1. 操作符重载的概念

操作符重载的本质

操作符重载的本质是用特殊形式的函数扩展操作符的功能

• C++通过operator关键字定义操作符重载函数
• 操作符重载遵循相同的函数重载规则

操作符重载的规则

在进行操作符重载时，必须遵循以下三条规则

• 不能改变原操作符的优先级
• 不能改变操作数的个数
• 不能改变操作符的原有语义

操作符重载的实现

全局函数和成员函数都可以实现对操作符的重载，重载为全局函数的语法规则为

/*
 * Sign为预定义的操作符，如：+, -, *, /;
 * lp和rp分别为左操作数和右操作数.
*/
Type operator Sign (const Type &lp, const Type &rp)
{
}

如果重载为全局函数，则需要在友元的辅助下才能实现，因此，一般选择将操作符重载为类的成员函数

class Type
{
public:
    Type operator Sign (const Type &rp)
    {
    }
};

• 比起全局操作符重载函数少一个参数（左操作数）
• 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;
}

2. 复数类的实现

下面，我们通过一个复数类，来分别实现算术运算操作符、比较操作符和赋值操作符的重载，以下是头文件和源文件的非重载部分代码。

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 <cmath>

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);
}

运算操作符重载

Complex.cpp

Complex Complex::operator + (const Complex &c)
{
    Complex ret;

    ret.a = a + c.a;
    ret.b = b + c.b;

    return ret;
}

Complex Complex::operator - (const Complex &c)
{
    Complex ret;

    ret.a = a - c.a;
    ret.b = b - c.b;

    return ret;
}

Complex Complex::operator * (const Complex &c)
{
    Complex ret;

    ret.a = a * c.a - b * c.b;
    ret.b = a * c.b + b * c.a;

    return ret;
}

Complex Complex::operator / (const Complex &c)
{
    Complex ret;
    double cm = c.a * c.a + c.b * c.b;

    ret.a = (a * c.a + b * c.b) / cm;
    ret.b = (b * c.a - a * c.b) / cm;

    return ret;
}

比较操作符重载

Complex.cpp

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

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

赋值操作符重载

Complex.cpp

Complex &Complex::operator = (const Complex &c)
{
    if (this != &c)
    {
        a = c.a;
        b = c.b;
    }

    return *this;
}

注意，赋值操作符有几点特殊之处：

• C++规定赋值操作符只能重载为成员函数
• 赋值操作符重载函数返回类型为引用，目的是为了实现连等，如a = b = c

复数类测试

main.cpp

#include "Complex.h"
#include <cstdio>

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;
}

3. 赋值操作符重载和拷贝构造函数

赋值操作符重载与深拷贝

赋值操作符重载和拷贝构造函数具有相同的作用和意义，那么，C++什么时候调用赋值操作符重载函数？什么时候调用拷贝构造函数？

ClassName c1;
ClassName c2 = c1; //调用拷贝构造函数
ClassName c3(c1);  //调用拷贝构造函数

ClassName c4;
c4 = c1;           //调用赋值操作符重载函数

和拷贝构造函数类似

• 编译器为每个类默认重载了赋值操作符
• 默认的赋值操作符重载函数仅完成浅拷贝
• 当需要进行深拷贝时，必须重载赋值操作符

作为一般性原则，重载赋值操作符，必然需要实现深拷贝！！！

#include <iostream>
#include <string>

using namespace std;

class Test
{
    int *m_pointer;
public:
    Test()
    {
        m_pointer = NULL;
    }

    Test(int i)
    {
        m_pointer = new int(i);
    }

    /*拷贝构造函数实现深拷贝*/
    Test(const Test &obj)
    {
        m_pointer = new int(*obj.m_pointer);
    }

    /*重载赋值操作符实现深拷贝*/
    Test &operator = (const Test &obj)
    {
        if (this != &obj)
        {
            delete m_pointer;
            m_pointer = new int(*obj.m_pointer);
        }

        return *this;
    }

    void print()
    {
        cout << "m_pointer = " << hex << m_pointer << endl;
    }

    ~Test()
    {
        delete m_pointer;
    }
};

int main()
{
    Test t1 = 1;
    Test t2;

    t2 = t1;

    t1.print();
    t2.print();

    return 0;
}

数组类IntArray改进

IntArray.h

#ifndef _INTARRAY_H_
#define _INTARRAY_H_

class IntArray
{
public:
    IntArray &operator = (const IntArray &obj); //Add
};

#endif

IntArray.cpp

IntArray &IntArray::operator = (const IntArray &obj)
{
    if (this != &obj)
    {
        int *pointer = new int[obj.m_length];

        if (pointer != NULL)
        {
            for (int i = 0; i < obj.m_length; i++)
            {
                pointer[i] = obj.m_pointer[i];
            }

            m_length = obj.m_length;
            delete m_pointer;
            m_pointer = pointer;
        }
    }

    return *this;
}