运算符重载，输出流运算符重载

运算符重载是很重要的，特别是在有些stl中必需重载小于号才能使用。

还有就是输出流的"<<"重载，方便直接cout输出，但是只能有友元函数来重载，因为在cout<<class T <<endl;中其左边类型已经被确定了，必需有这样的友元函数来实现：friend ostream &operator<<(ostream &,Complex &);

详见代码(复数类的实例)：

#include <iostream>
#include <cmath>
using namespace std;

class Complex {
private:
    double real, image ;
public:
    Complex(double r=0.0, double i=0.0) : real(r), image(i) {} 
    Complex(Complex &com) {
        real = com.real; 
        image = com.image; 
    }
    void Print() {
        cout<<"real="<<real<<'\t'<<"image="<<image<<'\n'; 
    }
    Complex operator+(Complex);  // 重载+运算符函数
    Complex operator+(double);   // 重载+运算符函数
    Complex operator=(Complex);  // 重载=运算符函数
    Complex operator+=(Complex); // 重载+=运算符函数
    double abs(void); //求绝对值函数
    Complex operator*(Complex);  // 重载*运算符函数
    Complex operator/(Complex);  // 重载/运算符函数
    bool operator<(Complex);
    bool operator>(Complex);
    friend ostream &operator<<(ostream &,Complex &);
    // operation+说明为类Complex类的友元函数,
    // friend只用于类说明中，定义时不加friend 
    friend Complex operator+(double, Complex);//***友元
};
ostream &operator<<(ostream &output,Complex &c) {
    output << c.real << "," << c.image ;
    return output;
}
//注意友元不是成员函数，但以直接访问私有成员 
Complex operator + (double d, Complex c) {
    return Complex(d + c.real, c.image); 
}
Complex Complex::operator+(Complex c) { //重载+
    Complex temp(real + c.real, image + c.image) ;
    return temp;
}
Complex Complex::operator+(double d) { //重载+ 
    return Complex(real + d, image); 
} 
Complex Complex::operator+=(Complex c) {//重载+ =
    Complex temp; //为了返回Complex类型的值，使+=可以连续使用 
    temp.real = real + c.real;
    temp.image = image + c.image;
    real = temp.real;
    image = temp.image;
    return temp;
}
Complex Complex::operator=(Complex c) {//重载=
    Complex temp; //定义temp为可返回Complex类型值，使=可连续使用 
    temp.real = c.real;  temp.image = c.image;
    real = temp.real;  image = temp.image;
    return temp;
}
double Complex::abs() {//求绝对值函数
    return sqrt(real * real + image * image); 
}
Complex Complex::operator*(Complex c) {//重载*
    return Complex(real*c.real-image*c.image, real*c.image+c.real*image);
}
Complex Complex::operator/(Complex c) { //重载/
   double d  = c.real  *c.real + c.image * c.image ;
   return Complex((real * c.real + image * c.image) / d , 
                  (image * c.real - real * c.image) / d); 
}
bool Complex::operator<(Complex c) {
    return abs() < c.abs();
}
bool Complex::operator>(Complex c) {
    return abs() > c.abs();
}

int main() 
{
    Complex c1(1.0,1.0), c2(2.0,2.0), c3(4.0,4.0), c;
    double d = 0.5 ;
    c1.Print();
    c=c2+c3;  c.Print();
    c+=c1;    c.Print();
    c=c+d;    c.Print(); //可用0.5代替d    
    c=c3*c2;  c.Print();
    c=c3/c1;  c.Print();
    std::cout<<"c3的模为："<<c3.abs()<<std::endl;
    c=c3=c2=c1;    c.Print();  //连续赋值
    c+=c3+=c2+=c1; c.Print();  //连续加赋值
    if(c > c1)
        cout<<"ok"<<endl;
    cout << c << endl;
    c=d+c;
    return 0;
}