本章小結

程式清單

一個簡單的基類

派生一個類

多态公有繼承

抽象基類

使用動态記憶體配置設定和友元的繼承

本章小結

繼承通過使用已有的類（基類）定義新的類（派生類），使得能夠根據需要修改程式設計代碼。公有繼承建立is-a關系，這意味若派生類對象也應該是某種基類對象。作為is-a模型的一部分，派生類繼承基類的資料成員和大部分方法，但不繼承基類的構造函數、析構函數和指派運算符。派生類可以直接通路基類的公有成員和保護成員，并能夠通過基類的公有方法和保護方法通路基類的私有成員。可以在派化類中新增資料成員和方法，還可以将派生類用作基類，來做進一步的開發。每個派生類都必須有自己的構造函數。程式建立派生類對象時，将首先調用基類的構造函數，然後調用派生類的構造函數；程式删除對象時，将首先調用派生類的析構函數，然後調用基類的析構函數。

如果要将類用作基類，則可以将成員聲明為保護的，而不是私有的，這樣，派生類将可以直接通路這些成員。然而，使用私有成員通常可以減少出現程式設計問題的可能性。如果希望派生類可以重新定義基類的方法，則可以使用關鍵字virtual将它聲明為虛的。這樣對于通過指針或引用通路的對象，能夠根據對象類型來處理，而不是根據引用或指針的類型來處理。具體地說，基類的析構函數通常應當是虛的。

可以考慮定義一個ABC：隻定義接口，而不涉及實作。例如，可以定義抽象類Shape,然後使用它派生出具體的形狀類，如Circle和Square。ABC必須至少包含一個純虛方法，可以在聲明中的分号前面加h=0 來聲明純虛方法，

virtual double area{} const = 0；

不一定非得定義純虛方法。對于包含純虛成員的類，不能使用它來建立對象。純虛方法用于定義派生類的通用接口。

程式清單

一個簡單的基類

13.1 tabtenn0.h

//tabtenn0.h -- a table-tennis base class
#ifndef TABTENN0_H_
#define TABTENN0_H_

#include <string>

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 <iostream>
#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 <iostream>
#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;
}

派生一個類

13.4 tabtenn1.h

//tabtenn1.h -- a table-tennis base class
#ifndef TABTENN1_H_
#define TABTENN1_H_

#include <string>

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 <iostream>
#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 <iostream>
#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;
}

多态公有繼承

13.7 brass.h

//brass.h -- bank account classes
#ifndef BRASS_H_
#define BRASS_H_
#include <string>

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 <iostream>
#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 沒有使用虛方法特性

//usebrass1.cpp -- testing bank account classes
//compile with brass.cpp
#include <iostream>
#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 使用虛方法特性

//usebrass2.cpp -- polymorphic example
//compile with brass.cpp

#include <iostream>
#include <string>
#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;
}

抽象基類

13.11 acctabc.h

//acctabc.h -- bank account classes
#ifndef ACCTABC_H_
#define ACCTABC_H_

#include <iostream>
#include <string>

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 <iostream>
#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 <iostream>
#include <string>
#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;
}

使用動态記憶體配置設定和友元的繼承

13.14 dma.h

//dma.h -- inheritance and dynamic memory allocation
#ifndef DMA_H_
#define DMA_H_
#include <iostream>

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 <cstring>

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 <iostream>
#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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