学习C++中,我们使用类,我们发现C++控制对类对象私有部分的访问,通常公有部分提供唯一的访问途径,这给C++带来了限制,友元能够帮我们很好的解决这个问题。
解决这种问题的方式: 1.严格按照A=B*2.75形式来写 2.使用非成员函数(友元)
Time operator*(double m, const Time & t);
这种函数却无法访问私有部分,我们在类声明中加上friend,从而形成友元。
friend Time operator*(double m, const Time & t); // goes in class declaration
另外交换乘法的顺序也可以把友元函数变成非友元函数。
Time operator*(double m, const Time & t)
{
return t * m; // use t.operator*(m)
}
有种常用的运算符重载<<运算符
void operator<<(ostream & os, const Time & t)
{
os << t.hours << " hours, " << t.minutes << " minutes";
}
则可以这样写```
```cpp
cout << trip;
输出
4 hours, 23 minutes
可以进行修改,加上个引用使返回值返回cout
ostream & operator<<(ostream & os, const Time & t)
{
os << t.hours << " hours, " << t.minutes << " minutes";
return os;
}
这样就可以连续使用<<了 下面来看一个实例,将运算符重载和友元结合在一起,可以进行消化
#include
using namespace std;
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);
};
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;
}
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;
return 0;
}
输出结果
Aida and Tosca:
3 hours, 35 minutes; 2 hours, 48 minutes
Aida + Tosca: 6 hours, 23 minutes
Aida * 1.17: 4 hours, 11 minutes
10.0 * Tosca: 28 hours, 0 minutes
你学废了么!!!