initializing a non-copyable member (or other object) in-place from a factory function

Question

A class must have a valid copy or move constructor for any of this syntax to be legal:

C x = factory();
C y( factory() );
C z{ factory() };

In

Answer 1

Issues like this were among the prime motivations for the change in C++17 to allow these initializations (and exclude the copies from the language, not merely as an optimization).