C++重载>>和<<（输入和输出运算符）详解

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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。

其实 C++ 标准库已经提供了 complex 类，能够很好地支持复数运算，但是这里我们又自己定义了一个 complex 类，这样做仅仅是为了教学演示。

本节要达到的目标是让复数的输入输出和 int、float 等基本类型一样简单。假设 num1、num2 是复数，那么输出形式就是：

cout<<num1<<num2<<endl;

输入形式就是：

cin>>num1>>num2;

cout 是 ostream 类的对象，cin 是 istream 类的对象，要想达到这个目标，就必须以全局函数（友元函数）的形式重载<<和>>，否则就要修改标准库中的类，这显然不是我们所期望的。

重载输入运算符>>

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：

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1. istream & operator>>(istream &in, complex &A){
2. in >> A.m_real >> A.m_imag;
3. return in;
4. }

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);

>>运算符可以按照下面的方式使用：

complex c;
cin>>c;

当输入1.45 2.34后，这两个小数就分别成为对象 c 的实部和虚部了。cin>> c;这一语句其实可以理解为：

operator<<(cin , c);

重载输出运算符<<

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：

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1. ostream & operator<<(ostream &out, complex &A){
2. out << A.m_real <<" + "<< A.m_imag <<" i ";
3. return out;
4. }

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：

friend ostream & operator<<(ostream &out, complex &A);

综合演示

结合输入输出运算符的重载，重新实现 complex 类：

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1. #include <iostream>
2. using namespace std;
3. class complex{
4. public:
5. complex(double real = 0.0, double imag = 0.0): m_real(real), m_imag(imag){ };
6. public:
7. friend complex operator+(const complex & A, const complex & B);
8. friend complex operator-(const complex & A, const complex & B);
9. friend complex operator*(const complex & A, const complex & B);
10. friend complex operator/(const complex & A, const complex & B);
11. friend istream & operator>>(istream & in, complex & A);
12. friend ostream & operator<<(ostream & out, complex & A);
13. private:
14. double m_real; //实部
15. double m_imag; //虚部
16. };
17. //重载加法运算符
18. complex operator+(const complex & A, const complex &B){
19. complex C;
20. C.m_real = A.m_real + B.m_real;
21. C.m_imag = A.m_imag + B.m_imag;
22. return C;
23. }
24. //重载减法运算符
25. complex operator-(const complex & A, const complex &B){
26. complex C;
27. C.m_real = A.m_real - B.m_real;
28. C.m_imag = A.m_imag - B.m_imag;
29. return C;
30. }
31. //重载乘法运算符
32. complex operator*(const complex & A, const complex &B){
33. complex C;
34. C.m_real = A.m_real * B.m_real - A.m_imag * B.m_imag;
35. C.m_imag = A.m_imag * B.m_real + A.m_real * B.m_imag;
36. return C;
37. }
38. //重载除法运算符
39. complex operator/(const complex & A, const complex & B){
40. complex C;
41. double square = A.m_real * A.m_real + A.m_imag * A.m_imag;
42. C.m_real = (A.m_real * B.m_real + A.m_imag * B.m_imag)/square;
43. C.m_imag = (A.m_imag * B.m_real - A.m_real * B.m_imag)/square;
44. return C;
45. }
46. //重载输入运算符
47. istream & operator>>(istream & in, complex & A){
48. in >> A.m_real >> A.m_imag;
49. return in;
50. }
51. //重载输出运算符
52. ostream & operator<<(ostream & out, complex & A){
53. out << A.m_real <<" + "<< A.m_imag <<" i ";;
54. return out;
55. }
56. int main(){
57. complex c1, c2, c3;
58. cin>>c1>>c2;
59. c3 = c1 + c2;
60. cout<<"c1 + c2 = "<<c3<<endl;
61. c3 = c1 - c2;
62. cout<<"c1 - c2 = "<<c3<<endl;
63. c3 = c1 * c2;
64. cout<<"c1 * c2 = "<<c3<<endl;
65. c3 = c1 / c2;
66. cout<<"c1 / c2 = "<<c3<<endl;
67. return 0;
68. }

#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