关于基类和派生类的动态绑定问题

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关于基类和派生类的动态绑定问题

Example 1

Quote是基类

class Quote{public:    //constructor and other member function    virtual std::ostream& debug (std::ostream& os)     {         os << bookNo << " " << price << " ";         return os;     }//member dataprivate: }

Bulk_quote是派生类

class Bulk_quote : public Quote{public:    // constructor    std::ostream& debug(std::ostream& os) override    {        Quote::debug(os) << quantity << " " << discount;        return os;    }};

int main(){    Quote a("12232", 12.3);    Bulk_quote b("3344", 22.2, 8, 0.13);//    Disc_quote c;    Quote *p1 = &a;    Quote *p2 = &b;    Quote &pt1 = a;    Quote &pt2 = b;    ///////////////////////    a.debug(std::cout)<< endl;    b.debug(std::cout)<< endl;    p1->debug(std::cout)<< endl;    p2->debug(std::cout)<< endl;    pt1.debug(std::cout)<< endl;    pt2.debug(std::cout)<< endl;}

运行结果

p2和pt2的静态类型是基类的指针或者引用，但是被指向或者绑定在了派生类上，而debug()成员函数是virtual function，所以在用指针和引用的时候编译器就要判断选择基类还是派生类

Example 2

class Base{public:    virtual void fn1 () {std::cout << "Base::fn1() operated" << std::endl;}    void fn2() {std::cout << "Base::fn2() operated" << std::endl;}};class Derived: public Base{public:    void fn1() override {std::cout << "Derived::fn1() operated" << std::endl;}    void fn2() {std::cout << "Derived::fn2() operated" << std::endl;}};

int main(){    Derived der;    Derived *der_ptr;    Base *base_ptr;    der_ptr = &der;    base_ptr = &der;    der_ptr->fn1();    der_ptr->fn2();    base_ptr->fn1();    base_ptr->fn2();}

运行结果

这里写图片描述

动态绑定只对virtual function 有用，由于fn2 是基类中的普通成员函数，则 base_ptr 虽然指向一个派生类，其调用的函数还是派生类对象中的基类部分中的成员函数

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