# C++解析(17)：操作符重载

2022年11月23日
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## 1.操作符重载

``#include <stdio.h>class Complex{    int a;    int b;public:    Complex(int a = 0, int b = 0)    {        this->a = a;        this->b = b;    }    int getA() { return a; }    int getB() { return b; }    friend Complex Add(const Complex& p1, const Complex& p2);};Complex Add(const Complex& p1, const Complex& p2){    Complex ret;    ret.a = p1.a + p2.a;    ret.b = p1.b + p2.b;    return ret;}int main(){    Complex c1(1, 2);    Complex c2(3, 4);    Complex c3 = Add(c1, c2); // c1 + c2    printf("c3.a = %d, c3.b = %d\n", c3.getA(), c3.getB());    return 0;}``

``[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.outc3.a = 4, c3.b = 6``

• C++中的重载能够扩展操作符的功能
• 操作符的重载以函数的方式进行
• 本质——用特殊形式的函数扩展操作符的功能

operator的本质是通过函数重载操作符

``#include <stdio.h>class Complex{    int a;    int b;public:    Complex(int a = 0, int b = 0)    {        this->a = a;        this->b = b;    }    int getA() { return a; }    int getB() { return b; }    friend Complex operator + (const Complex& p1, const Complex& p2);};Complex operator + (const Complex& p1, const Complex& p2){    Complex ret;    ret.a = p1.a + p2.a;    ret.b = p1.b + p2.b;    return ret;}int main(){    Complex c1(1, 2);    Complex c2(3, 4);    Complex c3 = operator + (c1, c2); // c1 + c2    printf("c3.a = %d, c3.b = %d\n", c3.getA(), c3.getB());    return 0;}``

``[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.outc3.a = 4, c3.b = 6``

• 比全局操作符重载函数少一个参数（左操作符）
• 不需要依赖友元就可以完成操作符重载
• 编译器优先在成员函数中寻找操作符重载函数

``#include <stdio.h>class Complex{    int a;    int b;public:    Complex(int a = 0, int b = 0)    {        this->a = a;        this->b = b;    }    int getA() { return a; }    int getB() { return b; }    Complex operator + (const Complex& p)    {        Complex ret;        printf("Complex operator + (const Complex& p)\n");        ret.a = this->a + p.a;        ret.b = this->b + p.b;        return ret;    }    friend Complex operator + (const Complex& p1, const Complex& p2);};Complex operator + (const Complex& p1, const Complex& p2){    Complex ret;    printf("Complex operator + (const Complex& p1, const Complex& p2)\n");    ret.a = p1.a + p2.a;    ret.b = p1.b + p2.b;    return ret;}int main(){    Complex c1(1, 2);    Complex c2(3, 4);    Complex c3 = c1 + c2; // c1.operator + (c2)    printf("c3.a = %d, c3.b = %d\n", c3.getA(), c3.getB());    return 0;}``

``[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.outComplex operator + (const Complex& p)c3.a = 4, c3.b = 6``

## 2.完善的复数类

• 统一复数与实数的运算方式
• 统一复数与实数的比较方式

``// Complex.h#ifndef _COMPLEX_H_#define _COMPLEX_H_class Complex{    double a;    double b;public:    Complex(double a = 0, double b = 0);    double getA();    double getB();    double getModulus();    Complex operator + (const Complex& c);    Complex operator - (const Complex& c);    Complex operator * (const Complex& c);    Complex operator / (const Complex& c);    bool operator == (const Complex& c);    bool operator != (const Complex& c);    Complex& operator = (const Complex& c);};#endif``

``// Complex.cpp#include "Complex.h"#include "math.h"Complex::Complex(double a, double b){    this->a = a;    this->b = b;}double Complex::getA(){    return a;}double Complex::getB(){    return b;}double Complex::getModulus(){    return sqrt(a * a + b * b);}Complex Complex::operator + (const Complex& c){    double na = a + c.a;    double nb = b + c.b;    Complex ret(na, nb);    return ret;}Complex Complex::operator - (const Complex& c){    double na = a - c.a;    double nb = b - c.b;    Complex ret(na, nb);    return ret;}Complex Complex::operator * (const Complex& c){    double na = a * c.a - b * c.b;    double nb = a * c.b + b * c.a;    Complex ret(na, nb);    return ret;}Complex Complex::operator / (const Complex& c){    double cm = c.a * c.a + c.b * c.b;    double na = (a * c.a + b * c.b) / cm;    double nb = (b * c.a - a * c.b) / cm;    Complex ret(na, nb);    return ret;}bool Complex::operator == (const Complex& c){    return (a == c.a) && (b == c.b);}bool Complex::operator != (const Complex& c){    return !(*this == c);}Complex& Complex::operator = (const Complex& c){    if( this != &c )    {        a = c.a;        b = c.b;    }    return *this;}``

``// test.cpp#include <stdio.h>#include "Complex.h"int main(){    Complex c1(1, 2);    Complex c2(3, 6);    Complex c3 = c2 - c1;    Complex c4 = c1 * c3;    Complex c5 = c2 / c1;    printf("c3.a = %f, c3.b = %f\n", c3.getA(), c3.getB());    printf("c4.a = %f, c4.b = %f\n", c4.getA(), c4.getB());    printf("c5.a = %f, c5.b = %f\n", c5.getA(), c5.getB());    Complex c6(2, 4);    printf("c3 == c6 : %d\n", c3 == c6);    printf("c3 != c4 : %d\n", c3 != c4);    (c3 = c2) = c1;    printf("c1.a = %f, c1.b = %f\n", c1.getA(), c1.getB());    printf("c2.a = %f, c2.b = %f\n", c2.getA(), c2.getB());    printf("c3.a = %f, c3.b = %f\n", c3.getA(), c3.getB());    return 0;}``

``[root@bogon Desktop]# g++ test.cpp Complex.cpp[root@bogon Desktop]# ./a.outc3.a = 2.000000, c3.b = 4.000000c4.a = -6.000000, c4.b = 8.000000c5.a = 3.000000, c5.b = 0.000000c3 == c6 : 1c3 != c4 : 1c1.a = 1.000000, c1.b = 2.000000c2.a = 3.000000, c2.b = 6.000000c3.a = 1.000000, c3.b = 2.000000``

• C++规定赋值操作符（=）只能重载为成员函数
• 操作符重载不能改变原操作符的优先级
• 操作符重载不能改变操作符的个数
• 操作符重载不应改变操作符的原有语义

## 3.小结

• 操作符重载是C++的强大特性之一
• 操作符重载的本质是通过函数扩展操作符的功能
• operator关键字是实现操作符重载的关键
• 操作符重载遵循相同的函数重载规则
• 全局函数和成员函数都可以实现对操作符的重载
• 复数的概念可以通过自定义类实现
• 复数中的运算操作可以通过操作符重载实现
• 赋值操作符只能通过成员函数实现
• 操作符重载的本质为函数定义

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