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目录

本章小结

       继承通过使用已有的类（基类）定义新的类（派生类），使得能够根据需要修改编程代码。公有继承建立is-a关系，这意味若派生类对象也应该是某种基类对象。作为is-a模型的一部分，派生类继承基类的数据成员和大部分方法，但不继承基类的构造函数、析构函数和赋值运算符。派生类可以直接访问基类的公有成员和保护成员，并能够通过基类的公有方法和保护方法访问基类的私有成员。可以在派化类中新增数据成员和方法，还可以将派生类用作基类，来做进一步的开发。每个派生类都必须有自己的构造函数。程序创建派生类对象时，将首先调用基类的构造函数，然后调用派生类的构造函数；程序删除对象时，将首先调用派生类的析构函数，然后调用基类的析构函数。

       如果要将类用作基类，则可以将成员声明为保护的，而不是私有的，这样，派生类将可以直接访问这些成员。然而，使用私有成员通常可以减少出现编程问题的可能性。如果希望派生类可以重新定义基类的方法，则可以使用关键字virtual将它声明为虚的。这样对于通过指针或引用访问的对象，能够根据对象类型来处理，而不是根据引用或指针的类型来处理。具体地说，基类的析构函数通常应当是虚的。

       可以考虑定义一个ABC：只定义接口，而不涉及实现。例如，可以定义抽象类Shape,然后使用它派生出具体的形状类，如Circle和Square。ABC必须至少包含一个纯虚方法，可以在声明中的分号前面加h=0 来声明纯虚方法，

       virtual double area{} const = 0；

       不一定非得定义纯虚方法。对于包含纯虚成员的类，不能使用它来创建对象。纯虚方法用于定义派生类的通用接口。

程序清单

一个简单的基类

13.1 tabtenn0.h

//tabtenn0.h -- a table-tennis base class#ifndef TABTENN0_H_#define TABTENN0_H_#include 
   
    using std::string;class TableTennisPlayer{private:	string firstname;	string lastname;	bool hasTable;public:	TableTennisPlayer(const string & fn = "none", const string & ln = "none", bool ht = false);	void Name() const;	bool HasTable() const { return hasTable; }	void ResetTable(bool v) { hasTable = v; }};#endif // !TABTENN0_H_
   

13.2 tabtenn0.cpp

//tabtenn0.cpp -- simple base-class methods#include 
   
    #include "tabtenn0.h"TableTennisPlayer::TableTennisPlayer(const string & fn, const string & ln , bool ht):firstname(fn),lastname(ln),hasTable(ht) {}void TableTennisPlayer::Name() const{	std::cout << lastname << ", " << firstname;}
   

13.3 usett0.cpp

//usett0.cpp -- using a base class#include 
   
    #include "tabtenn0.h"int main(){	using namespace std;	TableTennisPlayer player1("Chuck", "Blizzard", true);	TableTennisPlayer player2("Tara", "Boomdea", false);	player1.Name();	if (player1.HasTable())		cout << ": has a table.\n";	else		cout << ": hasn't a table.\n";	player2.Name();	if (player2.HasTable())		cout << ": has a table.\n";	else		cout << ": hasn't a table.\n";	return 0;}
   

派生一个类

13.4 tabtenn1.h

//tabtenn1.h -- a table-tennis base class#ifndef TABTENN1_H_#define TABTENN1_H_#include 
   
    using std::string;class TableTennisPlayer{private:	string firstname;	string lastname;	bool hasTable;public:	TableTennisPlayer(const string & fn = "none", const string & ln = "none", bool ht = false);	void Name() const;	bool HasTable() const { return hasTable; }	void ResetTable(bool v) { hasTable = v; }};class RatedPlayer :public TableTennisPlayer{private:	unsigned int rating;public:	RatedPlayer(unsigned int r = 0, const string & fn = "none", const string & ln = "none", bool ht = false);	RatedPlayer(unsigned int r, const TableTennisPlayer & tp);	unsigned int Rating() const { return rating; }	void ResetRating(unsigned int r) { rating = r; }};#endif // !TABTENN0_H_
   

13.5 tabtenn1.cpp

//tabtenn1.cpp -- simple base-class methods#include 
   
    #include "tabtenn1.h"TableTennisPlayer::TableTennisPlayer(const string & fn, const string & ln, bool ht)	:firstname(fn), lastname(ln), hasTable(ht) {}void TableTennisPlayer::Name() const{	std::cout << lastname << ", " << firstname;}RatedPlayer::RatedPlayer(unsigned int r, const string & fn, const string & ln, bool ht) :TableTennisPlayer(fn, ln, ht){	rating = r;}RatedPlayer::RatedPlayer(unsigned int r, const TableTennisPlayer & tp):TableTennisPlayer(tp),rating(r) {}
   

13.6 usett.cpp

//usett1.cpp -- using a base class and derived class#include 
   
    #include "tabtenn1.h"int main(){	using namespace std;	TableTennisPlayer player1("Tara", "Boomdea", false);	RatedPlayer rplayer1(1140, "Mallory", "Duck", true);	rplayer1.Name();	if (player1.HasTable())		cout << ": has a table.\n";	else		cout << ": hasn't a table.\n";	player1.Name();	if (player1.HasTable())		cout << ": has a table.\n";	else		cout << ": hasn't a table.\n";	cout << "Name: ";	rplayer1.Name();	cout << ": Rating: " << rplayer1.Rating() << endl;	RatedPlayer rplayer2(1212, player1);	cout << "Name: ";	rplayer2.Name();	cout << "; Rating: " << rplayer2.Rating() << endl;	return 0;}
   

多态公有继承

13.7  brass.h

//brass.h -- bank account classes#ifndef BRASS_H_#define BRASS_H_#include 
   
    using namespace std;class Brass{private:	string fullName;	long acctNum;	double balance;public:	Brass(const 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() {}};class BrassPlus :public Brass{private:	double maxLoan;	double rate;	double owesBank;public:	BrassPlus(const 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_
   

13.8 brass.cpp

//brass.cpp -- bank account class methods#include 
   
    #include "brass.h"using namespace std;typedef ios_base::fmtflags format;typedef streamsize precis;format setFormat();void restore(format fm, precis p);Brass::Brass(const string & s /* = "Nullbody" */, long an /* = -1 */, double bal /* = 0.0 */){	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){	format initialState = setFormat();	precis prec = cout.precision(2);	if (amt < 0)		cout << "Withdrawal amout must be positive; withdrawal canceled.\n";	else if (amt <= balance)		balance -= amt;	else		cout << "Withdrawal amount 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);}BrassPlus::BrassPlus(const string & s, long an, double bal, double ml, double r) :Brass(s, an, bal){	maxLoan = ml;	owesBank = 0.0;	rate = r;}void BrassPlus::ViewAcct() const {	format initialState = setFormat();	precis prec = cout.precision(2);	Brass::ViewAcct();	cout << "Maximum loan: $" << maxLoan << endl;	cout << "Owed to bank: $" << owesBank << endl;	cout.precision(3);	cout << "Loan Rate: " << 100 * rate << "%\n";	restore(initialState, prec);}void BrassPlus::Withdraw(double amt){	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(){	return cout.setf(ios_base::fixed, ios_base::floatfield);}void restore(format f, precis p){	cout.setf(f, ios_base::floatfield);	cout.precision(p);}
   

13.9 usebrass1.cpp 没有使用虚方法特性

//usebrass1.cpp -- testing bank account classes//compile with brass.cpp#include 
   
    #include "brass.h"int main(){	using namespace std;	Brass Piggy("Porcelot Pigg", 381299, 4000.00);	BrassPlus Hoggy("Horatio Hogg", 382288, 3000.00);	Piggy.ViewAcct();	cout << endl;	Hoggy.ViewAcct();	cout << endl;	cout << "Depositing $1000 into the hogg Account:\n";	Hoggy.Deposit(1000.00);	cout << "New balance: $" << Hoggy.Balance() << endl;	cout << "Withdrawing $4200 from the Pigg Account:\n";	Piggy.Withdraw(4200.00);	cout << "Pigg account balance: $" << Piggy.Balance() << endl;	cout << "Withdrawing $4200 from the Hogg Account:\n";	Hoggy.Withdraw(4200.00);	Hoggy.ViewAcct();	return 0;}
   

13.10 usebrass2.cpp 使用虚方法特性

//usebrass2.cpp -- polymorphic example//compile with brass.cpp#include 
   
    #include 
    
     #include "brass.h"const int CLIENTS = 4;int main(){	using namespace std;	Brass * p_clients[CLIENTS];	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];	}	cout << "Done.\n";	return 0;}
    
   

抽象基类

13.11 acctabc.h

//acctabc.h -- bank account classes#ifndef ACCTABC_H_#define ACCTABC_H_#include 
   
    #include 
    
     using namespace std;class AcctABC{private:	string fullName;	long acctNum;	double balance;protected:	struct Formatting	{		ios_base::fmtflags flag;		streamsize pr;	};	const string & FullName() const { return fullName; }	long AcctNum() const { return acctNum; }	Formatting SetFormat() const;	void Restore(Formatting & f) const;public:	AcctABC(const string & s = "Nullbody", long an = -1, double baF = 0.0);	void Deposit(double amt);	virtual void Withdraw(double amt) = 0;	double Balance() const { return balance; }	virtual void ViewAcct() const = 0;	virtual ~AcctABC() {}};class Brass :public AcctABC{public:	Brass(const string & s = "Nullbody", long an = -1, double bal = 0.0) :AcctABC(s, an, bal) {}	virtual void ViewAcct()const;	virtual void Withdraw(double amt);	virtual ~Brass() {}};class BrassPlus :public AcctABC{private:	double maxLoan;	double rate;	double owesBank;public:	BrassPlus(const string & s = "Nullbody", long an = -1, double bal = 0.0, double ml = 500, double r = 0.10);	BrassPlus(const Brass & ba, double ml = 500, double r = 0.10);	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 // !ACCTABC_H_
    
   

13.12 acctabc.cpp

//acctabc.cpp -- bank account class methods#include 
   
    #include "acctabc.h"using namespace std;AcctABC::AcctABC(const string & s /* = "Nullbody" */, long an /* = -1 */, double bal /* = 0.0 */){	fullName = s;	acctNum = an;	balance = bal;}void AcctABC::Deposit(double amt){	if (amt < 0)		cout << "Negative deposit not allowed; deposit is cancelled:\n";	else		balance += amt;}void AcctABC::Withdraw(double amt){	balance -= amt;}AcctABC::Formatting AcctABC::SetFormat() const{	Formatting f;	f.flag = cout.setf(ios_base::fixed, ios_base::floatfield);	f.pr = cout.precision(2);	return f;}void AcctABC::Restore(Formatting & f) const{	cout.setf(f.flag, ios_base::floatfield);	cout.precision(f.pr);}void Brass::Withdraw(double amt){	if (amt < 0)		cout << "Withdrawal amout must be positive; withdrawal canceled.\n";	else if (amt <= Balance())		AcctABC::Withdraw(amt);	else		cout << "Withdrawal amount of $" << amt << " exceeds your balance.\n" << "Withdrawal canceled.\n";}void Brass::ViewAcct() const{	Formatting f = SetFormat();	cout << "Brass Client: " << FullName() << endl;	cout << "Account Number: " << AcctNum() << endl;	cout << "Balance: $" << Balance() << endl;	Restore(f);}BrassPlus::BrassPlus(const string & s, long an, double bal, double ml, double r) :AcctABC(s, an, bal){	maxLoan = ml;	owesBank = 0.0;	rate = r;}BrassPlus::BrassPlus(const Brass & ba, double ml /* = 500 */, double r /* = 0.10 */) :AcctABC(ba){	maxLoan = ml;	owesBank = 0.0;	rate = r;}void BrassPlus::ViewAcct() const{	Formatting f = SetFormat();	cout << "BrassPlus Client: " << FullName() << endl;	cout << "Account Number: " << AcctNum() << endl;	cout << "Balance: $" << Balance() << endl;	cout << "Maximum loan: $" << maxLoan << endl;	cout << "Owed to bank: $" << owesBank << endl;	cout.precision(3);	cout << "Loan Rate: " << 100 * rate << "%\n";	Restore(f);}void BrassPlus::Withdraw(double amt){	Formatting f = SetFormat();	double bal = Balance();	if (amt <= bal)		AcctABC::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);		AcctABC::Withdraw(amt);	}	else		cout << "Credit limit exceeded. Transaction cancelled.\n";	Restore(f);}
   

13.13 usebrass3.cpp

//usebrass3.cpp -- polymorphic example using an abstract base class//compile with brass.cpp#include 
   
    #include 
    
     #include "acctabc.h"const int CLIENTS = 4;int main(){	using namespace std;	Brass * p_clients[CLIENTS];	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];	}	cout << "Done.\n";	return 0;}
    
   

使用动态内存分配和友元的继承

13.14 dma.h

//dma.h -- inheritance and dynamic memory allocation#ifndef DMA_H_#define DMA_H_#include 
   
    class baseDMA{private:	char * label;	int rating;public:	baseDMA(const char * l = "null", int r = 0);	baseDMA(const baseDMA & rs);	virtual ~baseDMA();	baseDMA & operator=(const baseDMA & rs);	friend std::ostream & operator<<(std::ostream &os, const baseDMA & rs);};class lacksDMA :public baseDMA{private:	enum { COL_LEN = 40 };	char color[COL_LEN];public:	lacksDMA(const char * c = "blank", const char * l = "null", int r = 0);	lacksDMA(const char * c, const baseDMA & rs);	friend std::ostream & operator<<(std::ostream & os, const lacksDMA & rs);};class hasDMA :public baseDMA{private:	char * style;public:	hasDMA(const char*s = "none", const char * l = "null", int r = 0);	hasDMA(const char * s, const baseDMA & rs);	hasDMA(const hasDMA & hs);	~hasDMA();	hasDMA & operator=(const hasDMA & rs);	friend std::ostream & operator<<(std::ostream & os, const hasDMA & rs);};#endif // !DMA_H_
   

13.15 dma.cpp

//dma.cpp -- dma class methods#include "dma.h"#include 
   
    using namespace std;baseDMA::baseDMA(const char * l /* = "null" */, int r /* = 0 */){	label = new char[strlen(l) + 1];	strcpy(label, l);	rating = r;}baseDMA::baseDMA(const baseDMA &rs){	label = new char[strlen(rs.label) + 1];	strcpy(label, rs.label);	rating = rs.rating;}baseDMA::~baseDMA(){	delete[] label;}baseDMA & baseDMA::operator=(const baseDMA & rs){	if (this == &rs)		return *this;	delete[]label;	label = new char[strlen(rs.label) + 1];	strcpy(label, rs.label);	rating = rs.rating;	return *this;}ostream & operator<<(ostream & os, const baseDMA & rs){	os << "Label: " << rs.label << endl;	os << "Rating: " << rs.rating << endl;	return os;}lacksDMA::lacksDMA(const char * c /* = "blank" */, const char * l /* = "null" */, int r /* = 0 */) :baseDMA(l, r){	strncpy(color, c, 39);	color[39] = '\0';}lacksDMA::lacksDMA(const char * c /* = "blank" */, const baseDMA & rs) :baseDMA(rs){	strncpy(color, c, COL_LEN - 1);	color[COL_LEN - 1] = '\0';}ostream & operator << (ostream & os, const lacksDMA & ls){	os << (const baseDMA &)ls;	os << "Color: " << ls.color << endl;	return os;}hasDMA::hasDMA(const char * s, const char * l, int r) : baseDMA(l, r){	style = new char[strlen(s) + 1];	strcpy(style, s);}hasDMA::hasDMA(const char * s, const baseDMA & rs) :baseDMA(rs){	style = new char[strlen(s) + 1];	strcpy(style, s);}hasDMA::hasDMA(const hasDMA & hs) :baseDMA(hs){	style = new char[strlen(hs.style) + 1];	strcpy(style, hs.style);}hasDMA::~hasDMA(){	delete[]style;}hasDMA & hasDMA::operator=(const hasDMA & hs){	if (this == &hs)		return *this;	baseDMA::operator=(hs);	delete[]style;	style = new char[strlen(hs.style) + 1];	strcpy(style, hs.style);	return *this;}ostream & operator<<(ostream & os, const hasDMA & hs){	os << (const baseDMA &)hs;	os << "Style: " << hs.style << endl;	return os;}
   

13.16 usedma.cpp

//usedma.cpp -- inheritance, friends, and DMA//compile with dma.cpp#include 
   
    #include "dma.h"int main(){	using namespace std;	baseDMA shirt("Portabelly", 8);	lacksDMA balloon("red", "Blimpo", 4);	hasDMA map("Mercator", "Buffalo Keys", 5);	cout << "Displaying baseDMA object:\n" << shirt << endl;	cout << "Displaying lacksDMA object:\n" << balloon << endl;	cout << "Displaying hasDMA object:\n" << map << endl;	lacksDMA balloon2(balloon);	cout << "Result of lacksDMA copy:\n" << balloon2 << endl;	hasDMA map2;	map2 = map;	cout << "Result of hasDMA assignment:\n" << map2 << endl;	return 0;}
   

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