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;

Answer 2

You also need to put A's constructor in C.cpp:

A.h

#include <memory>
#include <vector>

class B;

class A {
public:
     A();
    ~A();

private:
    std::unique_ptr<B> m_tilesets;
};

C.cpp

#include "A.h"

class B {

};

A::~A() {

}

A::A() {

}

See this answer. The constructor needs access to the complete type as well. This is so that it can call the deleter if an exception is thrown during construction.