no additional copy there. There's only the explicitly visible default constructor call.
But you did get me to take a second look at the post, and yeah the author's statement of "Finally, we copy construct an instance of A when the call to value_or terminates." is just incorrect. Try it out yourself: https://godbolt.org/z/hWqnrfGKE
If you replace the `.value_or(A())` with `.value()` there's no reduction in copies, only a single reduction in the constructed count which of course makes sense since the `A()` parameter isn't there.
The way you would optimize this to eliminate copies would be to do things like construct the optional in-place and take the parameter by const& instead, such as:
A f(const std::optional<A>& z) {
return z.value_or(A());
}
A g() {
std::optional<A> z(std::in_place, "message");
return f(z);
}
`std::in_place` being definitely a classic C++ "quirk" here and usually unlikely to be worth it but hey if you want to play "minimize the copy count" golf there you go. The value_or isn't actually adding any copies, though.
value_or's parameter type is U&&, so no there's no additional copy there. There's only the explicitly visible default constructor call.
But you did get me to take a second look at the post, and yeah the author's statement of "Finally, we copy construct an instance of A when the call to value_or terminates." is just incorrect. Try it out yourself: https://godbolt.org/z/hWqnrfGKE
If you replace the `.value_or(A())` with `.value()` there's no reduction in copies, only a single reduction in the constructed count which of course makes sense since the `A()` parameter isn't there.
The way you would optimize this to eliminate copies would be to do things like construct the optional in-place and take the parameter by const& instead, such as:
`std::in_place` being definitely a classic C++ "quirk" here and usually unlikely to be worth it but hey if you want to play "minimize the copy count" golf there you go. The value_or isn't actually adding any copies, though.