Can't use std::unique_ptr with T being a forward declaration

Question

Now first, I am aware of the general issues with unique_ptr<> and forward declarations as in Forward declaration with unique_ptr? .

Consider these three files:

Answer 1

The implicitly defined special member functions are inline, leading to issues with incomplete types. As the link from Chris's answer shows, all non-delegating constructors could potentially invoke the destructor. This includes copy(deleted in this case) and move constructors as well. So, when you are dealing with non-static members involving incomplete types, explicitly default the definitions in the source file, thus ensuring that they aren't defined inline.

In header:

A();
~A();
A(const A&);
A(A&&);

In source:

A::A() = default;
A::~A() = default;
A::A(const A&) = default;
A::A(A&&) = default;