How do I avoid forward declarations?

Let's say I have two classes, A and B:

class B;

class A
{
private:
    int an_int;
    B *something_else;
public:
    A(int n) : an_int(n), something_else(nullptr) {}
};

class B
{
private:
    int an_int;
    A *something_else;
public:
    B(int n) : an_int(n), something_else(nullptr) {}
};

How can I make it so that I don't have to prototype B in order to have a pointer to a B object in class A?