C++之操作符重载(+、-、*、/、=、==、!=)

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#C++ #操作符 #重载

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本文介绍了C++中操作符重载的意义、语法及具体实现，包括如何将操作符重载为全局函数和成员函数，并提供了复数类的示例代码。

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一、操作符重载的意义

　　C++中重载操作符能够扩展操作符的功能，操作符重载是以函数的方式进行的。其实操作符重载的本质为用特殊形式的函数扩展操作符的功能。

二、操作符重载的语法

　　重载操作符时是通过operator关键字来定义特殊的函数，其本质是通过函数重载操作符。其语法规则如下：

Type operator Sign(const Type& p1, const Type& p2)
{
    Type ret;

    return ret;
}

Sign为系统中预定义的操作符，如：+、-、*、/，等

三、操作符重载具体实现

　　操作符重载函数可以是全局函数，也可以是成员函数，下面分别给出实现：
　　
1.重载为全局函数
　　重载为全局函数时，必须给出所有的操作数。当然，全局函数也可以作为友元函数。下面代码为通过友元函数重载复数类的“+”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 = c1 + c2; // operator + (c1, c2)

    printf("c3.a = %d, c3.b = %d\n", c3.getA(), c3.getB());

    return 0;
}

运行结果如下：

这里写图片描述

　　下面给出复数类“==”和“！=”操作符非友元全局函数重载：

#include <iostream>

using namespace std;

class Complex
{
    double m_a;
    double m_b;
public:
    Complex(double a = 0, double b = 0)
    {
        m_a = a;
        m_b = b;
    }
    double getA() const
    {
        return m_a;
    }
    double getB() const
    {
        return m_b;
    }
};

bool operator == (const Complex& l, const Complex& r)
{
    return (l.getA() == r.getA()) && (l.getB() == l.getB());
}
bool operator != (const Complex& l, const Complex& r)
{
    return !(l == r);
}

int main()
{
    Complex c1(1, 2);
    Complex c2(1, 2);

    cout << "(c1 == c2) : " << (c1 == c2) << endl;
    cout << "(c1 != c2) : " << (c1 != c2) << endl;

    return 0;
}

运行结果如下所示：

这里写图片描述

2.重载为成员函数
　　操作符重载为成员函数时，比重载为全局操作符重载函数少一个参数（左操作数为this指针），编译器优先在成员函数中寻找操作符重载函数。下面给出复数类各操作符重载为成员函数的代码：
　　
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);
}

// 重载+操作符
Complex Complex::operator + (const Complex& c)
{
    double na = a + c.a;
    double nb = b + c.b;
    Complex ret(na, nb);

    return ret;
}

// 重载-操作符
Complex Complex::operator - (const Complex& c)
{
    double na = a - c.a;
    double nb = b - c.b;
    Complex ret(na, nb);

    return ret;
}

// 重载*（乘法）操作符
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;
}

// 重载/(除法)操作符
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;
}

// 重载==操作符
bool Complex::operator == (const Complex& c)
{
    return (a == c.a) && (b == c.b);
}

// 重载!=操作符
bool Complex::operator != (const Complex& c)
{
    return !(*this == c);
}

// 重载=（赋值）操作符
Complex& Complex::operator = (const Complex& c)
{
    if( this != &c )
    {
        a = c.a;
        b = c.b;
    }

    return *this;
}

main.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;
}