22. 向上造型

B 父类
D 子类

一个D的对象，可以交给B的变量.
一个D的指针，可以交给B的指针.
一个D的reference, 可以交给B的reference.

如果B是A的子类，那么B的对象可以被当做A的对象来看待/使用.

#include <iostream>using namespace std;class A{ public:   int i; public:   A():i(10) {}  };class B:public A{    };int main(){   A a;   B b;   cout << a.i << " " << b.i << endl; // 10 10   cout << sizeof(a) << " " << sizeof(b) << endl; //4 4    int *p = (int*)&a;   cout << p << " " << *p << endl; //0x7fff57659bf0 10   p = (int*)&b;   cout << p << " " << *p << endl; //0x7fff57659be8 10   return 0;}int main(){   A a;   B b;   cout << a.i << " " << b.i << endl; // 10 10   cout << sizeof(a) << " " << sizeof(b) << endl; //4 4    int *p = (int*)&a;   cout << p << " " << *p << endl; //0x7fff57659bf0 10   *p = 20;   cout << a.i << endl; // 20   p = (int*)&b;   cout << p << " " << *p << endl; //0x7fff57659be8 10   return 0;}

#include <iostream>using namespace std;class A{ public:   int i; public:   A():i(10) {}  };class B:public A{      private:    int j;  public:    B():j(30) {}    void f() { cout << " B.j = " << j << endl; }};int main(){   A a;   B b;   cout << a.i << " " << b.i << endl; // 10 10   cout << sizeof(a) << " " << sizeof(b) << endl; //4 8    int *p = (int*)&a;   cout << p << " " << *p << endl; //0x7fff57659bf0 10   *p = 20;   cout << a.i << endl; // 20   p = (int*)&b;   cout << p << " " << *p << endl; //0x7fff57659be8 10   p++;   *p = 50;   b.f(); //B.j = 50   return 0;}

子类的对象，当做父类的对象来看待，叫做upcasting(向上造型).

Manager是特殊的Employee.
Employee : 父类
Manager : 子类

Manager pete(“Pete”, “444-55-6666”, “Bakery”);
Employee* ep = &pete; // upcasting(向上造型)
Employee& er = pete; //upcasting(向上造型)