Unique_ptr and forward declaration

Question

Say I have two classes:

\"Foo.h\"

#pragma once    
class Foo
{
public:
    Foo()
    {

    };

    ~Foo()
    {

    };
};

\"A.h\"

Answer 1

It is no possible for 'A' to not have a constructor.

If you comment the constructor you wrote, the compiler will make a default constructor for you and it won't necessarily be in the same place you defined the one you made. Causing said problem.