企业官网建设流程全解析

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.