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C++重载>>和<<(输入和输出运算符)详解

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。 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<<>>,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:
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:
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:
#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

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