Изучение конструкторов ООП

#include <stdio.h>
#include <iostream>

struct A1{
public:
    A1(){
        std::cout << "A1" << std::endl;
    }
    ~A1(){
        std::cout << "~A1" << std::endl;
    }
};

struct A2{
public:
    A2(){
        std::cout << "A2" << std::endl;
    }
    ~A2(){
        std::cout << "~A2" << std::endl;
    }
};

struct B{
public:
    B(){
        std::cout << "B" << std::endl;
    }
    ~B(){
        std::cout << "~B" << std::endl;
    }
private:
    A1 a;
};

struct C : public B{
public:
    C(){
        std::cout << "C" << std::endl;
    }
    ~C(){
        std::cout << "~C" << std::endl;
    }
private:
    A2 a;
};

int main()
{
    C c;
}