c++学习笔记（三）多态性

一、概述

本文介绍了C++编程中的多态性的概念及用法，多态性即是同样的消息发给不同派生类对象会产生多种形式的结果。

二、使用Virtual函数和动态绑定实现多态性

通过基类的指针或引用（指针句柄或引用句柄）调用一个virtual函数时，C++动态地（在运行时）根据对象初始化的类选择正确的函数来执行。

代码如下：

//CommissionEmployee.h#ifndef COMMISSION_H#define COMMISSION_H#include <string>#include <iostream>using std::string;using std::cout;class CommissionEmployee{public:CommissionEmployee( const string &yourName ,const string &yourNumber,double gSales = 0.0,double cRate=0.0):name(yourName),number(yourNumber){setGrossSales(gSales);setCommissionRate(cRate);}void setName(const string &yourName ){name =yourName;}string getName()const{return name;}void setNumber(const string &yourNumb ){name =yourNumb;}string getNumber()const{return number;}void setGrossSales(double sales ){grossSales = sales>0? sales:0;}double getGrossSales()const{return grossSales;}void setCommissionRate( double rate){commissionRate = (rate<1.0&&rate>0.0)? rate:0.0;}double getCommissionRate()const{return commissionRate;}virtual double earning()const{return getGrossSales()*getCommissionRate();}virtual void print()const{cout<<"Name: "<<getName()<<"\nNumber: "<<getNumber()<<"\nGross sales: "<<getGrossSales()<<"\nCommission rate: "<<getCommissionRate();}private:string name;string number;double grossSales;double commissionRate;};#endif

//BasePlusCommissionEmployee.h#ifndef BASEPULS_H#define BASEPULS_H#include <string>using std::string;#include "CommissionEmployee.h"class BasePlusCommissionEmployee : public CommissionEmployee{public:BasePlusCommissionEmployee(const string &yourName,const string &yourNumber,double sales,double rate,double salary):CommissionEmployee(yourName,yourNumber,sales ,rate){setBaseSalary(salary);}void setBaseSalary(double yourSalary){baseSalary = yourSalary;}double getBaseSalary()const{return baseSalary;}virtual double earning()const{return getBaseSalary()+CommissionEmployee::earning();}virtual void print()const{CommissionEmployee::print();cout<<"\nbase salary: "<<getBaseSalary();}private:double baseSalary;};#endif 

以上为BasePluseCommissionEmployee类和CommissionEmployee类，前者是后者的派生，两者均含有函数doubleearning()const和void print()const，派生类

BasePluseCommissionEmployee类增加一个数据成员double baseSalary，并且重写了double earning()const和void print()const。main()函数如下：

#include <iostream>#include <iomanip>using namespace std;#include "BasePlusCommissionEmployee.h"#include "CommissionEmployee.h"int main(){CommissionEmployee commissionEmployee("Jay","1111",10000, 0.06);                 //初始化基类对象CommissionEmployee *commissionEmployeePtr =NULL; BasePlusCommissionEmployee basePlusCommissionEmployee("Ken","2222" ,8000,0.05 ,400);             //初始化派生类对象BasePlusCommissionEmployee *basePlueCommissionEmployeePtr =NULL;cout<<"使用句柄： \n";commissionEmployee.print();cout<<"\n\n";basePlusCommissionEmployee.print();cout<<"\n\n使用指针： \n";commissionEmployeePtr = &commissionEmployee;   //指向基类对象的基类指针basePlueCommissionEmployeePtr = &basePlusCommissionEmployee;  //指向派生类对象的派生类指针commissionEmployeePtr->print();cout<<"\n\n";basePlueCommissionEmployeePtr->print();cout<<"\n\n";commissionEmployeePtr = &basePlusCommissionEmployee;  //指向派生类的基类指针//BasePlusCommissionEmployee = &commissionEmployee;   //编译不通过commissionEmployeePtr->print();                       //指向派生类的基类指针动态地调用了派生类的成员函数      //如果不设置为virtual函数，则此处调用的是基类的成员函数cout<<"\n\n";return 0;}

 

可以看出将基类和派生类的成员函数doubleearning()const和void print()const前加上Virtual声明为虚函数，并且将派生类对象地址赋给基类指针，基类指针调用earning()和print()函数时，即可调用派生类的相应成员函数；相反，当不使用Virtual时，基类指针调用的是基类自己的earning()和print()函数。当基类中的虚函数无意义时，可以将其声明为纯虚函数，及没有任何实现的函数

如Virtualearing() const =0或Virtual earing() =0;

这样在派生类中必须给出这一虚函数的各自的实现，也即各自重载基类的纯虚函数

结果

三、使用向下强制类型转换dynam_cast实现多态性

<span style="font-size:10px;">BasePlusCommissionEmployee *derivePtr =                                //使用向下强制转换符dynamic_castdynamic_cast<BasePlusCommissionEmployee *>(commissionEmployeePtr); //将基类指针转换为派生类指针derivePtr derivePtr->print();  </span>

此方法也可实现调用派生类的成员函数

四、Virtual析构函数

当删除一个具有非虚析构函数的派生类对象，而一个基类指针却指向了该对象，则对它应用delete运算符，C++标准会指出这一行为未定义。这种问题的简单方法，即在基类中创建virtua析构函数（在析构函数前加上virtual关键字），即使所有派生类的析构函数与基类的析构函数不同名。这样，如果对一个基类指针用delete运算符来显示删除它所指的类层次中的某个对象，那么系统会根据指针所指对象调用相应类的析构函数。基类的析构函数在派生类析构函数调用之后调用。