代码含义

#include <iostream>
using namespace std;

class A{
public:
    A() { cout<<"构造A"<<endl;}
    ~A(){ cout<<"==>销毁A"<<endl;}
};

class B:public A{
public:
    B() { cout<<"构造B"<<endl;}
    ~B(){ cout<<"==>销毁B"<<endl;}
};

class C:public B{
public:
    C() { cout<<"构造C"<<endl;}
    ~C(){ cout<<"==>销毁C"<<endl;}
    B b;
    A a;
};

void main(){
    C c;
}  请教代码中的 B b C c A a 是什么意思