CS32 Class Composition question

#include <iostream>
#include <string>
using namespace std;

class A;
class B;

class A
{
public:
    A()
        :b(this)
    {
        n = "test";
        return;
    }
    void print_str()
    {
        cout << this->n;
    }
private:
    string n;
    B b;
};


class B
{
public:
    B(A* ptr_A)
    {
        m_ptr_A = ptr_A;
    }
    ~B()
    {
        m_ptr_A->print_str();
    }
private:
    A* m_ptr_A;
};




int main()
{
    A a;
}