学步园

作者：gzshun. 原创作品，转载请标明出处！
来源：http://blog.csdn.net/gzshun

运算符重载就是赋予已有的运算符多重含义。C++中通过重新定义运算符，使它能够用于特定类的对象执行特定的功能，从而增强了C++的扩充能力。

一、运算符重载的特点

在实现运算符重载的方法有2种：
1.用类的成员函数来实现；
2.通过类的友元函数来实现。
一个很重要的特点：
1.用类的成员函数来实现
成员函数的形参个数 = 几目运算符 - 1
假设重载运算符=，+，或+=，这些运算符是双目运算符，所以类的成员函数的形参必须为1.
2.通过类的幽怨函数来实现
友元函数的形参个数 = 几目运算符
假设重载运算符=，+，或+=，这些运算符是双目运算符，所以友元函数的形参必须为2.

二、C++不允许重载的运算符
不是所有的运算符都可以重载。在C++中不允许重载的运算符有：三目运算符"?:"，成员操作符"."、成员指针操作符"*"、作用域操作符"::"以及sizeof运算符。

三、运算符重载的基本用法-例子(+, -, +=)

#include <iostream>

using namespace std;

class CComplex
{
public:
    CComplex(double r = 0, double i = 0) : mRealPart(r), mImagePart(i)
    {
    }
    void Print() const
    {
        cout << mRealPart << " + " << mImagePart << "i" << endl;
    }
    CComplex operator+(CComplex &c);
    CComplex operator+(double r);
    friend CComplex operator+(double r, CComplex &c);
    friend CComplex operator-(CComplex &c);
    friend CComplex operator-(CComplex &c, double r);
    CComplex operator-(CComplex &c);
    void operator+=(CComplex &c);
private:
    double mRealPart;
    double mImagePart;
};

//类的成员函数 重载双目运算符"+"
CComplex CComplex::operator+(CComplex &c)
{
    cout << "调用CComplex::operator+(CComplex &c)函数" << endl;
    CComplex tmp;
    tmp.mRealPart = this->mRealPart + c.mRealPart;
    tmp.mImagePart = this->mImagePart + c.mImagePart;
    return tmp;
}

//类的成员函数 重载双目运算符"+"
CComplex CComplex::operator+(double r)
{
    cout << "调用CComplex::operator+(double r)函数" << endl;
    CComplex tmp;
    tmp.mRealPart = this->mRealPart + r;
    tmp.mImagePart = this->mImagePart;
    return tmp;
}

//类的友元函数 重载双目运算符"+"
CComplex operator+(double r, CComplex &c)
{
    cout << "调用friend CComplex operator+(double r, CComplex &c)函数" << endl;
    CComplex tmp;
    tmp.mRealPart = r + c.mRealPart;
    tmp.mImagePart = c.mImagePart;
    return tmp;
}

//类的友元函数 重载单目运算符"-"
CComplex operator-(CComplex &c)
{
    cout << "调用friend CComplex operator-(CComplex &c)函数" << endl;
    return CComplex(-c.mRealPart, -c.mImagePart);
}

//类的友元函数 重载双目运算符"-"
CComplex operator-(CComplex &c, double r)
{
    cout << "调用friend CComplex operator-(CComplex &c, double r)函数" << endl;
    CComplex tmp;
    tmp.mRealPart = c.mRealPart - r;
    tmp.mImagePart = c.mImagePart;
    return tmp;
}

//类的成员函数 重载双目运算符"-"
CComplex CComplex::operator-(CComplex &c)
{
    cout << "调用CComplex operator-(CComplex &c)函数" << endl;
    CComplex tmp;
    tmp.mRealPart = this->mRealPart - c.mRealPart;
    tmp.mImagePart = this->mImagePart - c.mImagePart;
    return tmp;
}

//类的成员函数 重载双目运算符"+="
void CComplex::operator+=(CComplex &c)
{
    cout << "调用void operator+=(CComplex &c)函数" << endl;
    this->mRealPart += c.mRealPart;
    this->mImagePart += c.mImagePart;
}

int main()
{
    CComplex c1(11, 12);
    CComplex c2(13, 14);
    CComplex c;

    c = c1 + c2;
    c.Print();

    c = c1 + 6.0;
    c.Print();

    c = 8.0 + c2;
    c.Print();

    c = -c1;
    c.Print();

    c = c1 - 6.0;
    c.Print();

    c = c2 - c1;
    c.Print();

    c1 += c2;
    c1.Print();

    return 0;
}

执行结果：
调用CComplex::operator+(CComplex &c)函数
24 + 26i
调用CComplex::operator+(double r)函数
17 + 12i
调用friend CComplex operator+(double r, CComplex &c)函数
21 + 14i
调用friend CComplex operator-(CComplex &c)函数
-11 + -12i
调用friend CComplex operator-(CComplex &c, double r)函数
5 + 12i
调用CComplex operator-(CComplex &c)函数
2 + 2i
调用void operator+=(CComplex &c)函数

24 + 26i

四、自增与自减的重载

#include <iostream>

using namespace std;

class CCounter
{
public:
    CCounter(int x = 0) : mCounter(x)
    {
    }
    CCounter operator++();
    CCounter operator++(int);
    void Print() const
    {
        cout << "mCounter = " << mCounter << endl;
    }
private:
    int mCounter;
};

CCounter CCounter::operator++()
{
    cout << "调用CCounter operator++()函数 前缀自增" << endl;
    ++mCounter;
    return *this;
}

CCounter CCounter::operator++(int)
{
    cout << "调用CCounter operator++(int)函数 后缀自增" << endl;
    mCounter++;
    return *this;
}

int main()
{
    int i;
    CCounter demo1, demo2;

    for(i = 0; i < 2; i++)
    {
        ++demo1;
    }
    demo1.Print();

    for(i = 0; i < 2; i++)
    {
        demo2++;
    }
    demo2.Print();

    return 0;
}

执行结果：
调用CCounter operator++()函数 前缀自增
调用CCounter operator++()函数 前缀自增
mCounter = 2
调用CCounter operator++(int)函数 后缀自增
调用CCounter operator++(int)函数 后缀自增

mCounter = 2