1.派生类和基类是一种is-a的关系，即派生类对象也是一个基类对象，可以对基类对象执行任何操作。

2.多态：同一个方法（c++中称为函数）在派生类和基类中的行为方式是不同的，换句话说就是方法的行为处决于调用该方法的对象。例如一个称为clean的方法，有的类执行扫地，有的类执行擦窗，有的类执行倒垃圾......但都是调用这个clean的方法。

两个重要的机制可以实现多态

（1）.在派生类中重新定义基类的方法。

（2）.使用虚方法。如果是通过引用或指针调用，程序将根据引用或指针指向的对象类型来选择方法。

3.派生类构造函数在初始化基类私有数据时，采用的是成员初始化列表语法

BrassPlus::BrassPlus(const std::string &s, long an, double bal, double ml, double r):Brass(s,an,bal)

brass 源码

1.brass.h 头文件

#ifndef BRASS_H

#define BRASS_H

#include<string>

class Brass

{

private:

    std::string fullName;

    long acctNum;

    double balance;

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);//取现，取回存款

    double Balance()const;//余额

    virtual void ViewAcct()const;//显示账号

    virtual ~Brass(){}

};

#endif // BRASS_H

1.brass.h 源文件

#include "brass.h"

#include<iostream>

using namespace std;

//确定小数格式及精度

typedef std::ios_base::fmtflags format;

typedef std::streamsize precis;

format setFormat();

void restore(format f,precis p);

Brass::Brass(const std::string &s, long an, double bal)

{

  fullName=s;

  acctNum=an;

  balance=bal;

}

//存款

void Brass::Deposit(double amt)

{

  if(amt<0)

      cout<<"Negative deposit not allowed; "

         <<"deposit is cancelled.\n";

  else

      balance +=amt;

}

//取款

void Brass::Withdraw(double amt)

{

   format initialState = setFormat();

   precis prec=cout.precision(2); //precision 准确

   //positive 正数

   if (amt<0)

       cout<<"Withdrawal amout must be positive;"

          <<"withdrawal canceled.\n";

   else if (amt<=balance)//取款数小于余额

       balance-=amt;

   else

       cout<<"Withdrawal amout of $"<<amt

          <<"exceeds your balance.\n"

         <<"withdrawal canceled.\n";

           restore(initialState,prec);

}

double Brass::Balance() const

{

  return balance;

}

void Brass::ViewAcct() const

{

    format initialState = setFormat();

        precis prec = cout.precision(2);

        cout << "Client: " << fullName << endl;

        cout << "Account Number: " << acctNum << endl;

        cout << "Balance: $" << balance << endl;

        restore(initialState, prec); // Restore original format

}

format setFormat()

{

    // set up ###.## format 设置显示格式

    return cout.setf(std::ios_base::fixed,

                std::ios_base::floatfield);

}

void restore(format f, precis p)

{

    cout.setf(f, std::ios_base::floatfield);

    cout.precision(p);//精度

}

brassplus 头文件

#ifndef BRASSPLUS_H

#define BRASSPLUS_H

#include"brass.h"

#include<iostream>

using namespace std;

class BrassPlus : public Brass

{

private:

    double maxLoan;

    double rate;

    double owesBank;

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 // BRASSPLUS_H

brassplus 源文件

#include "brassplus.h"

#include"brass.h"

typedef std::ios_base::fmtflags format;

typedef std::streamsize precis;

format setFormat();

void restore(format f, precis p);

BrassPlus::BrassPlus(const std::string &s, long an, double bal, double ml, double r):Brass(s,an,bal)

{

    maxLoan = ml;//最大贷款额度

        owesBank = 0.0;

        rate = r;//利率

}

//用于隐含拷贝构造函数

BrassPlus::BrassPlus(const Brass &ba, double ml, double r):Brass(ba)

{

    maxLoan = ml;

        owesBank = 0.0;

        rate = r;

}

//重新定义 显示账号的信息

void BrassPlus::ViewAcct() const

{

    format initialState = setFormat();

        precis prec = cout.precision(2);

        Brass::ViewAcct();   // display base portion  如果没写Brass：：制定域将会无穷递归调用

        cout << "Maximum loan: $" << maxLoan << endl;

        cout << "Owed to bank: $" << owesBank << endl;

        cout.precision(3);  // ###.### format

        cout << "Loan Rate: " << 100 * rate << "%\n";

        restore(initialState, prec);

}

void BrassPlus::Withdraw(double amt)

{

    format initialState = setFormat();

        precis prec = cout.precision(2);

       //需通过基类公有访问基类的私有变量 获取余额

        double bal = Balance();

        if (amt <= bal)

            Brass::Withdraw(amt);

        else if ( amt <= bal + maxLoan - owesBank)//存款+最大信用额度-欠款

        {

            double advance = amt - bal;

            owesBank += advance * (1.0 + rate);

            cout << "Bank advance: $" << advance << endl;

            cout << "Finance charge: $" << advance * rate << endl;

            Deposit(advance);

            Brass::Withdraw(amt);

        }

        else

            cout << "Credit limit exceeded. Transaction cancelled.\n";//Credit贷款

        restore(initialState, prec);

}