《C++ Primer Plus（第六版）》（33）（第十五章 友元、异常和其他 编程题答案）

15.8 编程练习

1.对Tv和Remote类做如下修改：

a.让他们互为友元；

b.在Remote类中添加一个状态变量成员，该成员描述遥控器是川渝常规模式还是互动模式；

c.在Remote中添加一个显示模式的方法；

d.在Tv类中添加一个队Remote中新成员进行切换的方法，该方法应仅当TV处于打开状态才能使用。

编写一个小程序来测试这些新特性。

Test.h

//
//  Test.h
//  HelloWorld
//
//  Created by feiyin001 on 16/12/21.
//  Copyright (c) 2016年 FableGame. All rights reserved.
//

#ifndef _Test_H_
#define _Test_H_
#include <iostream>
#include <string>
#include <valarray>
using namespace std;
namespace FableGame
{
class Remote;
class Tv
{
public:
friend class Remote;
enum{Off, On};
enum{MinVal, MaxVal = 20};
enum{Antenna, Cable};
enum{TV,DVD};

Tv(int s = Off, int mc = 125):state(s), volume(5),maxchannel(mc), channel(2), mode(Cable), input(TV){}
void onoff(){state = (state == On)? Off:On;}
bool ison() const {return state == On;}
bool volup();
bool voldown();
void chanup();
void chandown();
void set_mode(){mode = (mode == Antenna)? Cable: Antenna;}
void set_input(){input = (input == TV)? DVD:TV;}
void settings()const;
bool changeInterAction(Remote& prev, bool interaction);
private:
int state;
int volume;
int maxchannel;
int channel;
int mode;
int input;
};
class Remote
{
private:
int mode;
bool interaction;//互动状态
public:
friend class Tv;
Remote(int m = Tv::TV):mode(m), interaction(false){}
bool volup(Tv& t){return t.volup();}
bool voldown(Tv& t){return t.voldown();}
void onoff(Tv&t){t.onoff();}
void chanup(Tv&t){t.chanup();}
void chandown(Tv&t){t.chandown();}
void set_chan(Tv&t, int c){t.channel = c;}
void set_mode(Tv&t){t.set_mode();}
void set_input(Tv&t){t.set_input();}
void showInteraction();
};
}
#endif

Test.cpp

//
//  Test.cpp
//  HelloWorld
//
//  Created by feiyin001 on 16/12/21.
//  Copyright (c) 2016年 FableGame. All rights reserved.
//

#include "Test.h"
#include <iostream>
#include <cstdlib>
using namespace std;
using namespace FableGame;

bool Tv::volup()
{
if (volume < MaxVal) {
volume++;
return true;
}
else
return false;
}

bool Tv::voldown()
{
if (volume > MinVal) {
volume--;
return true;
}
else
return false;
}

void Tv::chanup()
{
if (channel < maxchannel) {
channel++;
}
else
{
channel = 1;
}
}

void Tv::chandown()
{
if (channel > 1) {
channel--;
}
else
channel = maxchannel;
}

bool Tv::changeInterAction(Remote& prev, bool interaction)
{
if ( state == Off ) {
return false;
}
prev.interaction = interaction;
return true;
}

void Tv::settings()const
{
cout << "TV is" << (state == Off?"Off" : "On") << endl;
if (state == On) {
cout << "Volume setting = " << volume << endl;
cout << "Channel setting = " << channel << endl;
cout << "Mode = " << (mode == Antenna? "antenna":"cable")<<endl;
cout << "Input = " << (input == TV?"TV" : "DVD" ) << endl;
}
}

void Remote::showInteraction()
{
if (interaction) {
cout << "Remote And TV is in interaction" << endl;
}
else{
cout << "Remote And TV is not in interaction" << endl;
}
}

main.cpp

//
//  main.cpp
//  HelloWorld
//
//  Created by feiyin001 on 16/12/21.
//  Copyright (c) 2016年 FableGame. All rights reserved.
//
#include <iostream>
#include "Test.h"
using namespace std;
using namespace FableGame;

int main()
{
Tv s42;
Remote grey;
cout << "Initial settings for 42\" TV:\n";
s42.settings();
s42.changeInterAction(grey, true);
grey.showInteraction();
s42.onoff();
cout << "\nAdjusted settings for 42\" TV:\n";
s42.settings();
s42.changeInterAction(grey, true);
grey.showInteraction();

return 0;
}

2.修改程序清单15.11，使用两种异常类型都是从头文件<stdexcept>提供的logic_error类派生出来的类。让每个what（）方法都报告函数名和问题的性质。异常对象不用存储错误的参数值，而只需支持what（）方法。

Test.h

//
//  Test.h
//  HelloWorld
//
//  Created by feiyin001 on 16/12/30.
//  Copyright (c) 2016年 FableGame. All rights reserved.
//

#ifndef _Test_H_
#define _Test_H_
#include <iostream>
#include <stdexcept>
#include <string>
using namespace std;
namespace FableGame
{
class bad_hmean: public logic_error
{
public:
explicit bad_hmean(const string& s):logic_error(s){}
};
class bad_gmean: public logic_error
{
public:
explicit bad_gmean(const string& s):logic_error(s){}
};
}
#endif

main.cpp

//
//  main.cpp
//  HelloWorld
//
//  Created by feiyin001 on 16/12/30.
//  Copyright (c) 2016年 FableGame. All rights reserved.
//
#include <iostream>
#include "Test.h"
#include <cmath>
using namespace std;
using namespace FableGame;

double hmean(double a, double b);//调和平均数
double gmean(double a, double b);//几何平均数
int main()
{
double x,y,z;
cout << "Enter two numbers: \n";
while (cin >> x >> y) {
try
{
z = hmean(x, y);
cout << "Harmonic mean of " << x << " and " << y << " is " << z << endl;
z = gmean(x,y);
cout << "Geometric mean of " << x << " and " << y << " is " << z << endl;
cout << "Enter next set of numbers <q to quit>: ";
}
catch (bad_hmean & bg)
{
cout << bg.what();
cout << "Try again.\n";
continue;
}
catch(bad_gmean & hg)
{
cout << hg.what();
cout << "Sorry, you don't get to play any more.\n";
break;
}
cout << "Bye!\n";
}
return 0;
}

double hmean(double a, double b)
{
if (a == -b) {
throw bad_hmean("hmean() error: invalid arguments: a = -b\n");
}
return 2 * a * b/(a + b);
}

double gmean(double a, double b)
{
if (a < 0 || b < 0) {
throw bad_gmean("gmean() arguments should be >= 0\n");
}
return std::sqrt(a* b);
}

3.这个练习与编程练习2相同，但异常类是从一个这样的基类派生而来的：它是从logic_error派生而来的，并且存储两个参数值。异常类应该有一个这样的方法：报告这些值以及函数名。程序使用一个catch来捕捉基类异常，其中任何一种从该基类异常派生而来的异常都将导致循环结束。

Test.h

//
// Test.h
// HelloWorld
//
// Created by feiyin001 on 16/12/30.
// Copyright (c) 2016年 FableGame. All rights reserved.
//

#ifndef _Test_H_
#define _Test_H_
#include <iostream>
#include <stdexcept>
#include <string>
#include <sstream>
using namespace std;
namespace FableGame
{
string toStr(double val);

class bad_mean : public logic_error
{
public:
explicit bad_mean(const string& s, double a, double b);
virtual const char * what() const
{
return _what.c_str();
}
private:
double _a;
double _b;
string _what;
};

}
#endifTest.cpp
//
// Test.cpp
// HelloWorld
//
// Created by feiyin001 on 16/12/29.
// Copyright (c) 2016年 FableGame. All rights reserved.
//

#include "Test.h"
#include <iostream>
using namespace std;
using namespace FableGame;

FableGame::bad_mean::bad_mean(const string& s, double a, double b) :logic_error(s), _a(a), _b(b)
{
_what = logic_error::what();
_what = _what + "a:" + to_string(_a) + " b:" + to_string(_b) + "\n";
}

std::string FableGame::toStr(double val)
{
char buf[20];
sprintf_s(buf, "%f", val);
return string(buf);
};main.cpp
//
// main.cpp
// HelloWorld
//
// Created by feiyin001 on 16/12/30.
// Copyright (c) 2016年 FableGame. All rights reserved.
//
#include <iostream>
#include "Test.h"
#include <cmath>
using namespace std;
using namespace FableGame;

double hmean(double a, double b);//调和平均数
double gmean(double a, double b);//几何平均数
int main()
{
double x, y, z;
cout << "Enter two numbers: \n";
while (cin >> x >> y) {
try
{
z = hmean(x, y);
cout << "Harmonic mean of " << x << " and " << y << " is " << z << endl;
z = gmean(x, y);
cout << "Geometric mean of " << x << " and " << y << " is " << z << endl;
cout << "Enter next set of numbers <q to quit>: ";
}
catch (logic_error & bg)
{
cout << bg.what();
cout << "Sorry, you don't get to play any more.\n";
break;
}
cout << "Bye!\n";
}
return 0;
}

double hmean(double a, double b)
{
if (a == -b) {
throw bad_mean("hmean() error: invalid arguments: a = -b\n", a, b);
}
return 2 * a * b / (a + b);
}

double gmean(double a, double b)
{
if (a < 0 || b < 0) {
throw bad_mean("gmean() arguments should be >= 0\n", a, b);
}
return std::sqrt(a* b);
}
4.程序清单15.16在每个try后面都是用两个catch块，以确保nbad_index异常导致方法label_val()被调用。请修改该程序，在每个try块后面只使用一个catch块，并使用RTTI来确保合适时调用label_val().

原来的代码：

Test.h

//
// Test.h
// HelloWorld
//
// Created by feiyin001 on 16/12/30.
// Copyright (c) 2016年 FableGame. All rights reserved.
//

#ifndef _Test_H_
#define _Test_H_
#include <iostream>
#include <stdexcept>
#include <string>
#include <sstream>
using namespace std;
namespace FableGame
{
class Sales
{
public:
enum{MONTHS = 12};
class bad_index : public logic_error
{
private:
int bi;
public:
explicit bad_index(int ix, const std::string& s = "Index error in Sales object\n");
int bi_val() const { return bi; }
virtual ~bad_index() throw() {}
};
explicit Sales(int yy = 0);
Sales(int yy, const double* gr, int n);
virtual ~Sales(){}
int Year() const { return year; }
virtual double operator[](int i)const;
virtual double& operator[](int i);
private:
double gross[MONTHS];
int year;
};

class LabeledSales : public Sales
{
public:
class nbad_index : public Sales::bad_index
{
public:
nbad_index(const std::string& lb, int ix, const std::string& s = "Index error in LabeledSales object\n");
const std::string& label_val()const { return lbl; }
virtual ~nbad_index()throw(){}
private:
std::string lbl;
};
explicit LabeledSales(const string& lb = "none", int yy = 0);
LabeledSales(const string& lb, int yy, const double* gr, int n);
virtual ~LabeledSales(){}
const std::string& Label() const { return label; }
virtual double operator[](int i)const;
virtual double &operator[](int i);
private:
string label;
};
}
#endifTest.cpp
//
// Test.cpp
// HelloWorld
//
// Created by feiyin001 on 16/12/29.
// Copyright (c) 2016年 FableGame. All rights reserved.
//

#include "Test.h"
#include <iostream>
using namespace std;
using namespace FableGame;

FableGame::Sales::bad_index::bad_index(int ix, const std::string& s )
:logic_error(s), bi(ix)
{
}

FableGame::Sales::Sales(int yy /*= 0*/)
{
year = yy;
for (int i = 0; i < MONTHS; ++i)
{
gross[i] = 0;
}
}

FableGame::Sales::Sales(int yy, const double* gr, int n)
{
year = yy;
int lim = (n < MONTHS) ? n : MONTHS;
for (int i = 0; i < MONTHS; ++i)
{
if (i < lim)
{
gross[i] = gr[i];
}
else
{
gross[i] = 0;
}
}
}

double FableGame::Sales::operator[](int i) const
{
if (i < 0 || i >= MONTHS)
{
throw bad_index(i);
}
return gross[i];
}

double& FableGame::Sales::operator[](int i)
{
if (i < 0 || i >= MONTHS)
{
throw bad_index(i);
}
return gross[i];
}

FableGame::LabeledSales::nbad_index::nbad_index(const std::string& lb, int ix, const std::string& s)
:Sales::bad_index(ix,s)
{
lbl = lb;
}

FableGame::LabeledSales::LabeledSales(const string& lb /*= "none"*/, int yy /*= 0*/)
: Sales(yy)
{
label = lb;
}

FableGame::LabeledSales::LabeledSales(const string& lb, int yy, const double* gr, int n)
: Sales(yy, gr, n)
{
label = lb;
}

double FableGame::LabeledSales::operator[](int i) const
{
if (i < 0 || i >= MONTHS)
{
throw nbad_index(Label(), i);
}
return Sales::operator[](i);
}

double & FableGame::LabeledSales::operator[](int i)
{
if (i < 0 || i >= MONTHS)
{
throw nbad_index(Label(), i);
}
return Sales::operator[](i);
}
main.cpp
//
// main.cpp
// HelloWorld
//
// Created by feiyin001 on 16/12/30.
// Copyright (c) 2016年 FableGame. All rights reserved.
//
#include <iostream>
#include "Test.h"
#include <cmath>
using namespace std;
using namespace FableGame;

int main()
{
double vals1[12] =
{
1220, 1100, 1122, 2212, 1232, 2334,
2884, 2393, 3302, 2922, 3002, 3544
};
double vals2[12] =
{
12, 11, 22, 21, 32, 34,
28, 29, 33, 39, 32, 35
};
Sales sales1(2011, vals1, 12);
LabeledSales sales2("Blogstar", 2012, vals2, 12);
cout << "First try block:\n";
try
{
int i;
cout << "Year = " << sales1.Year() << endl;
for (i = 0; i < 12; ++i)
{
cout << sales1[i] << ' ';
if (i % 6 == 5)
{
cout << endl;
}
}
cout << "Year = " << sales2.Year() << endl;
cout << "Label = " << sales2.Label() << endl;
for (i = 0; i < 12; ++i)
{
cout << sales2[i] << ' ';
if (i % 6 == 5)
{
cout << endl;
}
}
cout << "End of try block 1.\n";
}
catch (LabeledSales::nbad_index& bad)
{
cout << bad.what();
cout << "Company: " << bad.label_val() << endl;
cout << "bad index: " << bad.bi_val() << endl;
}
catch (Sales::bad_index& bad)
{
cout << bad.what();
cout << "bad index: " << bad.bi_val() << endl;
}
cout << "\nNext try block:\n";
try
{
sales2[2] = 37.5;
sales1[20] = 23345;
cout << "End of try block 2. \n";
}
catch (LabeledSales::nbad_index& bad)
{
cout << bad.what();
cout << "Company: " << bad.label_val() << endl;
cout << "bad index: " << bad.bi_val() << endl;
}
catch (Sales::bad_index & bad)
{
cout << bad.what();
cout << "bad index: " << bad.bi_val() << endl;
}

cout << "done\n";
return 0;
}

修改代码：
//
// main.cpp
// HelloWorld
//
// Created by feiyin001 on 16/12/30.
// Copyright (c) 2016年 FableGame. All rights reserved.
//
#include <iostream>
#include "Test.h"
#include <cmath>
using namespace std;
using namespace FableGame;

int main()
{
double vals1[12] =
{
1220, 1100, 1122, 2212, 1232, 2334,
2884, 2393, 3302, 2922, 3002, 3544
};
double vals2[12] =
{
12, 11, 22, 21, 32, 34,
28, 29, 33, 39, 32, 35
};
Sales sales1(2011, vals1, 12);
LabeledSales sales2("Blogstar", 2012, vals2, 12);
cout << "First try block:\n";
try
{
int i;
cout << "Year = " << sales1.Year() << endl;
for (i = 0; i < 12; ++i)
{
cout << sales1[i] << ' ';
if (i % 6 == 5)
{
cout << endl;
}
}
cout << "Year = " << sales2.Year() << endl;
cout << "Label = " << sales2.Label() << endl;
for (i = 0; i < 12; ++i)
{
cout << sales2[i] << ' ';
if (i % 6 == 5)
{
cout << endl;
}
}
cout << "End of try block 1.\n";
}
catch (Sales::bad_index& bad)
{
cout << bad.what();
cout << "bad index: " << bad.bi_val() << endl;
LabeledSales::nbad_index* bad2 = dynamic_cast<LabeledSales::nbad_index*>(&bad);
if (bad2)
{
cout << "Company: " << bad2->label_val() << endl;
}
}
cout << "\nNext try block:\n";
try
{
sales2[2] = 37.5;
sales1[20] = 23345;
cout << "End of try block 2. \n";
}
catch (Sales::bad_index& bad)
{
cout << bad.what();
cout << "bad index: " << bad.bi_val() << endl;
LabeledSales::nbad_index* bad2 = dynamic_cast<LabeledSales::nbad_index*>(&bad);
if (bad2)
{
cout << "Company: " << bad2->label_val() << endl;
}
}

cout << "done\n";
return 0;
}



总结： 
貌似越到后面，就经常出现了排版混乱的情况，不知道是原版的问题还是翻译本的问题。

程序例子也越来越抽象了，而且是说明的东西居多。

其实初学者，甚至学得好深入的人，接触的可能还是很少。

很多细节，只有在非常底层或者特定的应用中才会使用到。

或者，用到的时候，百度一下就可以了。