2021-07-05 C++ Primer Plus 第十四章友元、异常和其它编程练习

编程练习

1.对Tv和Remote类做如下修改：

a. 让它们互为友元；

b.在Remote类中添加一个状态变量成员，该成员描述遥控器是出于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类中添加一个对Remote中新成员进行切换的方法，该方法应当仅次于TV处于打开状态下才能运行。

编写一个小程序来测试这些新特性。

TV.h

// tv.h -- Tv and Remote classes
#ifndef TV_H_
#define TV_H_
class Remote;
class Tv
{
public:
    friend class Remote;   // Remote can access Tv private parts
    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; // display all settings
    void shift_mode(Remote &r) const;
private:
    int state;             // on or off
    int volume;            // assumed to be digitized
    int maxchannel;        // maximum number of channels
    int channel;           // current channel setting
    int mode;              // broadcast or cable
    int input;             // TV or DVD
};

class Remote
{
private:
    int mode;              // controls TV or DVD
    bool mode_status;      // false:normal,true:interaction
public:
    friend class Tv;
    Remote(int m = Tv::TV) : mode(m),mode_status(false) {}
    void show_status() const;
    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();}
};
#endif

TV.cpp

// tv.cpp -- methods for the Tv class (Remote methods are inline)
#include <iostream>
#include "TV.h"

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;
}

void Tv::settings() const
{
    using std::cout;
    using std::endl;
    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 Tv::shift_mode(Remote &r) const{
    if (state == 1)
        if (r.mode_status)
            r.mode_status = false;
        else
            r.mode_status = true;
    else std::cout << "TV is off" << std::endl;
}

void Remote::show_status() const{
    if (mode_status)
        std::cout << "Remote mode status: interaction"
                  << std::endl;
    else
        std::cout << "Remote mode status: normal"
                  << std::endl;
}

main.cpp

#include <iostream>
#include "TV.h"

int main()
{
    using std::cout;
    Tv s42;
    cout << "Initial settings for 42\" TV:\n";
    s42.settings();
    s42.onoff();
    s42.chanup();
    cout << "\nAdjusted settings for 42\" TV:\n";
    s42.settings();

    Remote grey;
    grey.show_status();
    grey.set_chan(s42, 10);
    grey.volup(s42);
    grey.volup(s42);
    cout << "\n42\" settings after using remote:\n";
    s42.settings();
    s42.shift_mode(grey);
    grey.show_status();

    Tv s58(Tv::On);
    s58.set_mode();
    grey.set_chan(s58,28);
    cout << "\n58\" settings:\n";
    s58.settings();
    // std::cin.get();
    return 0;
}

2.修改程序清单15.11，使两种异常类型都是从头文件<stdexcept>提供的logic_error类派生出的类。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。

exc_mean.h

// exc_mean.h  -- exception classes for hmean(), gmean()
#include <iostream>
#include <execution>
using namespace std;
class bad_hmean : public invalid_argument
{
public:
    bad_hmean() : invalid_argument("a + b = 0") {}
};

class bad_gmean : public range_error
{
public:
    bad_gmean() : range_error("a < 0, b < 0") {}
};

error4.cpp

#ifndef ERROR_EXC_MEAN_H
#define ERROR_EXC_MEAN_H
#include "exc_mean.h"
#include <iostream>
#include <cmath> // or math.h, unix users may need -lm flag
// function prototypes
using namespace std;
double hmean(double a, double b);
double gmean(double a, double b);
int main()
{
    double x, y, z;

    cout << "Enter two numbers: ";
    while (cin >> x >> y)
    {
        try {                  // start of try block
            z = hmean(x,y);
            cout << "Harmonic mean of " << x << " and " << y
                 << " is " << z << endl;
            cout << "Geometric mean of " << x << " and " << y
                 << " is " << gmean(x,y) << endl;
            cout << "Enter next set of numbers <q to quit>: ";
        }// end of try block
        catch (bad_hmean &gh)    // start of catch block
        {
            cout << "Caught the error!" << endl;
            cout << gh.what() << endl;
            cout << "Try again.\n";
            continue;
        }
        catch (bad_gmean &bg)
        {
            cout << "Caught the error!" << endl;
            cout << bg.what() << endl;
//            cout << "Values used: " << hg.v1 << ", "
//                 << hg.v2 << endl;
            cout << "Sorry, you don't get to play any more.\n";
            break;
        } // end of catch block
    }
    cout << "Bye!\n";
    return 0;
}

double hmean(double a, double b)
{
    if (a == -b)
        throw bad_hmean();
    return 2.0 * a * b / (a + b);
}

double gmean(double a, double b)
{
    if (a < 0 || b < 0)
        throw bad_gmean();
    return std::sqrt(a * b);
}
#endif //ERROR_EXC_MEAN_H

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：报告这些值以函数名。程序使用一个catch块来捕获基类异常，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。

exc_mean.h

#include <iostream>
#include <exception>
using namespace std;
class bad : public logic_error
{
private:
    double v1,v2;
public:
    bad(double v1,double v2) : v1(v1), v2(v2), logic_error("logic error!") {}
    void show_error() { cout << "error value: " << v1 <<" "<<v2 << endl;}
};

error4.cpp

#ifndef ERROR_EXC_MEAN_H
#define ERROR_EXC_MEAN_H
#include "exc_mean.h"
#include <iostream>
#include <cmath> // or math.h, unix users may need -lm flag
// function prototypes
using namespace std;
double hmean(double a, double b);
double gmean(double a, double b);
int main()
{
    double x, y, z;

    cout << "Enter two numbers: ";
    while (cin >> x >> y)
    {
        try {                  // start of try block
            z = hmean(x,y);
            cout << "Harmonic mean of " << x << " and " << y
                 << " is " << z << endl;
            cout << "Geometric mean of " << x << " and " << y
                 << " is " << gmean(x,y) << endl;
            cout << "Enter next set of numbers <q to quit>: ";
        }// end of try block
        catch (bad &gh)    // start of catch block
        {
            cout << "Caught the error!" << endl;
            cout << gh.what() << endl;
            gh.show_error();
//            cout << ".\n";
            break;
        }
    }
    cout << "Bye!\n";
    return 0;
}

double hmean(double a, double b)
{
    if (a == -b)
        throw bad(a,b);
    return 2.0 * a * b / (a + b);
}

double gmean(double a, double b)
{
    if (a < 0 || b < 0)
        throw bad(a,b);
    return std::sqrt(a * b);
}
#endif //ERROR_EXC_MEAN_H

4.程序清单15.16在每个try后面都使用两个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()。

sales.h

#ifndef SALES_SALES_H
#define SALES_SALES_H
#include <stdexcept>
#include <string>

class Sales
{
public:
    enum {MONTHS = 12};   // could be a static const
    class bad_index : public std::logic_error
    {
    private:
        int bi;  // bad index value
    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
    {
    private:
        std::string lbl;
    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() {}
    };
    explicit LabeledSales(const std::string & lb = "none", int yy = 0);
    LabeledSales(const std::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:
    std::string label;
};



#endif //SALES_SALES_H

sales.cpp

#include "sales.h"using std::string;Sales::bad_index::bad_index(int ix, const string & s )        : std::logic_error(s), bi(ix){}Sales::Sales(int yy){    year = yy;    for (int i = 0; i < MONTHS; ++i)        gross[i] = 0;}Sales::Sales(int yy, const double * gr, int n){    year = yy;    int lim = (n < MONTHS)? n : MONTHS;    int i;    for (i = 0; i < lim; ++i)        gross[i] = gr[i];    // for i > n and i < MONTHS    for ( ; i < MONTHS; ++i)        gross[i] = 0;}double Sales::operator[](int i) const{    if(i < 0 || i >= MONTHS)        throw bad_index(i);    return gross[i];}double & Sales::operator[](int i){    if(i < 0 || i >= MONTHS)        throw bad_index(i);    return gross[i];}LabeledSales::nbad_index::nbad_index(const string & lb, int ix,                                     const string & s ) : Sales::bad_index(ix, s){    lbl = lb;}LabeledSales::LabeledSales(const string & lb, int yy)        : Sales(yy){    label = lb;}LabeledSales::LabeledSales(const string & lb, int yy, const double * gr, int n)        : Sales(yy, gr, n){    label = lb;}double LabeledSales::operator[](int i) const{    if(i < 0 || i >= MONTHS)        throw nbad_index(Label(), i);    return Sales::operator[](i);}double & LabeledSales::operator[](int i){    if(i < 0 || i >= MONTHS)        throw nbad_index(Label(), i);    return Sales::operator[](i);}