二元运算符重载

以非成员函数方式重载运算符

/***
overtwo.cpp
***/
#include<iostream>
using namespace std;class Box
{public:Box(double l = 2.0,double b = 2.0,double h = 2.0){length = l;breadth = b;height = h;}double getVolume(){return length*breadth*height;}private:double length;double breadth;double height;friend Box operator+(const Box& a,const Box& b);
};Box operator+(const Box& a,const Box& b)
{Box box;box.length = a.length+b.length;box.breadth = a.breadth+b.breadth;box.height = a.height + b.height;return box;
}int main()
{Box box1(3.3,1.2,1.5);Box box2(8.5,6.0,2.0);Box box3;double volume = 0.0;volume = box1.getVolume();cout << "Volume of box1 : " << volume << endl;volume = box2.getVolume();cout << "Volume of box2 : " << volume << endl;box3 = box1 + box2;volume = box3.getVolume();cout << "Volume of box3 : " << volume << endl;return 0;
}

运算结果：

exbot@ubuntu:~/wangqinghe/C++/20190808$ g++ overtwo.cpp -o overtwo

exbot@ubuntu:~/wangqinghe/C++/20190808$ ./overtwo

Volume of box1 : 5.94

Volume of box2 : 102

Volume of box3 : 297.36

当两个对象相加时是没有顺序要求的，但是要重载+让其与一个数字相加有顺序要求，可以通过加一个友元函数使另一个顺序的输出合法。

/***
overfriend.cpp
***/
#include<iostream>
using namespace std;class A
{private:int a;public:A();A(int n);A operator+(const A &obj);A operator+(const int b);friend A operator+(const int b,A obj);void display();
};A::A()
{a = 0;
}A::A(int n)
{a = n;
}A A::operator +(const A& obj)  //重载+号，用于对象相加
{return this->a+obj.a;
}A A::operator+(const int b) //对象和数相加
{return A(a+b);
}A operator+ (const int b, A obj) // 友元函数调用第二个重载+的成员函数，相当于obj.operator(b)
{return obj+b;
}void A::display()
{cout << a << endl;
}int main()
{A a1(1);A a2(2);A a3,a4,a5;a1.display();a2.display();int m = 1;a3 = a1 + a2; //可以调换顺序，相当于a3=a1.operator+(a2);
    a3.display();a4 = a1 + m; //加了一个友元函数可调换顺序
    a4.display();a5 = m + a1;a5.display();return 0;
}