Question

Assuming I have:

• class A which is non-copyable
• class B which has as a member, const A& a (and takes an A in its constructer and sets it in its initialization list)
• a function A GenerateA();

Does this mean that it should be valid to do:
B(GenerateA())
?

i.e, does the const ref mean that no copy of the A that generateA() returns is done? And does that mean that the scope of the returned temporary is extended for as long as B exists?

EDIT: Addon question from the comments:
Is it acceptable to return a A& from GenerateA() to a local A, if the lvalue is a const A&?

Thanks!

Answer 1

As it has already been stated by others, A GenerateA() cannot compile if A is not copyable.

Regarding the const ref : no, the lifetime of the temporary will not be extended to the lifetime of B. The standard [12.2.5] states :

A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the constructor exits. […] A temporary bound to the returned value in a function return statement (6.6.3) persists until the function exits.

So yes, extension of the lifetime of a temporary exists in some contexts (and is sometime truly useful : see this article), but not in the one you presented.

Regarding your last question, it’s not legal to return a reference to a local variable from GenerateA() (and binding the result to a const reference won’t be of any help).