C++ Primer Plus 学习笔记第十一章使用类运算符重载友元函数

骑猪看日落 2022-01-28 04:41 206阅读 0赞

运算符重载的限制：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70][]

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限制这么多 人生还有什么意义= =python倒是可以把+变- -变\*之类的

示例代码：

mytime2.h

#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

mytime2.cpp

#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";
    }

usetime2.cpp

#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;
      total.Show();
      cout << endl;
    
      diff = weeding - waxing;
      cout << "weeding time - waxing time = ";
      diff.Show();
      cout << endl;
    
      adjusted = total * 1.5;
      cout << "adjusted work time = ";
      adjusted.Show();
      cout << endl;
    
      return 0;
    }

友元（有缘= =）

友元函数

友元类

友元成员函数

处理外部函数调用类私有函数或对象

比如 Time \* 1.5 改成 1.5 \* Time

一般情况 Time::Time operator\* (double n)  const

这样的话就没办法适用1.5 \* Time

所以需要友元函数来跟Time对象做做关系，让外部函数能够访问Time对象的成员函数或对象

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关于<< 的重载

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这种方法只能适用于简单的 cout << time;

但是如果碰到 cout << "abc" << time 就没用了

那咋办捏

这样写：

ostream & operator<< (ostream & os, Time & t)

.......

return os;

![20190523115056236.png][]

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[20190523115056236.png]: /images/20220128/e421a3e058ac47bb89a2584058d6e73d.png