C++ copy constructor called at return

Question

error: use of deleted function 'A::A(const A&)'  return tmp;         ^~~ 

Why is the copy constructor called only when there is a virtual destructor in A? How to avoid this?

struct B {};  struct A{     std::unique_ptr<B> x;     virtual ~A() = default; };  A f() {     A tmp;     return tmp; } 

Answer 1

virtual ~A() = default; is a user declared destructor. Because of that, A no longer has a move constructor. That means return tmp; can't move tmp and since tmp is not copyable, you get a compiler error.

There are two ways you can fix this. You can add a move constructor like

struct A{     std::unique_ptr<B> x;      A() = default; // you have to add this since the move constructor was added     A(A&&) = default; // defaulted move     virtual ~A() = default; }; 

or you can create a base class that has the virtual destructor and inherit from that like

struct C {     virtual ~C() = default; };  struct A : C {     std::unique_ptr<B> x; }; 

This works because A no longer has a user declared destructor (Yes, C does but we only care about A) so it will still generate a move constructor in A. The important part of this is that C doesn't have a deleted move constructor, it just doesn't have one period, so trying to move it will cause a copy. That means C's copy constructor is called in A's implicitly generated move constructor since C(std::move(A_obj_to_move_from)) will copy as long as it doesn't have a deleted move constructor.