在C++中，标准库本身已经对左移运算符<<和右移运算符>>分别进行了重载，使其能够用于不同数据的输入输出，但是输入输出的对象只能是 C++ 内置的数据类型（例如 bool、int、double 等）和标准库所包含的类类型（例如 string、complex、ofstream、ifstream 等）。
如果我们自己定义了一种新的数据类型，需要用输入输出运算符去处理，那么就必须对它们进行重载。本节以前面的 complex 类为例来演示输入输出运算符的重载。
下面我们以全局函数的形式重载>>，使它能够读入两个 double 类型的数据，并分别赋值给复数的实部和虚部：
istream & operator>>(istream &in, complex &A){ in >> A.m_real >> A.m_imag; return in; }
istream 表示输入流，cin 是 istream 类的对象，只不过这个对象是在标准库中定义的。之所以返回 istream 类对象的引用，是为了能够连续读取复数，让代码书写更加漂亮，例如：
complex c1, c2; cin>>c1>>c2;
如果不返回引用，那就只能一个一个地读取了：
complex c1, c2; cin>>c1; cin>>c2;
另外，运算符重载函数中用到了 complex 类的 private 成员变量，必须在 complex 类中将该函数声明为友元函数：
friend istream & operator>>(istream & in , complex &a);
同样地，我们也可以模仿上面的形式对输出运算符>>进行重载，让它能够输出复数，请看下面的代码：
ostream & operator<<(ostream &out, complex &A){ out << A.m_real <<" + "<< A.m_imag <<" i "; return out; }
ostream 表示输出流，cout 是 ostream 类的对象。由于采用了引用的方式进行参数传递，并且也返回了对象的引用，所以重载后的运算符可以实现连续输出。
为了能够直接访问 complex 类的 private 成员变量，同样需要将该函数声明为 complex 类的友元函数：
(可以加上const修饰)
friend ostream & operator<<(ostream &out, const complex &A);
结合输入输出运算符的重载，重新实现 complex 类：
#include <iostream> using namespace std; class complex{ public: complex(double real = 0.0, double imag = 0.0): m_real(real), m_imag(imag){ }; public: friend complex operator+(const complex & A, const complex & B); friend complex operator-(const complex & A, const complex & B); friend complex operator*(const complex & A, const complex & B); friend complex operator/(const complex & A, const complex & B); friend istream & operator>>(istream & in, complex & A); friend ostream & operator<<(ostream & out, complex & A); private: double m_real; //实部 double m_imag; //虚部 }; //重载加法运算符 complex operator+(const complex & A, const complex &B){ complex C; C.m_real = A.m_real + B.m_real; C.m_imag = A.m_imag + B.m_imag; return C; } //重载减法运算符 complex operator-(const complex & A, const complex &B){ complex C; C.m_real = A.m_real - B.m_real; C.m_imag = A.m_imag - B.m_imag; return C; } //重载乘法运算符 complex operator*(const complex & A, const complex &B){ complex C; C.m_real = A.m_real * B.m_real - A.m_imag * B.m_imag; C.m_imag = A.m_imag * B.m_real + A.m_real * B.m_imag; return C; } //重载除法运算符 complex operator/(const complex & A, const complex & B){ complex C; double square = A.m_real * A.m_real + A.m_imag * A.m_imag; C.m_real = (A.m_real * B.m_real + A.m_imag * B.m_imag)/square; C.m_imag = (A.m_imag * B.m_real - A.m_real * B.m_imag)/square; return C; } //重载输入运算符 istream & operator>>(istream & in, complex & A){ in >> A.m_real >> A.m_imag; return in; } //重载输出运算符 ostream & operator<<(ostream & out, complex & A){ out << A.m_real <<" + "<< A.m_imag <<" i ";; return out; } int main(){ complex c1, c2, c3; cin>>c1>>c2; c3 = c1 + c2; cout<<"c1 + c2 = "<<c3<<endl; c3 = c1 - c2; cout<<"c1 - c2 = "<<c3<<endl; c3 = c1 * c2; cout<<"c1 * c2 = "<<c3<<endl; c3 = c1 / c2; cout<<"c1 / c2 = "<<c3<<endl; return 0; }
运行结果：
2.4 3.6↙
4.8 1.7↙
c1 + c2 = 7.2 + 5.3 i
c1 - c2 = -2.4 + 1.9 i
c1 * c2 = 5.4 + 21.36 i
c1 / c2 = 0.942308 + 0.705128 i
原文：http://www.cnblogs.com/wft1990/p/7118928.html