C++多态公有继承的一个例子

如果希望同一个方法在派生类和基类中的行为是不同的，换句话说，方法的行为应取决于调用该方法的对象，这种较复杂的行为称为多态——具有多种形态，即同一个方法的行为随上下文而异。
经常使用的两种重要的机制可用于实现多态公有继承；

在派生类中重新定义基类的方法

使用虚函数

代码实现的功能比较简单，在实际应用中肯定还是要加很多功能的，这里仅仅是一个开户功能和取款的功能，主要还是为了熟悉C++的一些特性为主。

brass.h

//brass.h -- bank account classes
#ifndef BRASS_H_
#define BRASS_H_
#include <string>
//Brass Account class
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);
  virtual void Withdraw(double amt);
  double Balance() const;
  virtual void ViewAcct() const;
  virtual ~Brass() {}
};

//Brass Plus Account Class
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

brass.cpp

//brass.cpp -- bank account class methods
#include <iostream>
#include "brass.h"
using std::cout;
using std::endl;
using std::string;

//formatting stuff
typedef std::ios_base::fmtflags format;
typedef std::streamsize precis;
format setFormat();
void restore(format f, precis p);

Brass::Brass(const string& s, long int an, double bal)
{
  fullName = s;
  acctNum = an;
  balance = bal;
}

void Brass::Deposit(double amt)
{
  if(amt < 0)
    cout << "Negative Deposit not allowed; "
        << "Deposit is cancelled. \n";
  else
    balance += amt;
}

void Brass::Withdraw(double amt)
{
  //set up ###.## format 
  format initialState = setFormat();
  precis prec = cout.precision(2);

  if(amt < 0)
    cout << "Withdrawal amount must be positive; "
        << "Withdraw cancelled. \n";
  else if(amt <= balance)
    balance -= amt;
  else
    cout << "Withdrawal amount of $" << amt << " exceeds your  balance. \n"
        << "Withdrawal cancelled. \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: " << fullName << endl;
  cout << "Account Number: " << acctNum << endl;
  cout << "Balance: $" << balance << endl;
  restore(initialState, prec); //restore original format
}

//BrassPlus Methods
BrassPlus::BrassPlus(const string& s, long int an, double bal, double ml, double r): Brass(s, an, bal)
{
  maxLoan = ml;
  owesBank = 0.0;
  rate = r;
}


BrassPlus::BrassPlus(const Brass& ba, double ml, double r): Brass(ba)//use implicit copy constructor
{
  maxLoan = ml;
  owesBank = 0.0;
  rate = r;
}

//redefine how ViewAcct() works
void BrassPlus::ViewAcct() const
{
  //set up ###.## format
  format initialState = setFormat();
  precis prec = cout.precision(2);

  Brass::ViewAcct(); // display base portion
  cout << "Maximum loan: $" << maxLoan << endl;
  cout << "Owed to bank: $" << owesBank << endl;
  cout.precision(3); //###.## format
  cout << "Loan Rate: " << 100 * rate << " %\n";
  restore(initialState, prec);
}

//redefine how Withdraw() works
void BrassPlus::Withdraw(double amt)
{
  //set up ###.## format
  format initialState = setFormat();
  precis prec = cout.precision(2);

  double bal = Balance();
  if(amt <= bal)
    Brass::Withdraw(amt);
  else if(amt <= bal + maxLoan - owesBank)
  {
    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";
  restore(initialState, prec);
}

format setFormat()
{
  //set up ###.## format
  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);
}

main.cpp

//usebrass2.cpp -- polymorphic example
//compile with brass.cpp
#include <iostream>
#include <string>
#include <boost/concept_check.hpp>
#include "brass.h" 
const int CLIENTS = 4;

int main()
{
  using std::cin;
  using std::cout;
  using std::endl;

  Brass * p_clients[CLIENTS];
  std::string temp;
  long tempnum;
  double tempbal;
  char kind;

  for(int i = 0; i < CLIENTS; i++)
  {
    cout << "Enter client's name : ";
    getline(cin, temp);
    cout << "Enter client's account number: ";
    cin >> 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
    {
      double tmax, trate;
      cout << "Enter the overdraft limit: $";
      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;
    }
    cout << endl;
    for(int i = 0; i < CLIENTS; i++)
    {
      p_clients[i]->ViewAcct();
      cout << endl;
    }

    for(int i = 0; i < CLIENTS; i++)
    {
      delete p_clients[i]; // free memory
    }

    cout << "Done.\n";
    return 0;
}

最后运行的结果

参考文献：
Publishing S. C++ Primer Plus, Fifth Edition[J]. Pearson Schweiz Ag, 2005.