友元函数对比与实例

计算时钟加减法

头函数 mytime2.h

// mytime2.h -- Time class after operator overloading
#ifndef MYTIME2_H_
#define MYTIME2_H_

class Time
{
private:
    int hours;
    int minutes;
public:
    Time();
    Time(int h, int m = 0);
    void AddMin(int m);
    void AddHr(int h);
    void Reset(int h = 0, int m = 0);
    Time operator+(const Time & t) const;
    Time operator-(const Time & t) const;
    Time operator*(double n) const;
    void Show() const;
};
#endif

函数定义

// mytime2.cpp  -- implementing Time methods
#include <iostream>
#include "mytime2.h"

Time::Time()
{
    hours = minutes = 0;
}

Time::Time(int h, int m )
{
    hours = h;
    minutes = m;
}

void Time::AddMin(int m)
{
    minutes += m;
    hours += minutes / 60;
    minutes %= 60;
}
void Time::AddHr(int h)
{
    hours += h;
}

void Time::Reset(int h, int m)
{
    hours = h;
    minutes = m;
}

Time Time::operator+(const Time & t) const
{
    Time sum;
    sum.minutes = minutes + t.minutes;
    sum.hours = hours + t.hours + sum.minutes / 60;
    sum.minutes %= 60;
    return sum;
}

Time Time::operator-(const Time & t) const
{
    Time diff;
    int tot1, tot2;
    tot1 = t.minutes + 60 * t.hours;
    tot2 = minutes + 60 * hours;
    diff.minutes = (tot2 - tot1) % 60;
    diff.hours = (tot2 - tot1) / 60;
    return diff;
}

Time Time::operator*(double mult) const
{
    Time result;
    long totalminutes = hours * mult * 60 + minutes * mult;
    result.hours = totalminutes / 60;
    result.minutes = totalminutes % 60;
    return result;
}

void Time::Show() const
{
    std::cout << hours << " hours, " << minutes << " minutes";
}

主函数

// usetime2.cpp -- using the third draft of the Time class
// compile usetime2.cpp and mytime2.cpp together
#include <iostream>
#include "mytime2.h"

int main()
{
    using std::cout;
    using std::endl;
    Time weeding(4, 35);
    Time waxing(2, 47);
    Time total;
    Time diff;
    Time adjusted;

    cout << "weeding time = ";
    weeding.Show();
    cout << endl;
 
    cout << "waxing time = ";
    waxing.Show();
    cout << endl;
    
    cout << "total work time = ";
    total = weeding + waxing;   // use operator+()
    total.Show();
    cout << endl;

    diff = weeding - waxing;    // use operator-()
    cout << "weeding time - waxing time = ";
    diff.Show();
    cout << endl;

    adjusted = total * 1.5;      // use operator+()
    cout << "adjusted work time = ";
    adjusted.Show();
    cout << endl;
    // std::cin.get();    
    return 0;
}

上面乘法运算符将一个Time值与另一个double值结合。但需要限制运算符的使用方法。左侧的操作符必须是调用对象。

也就是说： A=B * 2.75;

可以转化为 A=B.operator * (2.75)；

但如果 A=2.75 * B； 则需要函数原型为Time operator* （double m,const Time &t）;

这是个返回值为Time类型的非成员函数（声明在类外），不能直接访问类私有成员。

 

但是可以将Time对象t作为一个整体使用，让成员函数来处理私有制，因此可以不必是友元。

头文件

// mytime3.h -- Time class with friends
#ifndef MYTIME3_H_
#define MYTIME3_H_
#include <iostream>

class Time
{
private:
    int hours;
    int minutes;
public:
    Time();
    Time(int h, int m = 0);
    void AddMin(int m);
    void AddHr(int h);
    void Reset(int h = 0, int m = 0);
    Time operator+(const Time & t) const;
    Time operator-(const Time & t) const;
    Time operator*(double n) const;
    friend std::ostream & operator<<(std::ostream & os, const Time & t);

};

Time operator*(double m, const Time & t);

#endif

函数定义

// mytime3.cpp  -- implementing Time methods
#include "mytime3.h"

Time::Time()
{
    hours = minutes = 0;
}

Time::Time(int h, int m )
{
    hours = h;
    minutes = m;
}

void Time::AddMin(int m)
{
    minutes += m;
    hours += minutes / 60;
    minutes %= 60;
}

void Time::AddHr(int h)
{
    hours += h;
}

void Time::Reset(int h, int m)
{
    hours = h;
    minutes = m;
}

Time Time::operator+(const Time & t) const
{
    Time sum;
    sum.minutes = minutes + t.minutes;
    sum.hours = hours + t.hours + sum.minutes / 60;
    sum.minutes %= 60;
    return sum;
}

Time Time::operator-(const Time & t) const
{
    Time diff;
    int tot1, tot2;
    tot1 = t.minutes + 60 * t.hours;
    tot2 = minutes + 60 * hours;
    diff.minutes = (tot2 - tot1) % 60;
    diff.hours = (tot2 - tot1) / 60;
    return diff;
}

Time Time::operator*(double mult) const
{
    Time result;
    long totalminutes = hours * mult * 60 + minutes * mult;
    result.hours = totalminutes / 60;
    result.minutes = totalminutes % 60;
    return result;
}

std::ostream & operator<<(std::ostream & os, const Time & t)
{
    os << t.hours << " hours, " << t.minutes << " minutes";
    return os; 
}

Time operator*(double m, const Time & t)
{
	return t * m;
}   // inline definition

主函数

//usetime3.cpp -- using the fourth draft of the Time class
// compile usetime3.cpp and mytime3.cpp together
#include <iostream>
#include "mytime3.h"

int main()
{
    using std::cout;
    using std::endl;
    Time aida(3, 35);
    Time tosca(2, 48);
    Time temp;

    cout << "Aida and Tosca:\n";
    cout << aida<<"; " << tosca << endl;
    temp = aida + tosca;     // operator+()
    cout << "Aida + Tosca: " << temp << endl;
    temp = aida* 1.17;  // member operator*()
    cout << "Aida * 1.17: " << temp << endl;
    cout << "10.0 * Tosca: " << 10.0 * tosca << endl;
	// std::cin.get();
    return 0; 
}

友元函数

头文件

// mytime3.h -- Time class with friends
#ifndef MYTIME3_H_
#define MYTIME3_H_
#include <iostream>

class Time
{
private:
    int hours;
    int minutes;
public:
    Time();
    Time(int h, int m = 0);
    void AddMin(int m);
    void AddHr(int h);
    void Reset(int h = 0, int m = 0);
    Time operator+(const Time & t) const;
    Time operator-(const Time & t) const;
    Time operator*(double n) const;
    friend Time operator*(double m, const Time & t)
        { return t * m; }   // inline definition
    friend std::ostream & operator<<(std::ostream & os, const Time & t);

};
#endif

函数定义

// mytime3.cpp  -- implementing Time methods
#include "mytime3.h"

Time::Time()
{
    hours = minutes = 0;
}

Time::Time(int h, int m )
{
    hours = h;
    minutes = m;
}

void Time::AddMin(int m)
{
    minutes += m;
    hours += minutes / 60;
    minutes %= 60;
}

void Time::AddHr(int h)
{
    hours += h;
}

void Time::Reset(int h, int m)
{
    hours = h;
    minutes = m;
}

Time Time::operator+(const Time & t) const
{
    Time sum;
    sum.minutes = minutes + t.minutes;
    sum.hours = hours + t.hours + sum.minutes / 60;
    sum.minutes %= 60;
    return sum;
}

Time Time::operator-(const Time & t) const
{
    Time diff;
    int tot1, tot2;
    tot1 = t.minutes + 60 * t.hours;
    tot2 = minutes + 60 * hours;
    diff.minutes = (tot2 - tot1) % 60;
    diff.hours = (tot2 - tot1) / 60;
    return diff;
}

Time Time::operator*(double mult) const
{
    Time result;
    long totalminutes = hours * mult * 60 + minutes * mult;
    result.hours = totalminutes / 60;
    result.minutes = totalminutes % 60;
    return result;
}

std::ostream & operator<<(std::ostream & os, const Time & t)
{
    os << t.hours << " hours, " << t.minutes << " minutes";
    return os; 
}

主函数

//usetime3.cpp -- using the fourth draft of the Time class
// compile usetime3.cpp and mytime3.cpp together
#include <iostream>
#include "mytime3.h"

int main()
{
    using std::cout;
    using std::endl;
    Time aida(3, 35);
    Time tosca(2, 48);
    Time temp;

    cout << "Aida and Tosca:\n";
    cout << aida<<"; " << tosca << endl;
    temp = aida + tosca;     // operator+()
    cout << "Aida + Tosca: " << temp << endl;
    temp = aida* 1.17;  // member operator*()
    cout << "Aida * 1.17: " << temp << endl;
    cout << "10.0 * Tosca: " << 10.0 * tosca << endl;
	// std::cin.get();
    return 0; 
}