Swapping private unique pointers with a public method

Question

I have a class with a private unique_ptr member and I want to expose a public swap function which will swap ownership of its unique_ptr member with that of the provided other. How can I do so?

class A{ // a is polymorphic type, derivatives A1, A2, etc
    private:
        unique_ptr<B> b;
    public:
        void swap_b(A??? other) {
            A.b.swap_c(other.b) // I want to swap A.b.c with other.b.c
        }
};

class B{ // b is polymorphic type
    private:
        unique_ptr<C> c; // c is also polymorphic type
    public:
        void swap_C(B??? other) {
            B.c.swap(other.c)
        }
};

int main() {
    unique_ptr<A> alphaOne = make_unique<A1>(...);
    unique_ptr<A> alphaTwo = make_unique<A2>(...);
    alphaOne.swap(alphaTwo); // the unique_ptr alphaOne.b.c should swap ownership with alphaTwo.b.c
}

I have a vector<unique_ptr<A>>, from which I want to grab any two elements and call swap on them to swap their b.c unique_ptrs. How can I achieve this?

Answer 1

I am unsure if I am missing something, but a simple reference should suffice:

#include <memory>
using namespace std;

class C {};

class B {
   private:
    unique_ptr<C> c;

   public:
    void swap(B& other){ c.swap(other.c); }
};

class A {
   private:
    unique_ptr<B> b;

   public:
    virtual ~A() = default; // See note 1.

    void swap(A& other){ b.swap(other.b); }
};

class A1:public A{};
class A2:public A{};


int main() {
    unique_ptr<A> alphaOne = make_unique<A1>();
    unique_ptr<A> alphaTwo = make_unique<A2>();
    alphaOne.swap(alphaTwo);
}

1. The destructor of the base class should be virtual when you destroy instances via a base class pointer.