有两种方式可以实现多态公有继承

1) 在派生类中重新定义基类的方法

2) 使用虚方法

如下是使用虚方法

brass.h

#ifndef BRASS_H#define BRASS_H#include 
    
     class Brass{private:    std::string fullName;  //客户姓名    long acctNum;  //账号    double balance;  //当前结余public:    Brass(const std::string & s="Nullbody",long an=-1,          double bal=0.0);    void Deposit(double amt);  //存款    virtual void withdraw(double amt);  //取款    double Balance() const; //显示余额    virtual void ViewAcct() const;    virtual ~Brass() {}};//Brass Plus Account Classclass BrassPlus:public Brass{private:    double maxLoan;  //透支的上线    double rate;  //透支贷款利率    double owesBank;  //当前的透支额度public:    BrassPlus(const std::string &s="Nullbody",long an=-1,              double bal=0.0,double ml=500,double r=0.11125);    BrassPlus(const Brass &ba,double ml=500,                              double r=0.11125);    virtual void ViewAcct() const;    virtual void withdraw(double amt);    void ResetMax(double m){maxLoan=m;};    void ResetRate(double r){rate=r;};    void ResetOwes() {owesBank=0;};};#endif // BRASS_H
    

main.cpp

#include 
    
     #include "brass.h"using namespace std;//formatting stufftypedef std::ios_base::fmtflags format;typedef std::streamsize precis;format setFormat();void restore(format f,precis p);//Brass methodsBrass::Brass(const string &s, long an, double bal){    fullName=s;    acctNum=an;    balance=bal;}//储蓄函数void Brass::Deposit(double amt){    if(amt<0)    {        cout<<"Negative deposit not allowed;"           <<"deposit is cancelled\n";    }    else        balance=balance+amt;}void Brass::withdraw(double amt){    //set up ###.##format    format initialState=setFormat();    precis prec=cout.precision(2);    if(amt<0)    {        cout<<"withdrawal amouut must be positive;"           <<"withdrawal canceled\n";    }    else if(amt<=balance)    {        balance=balance-amt;    }    else    {        cout<<"withrdrawl amout of $"<
     
      <<" exceeds your balance\n"          <<"withdrawal canceled\n";    }    restore(initialState,prec);}double Brass::Balance() const{    return balance;}void Brass::ViewAcct() const{    //set up ###.## format    format initialState=setFormat();    precis prec=cout.precision(2);    cout<<"Client:"<
      
       <
       
        >tempnum;        cout<<"Enter opening balance:$";        cin>>tempbal;        cout<<"Enter 1 for Brass Account or"           <<"2 for BrassPlus Account:";        while(cin>>kind &&(kind!='1' && kind!='2'))            cout<<"Enter either 1 or 2";        if(kind == '1')        {            p_clients[i]=new Brass(temp,tempnum,tempbal);        }        else  //输入的kind是2        {            double tmax,trate;            cout<<"Enter the overdraft limit:$";  //overdraft 透支            cin>>tmax;            cout<<"Enter the interest rate"               <<"as a decimal fraction:";            cin>>trate;            p_clients[i]=new BrassPlus(temp,tempnum,tempbal,tmax,trate);        }        while(cin.get()!='\n')            continue;    }  //for(int i=0;i
        
         ViewAcct();        cout<
        
       
      
     
    

运行效果如下

开发环境Qt Creator 4.0.3 (Community)