运算符重载总结（大全）

#include <iostream>
//#include <string>
#include <sstream>
//#include <stdbool.h>
#include <stdexcept>
/* run this program using the console pauser or add your own getch, system("pause") or input loop */
class A
{
    public:
        A();
        A(int initialValue);
        A(const A& src);
        
        //友元函数是全局函数 ，返回的是对象本身，不是引用 
        friend A operator +(const A& lhs,const A& rhs);
        friend A operator -(const A& lhs,const A& rhs);
        friend A operator *(const A& lhs,const A& rhs);
        friend A operator /(const A& lhs,const A& rhs);
        // 局部函数,简写运算符
        A& operator = (const A& rhs);
//        A& operator +(const A& rhs);
//        A& operator -(const A& rhs);
//        A& operator *(const A& rhs);
//        A& operator /(const A& rhs);
        //局部函数,简写运算符 
        A& operator +=(const A&rhs);
        A& operator -=(const A&rhs);
        A& operator *=(const A&rhs);
        A& operator /=(const A&rhs);
        //比较运算符
        friend bool operator ==(const A& lhs,const A& rhs);
        friend bool operator !=(const A& lhs,const A& rhs);
        friend bool operator >=(const A& lhs,const A& rhs);
        friend bool operator <=(const A& lhs,const A& rhs);
        friend bool operator >(const A& lhs,const A& rhs);
        friend bool operator <(const A& lhs,const A& rhs);
        //普通成员函数 
        void setValue(int value);
        void show();
         
    private:
        int mValue;        
};
A::A()
:mValue(0)
{
    
}
A::A(int initialValue)
:mValue(initialValue)
{
    
}
A::A(const A& src)
:mValue(src.mValue)
{
    
}
void A::setValue(int value)
{
    mValue = value;
}
 
A operator +(const A& lhs,const A& rhs)
{
    std::cout << "AAA" << std::endl;    //检测语句 
    A newA;
    newA.setValue(lhs.mValue + rhs.mValue);
    return newA;
}
A operator -(const A& lhs,const A& rhs)
{
    std::cout << "BBB" << std::endl;    //检测语句 
    A newA;
    newA.setValue(lhs.mValue - rhs.mValue);
    return newA;
}
A operator *(const A& lhs,const A& rhs)
{
    std::cout << "CCC" << std::endl;    //检测语句 
    A newA;
    newA.setValue(lhs.mValue * rhs.mValue);
    return newA;
}
A operator /(const A& lhs,const A& rhs)
{
    std::cout << "DDD" << std::endl;    //检测语句 
    if(!rhs.mValue)
    {
        throw std::invalid_argument("denominator is zero
");
    }
    A newA;
    newA.setValue(lhs.mValue / rhs.mValue);
    return newA;
}

A& A::operator = (const A& rhs)
{
    std::cout << "EEE" << std::endl;    //检测语句 
    if(this == &rhs)
    {
        std::cout << "自赋值" << std::endl; 
        return *this;
    }
    this->mValue = rhs.mValue;
    return * this;
}
//A& A::operator +(const A& rhs)
//{
//    std::cout << "!!!!!" << std::endl;    //检测语句 
//    this->setValue(this->mValue + rhs.mValue);
//    return *this;
//}
//A& A::operator -(const A& rhs)
//{
//    std::cout << "@@@@@" << std::endl;    //检测语句 
//    this->setValue(this->mValue - rhs.mValue);
//    return *this;
//}
//A& A::operator *(const A& rhs)
//{
//    std::cout << "#####" << std::endl;    //检测语句 
//    this->setValue(this->mValue + rhs.mValue);
//    return *this;
//} 
//A& A::operator /(const A& rhs)
//{
//    std::cout << "￥￥￥￥" << std::endl;    //检测语句 
//    if(!rhs.mValue)
//    {
//        throw std::invalid_argument("denominator is zero
");
//    }
//    this->setValue(this->mValue / rhs.mValue);
//    return *this;
//}
A& A::operator +=(const A&rhs)
{
    std::cout << "FFF" << std::endl;    //检测语句 
    this->setValue(this->mValue + rhs.mValue);
    return *this;
}
A& A::operator -=(const A&rhs)
{
    std::cout << "GGG" << std::endl;    //检测语句 
    this->setValue(this->mValue - rhs.mValue);
    return *this;
}
A& A::operator *=(const A&rhs)
{
    std::cout << "HHH" << std::endl;    //检测语句 
    this->setValue(this->mValue * rhs.mValue);
    return *this;
}
A& A::operator /=(const A&rhs)
{
    std::cout << "III" << std::endl;    //检测语句 
    if(!rhs.mValue)
    {
        throw std::invalid_argument("denominator is zero!
");
    }
    this->setValue(this->mValue / rhs.mValue);
    return *this;
}
bool operator ==(const A& lhs,const A& rhs)
{
    std::cout << "QQQ" << std::endl;    //检测语句 
    return lhs.mValue == rhs.mValue;
}
bool operator !=(const A& lhs,const A& rhs)
{
    std::cout << "PPP" << std::endl;    //检测语句 
    return lhs.mValue != rhs.mValue;
}
bool operator >=(const A& lhs,const A& rhs)
{
    std::cout << "TTT" << std::endl;    //检测语句 
    return lhs.mValue >= rhs.mValue;
} 
bool operator <=(const A& lhs,const A& rhs)
{
    std::cout << "OOO" << std::endl;    //检测语句 
    return lhs.mValue <= rhs.mValue;
} 
bool operator >(const A& lhs,const A& rhs)
{
    std::cout << "KKK" << std::endl;    //检测语句 
    return lhs.mValue > rhs.mValue;
}
bool operator <(const A& lhs,const A& rhs)
{
    std::cout << "XXX" << std::endl;    //检测语句 
    return lhs.mValue < rhs.mValue;
}
void A::show()
{
    std::cout << "value : " << mValue << std::endl;
}

int main(int argc, char** argv) 
{
    A a1(1);
    A a2(2);
    A a3(3);
    A a4(4);
    A a5,a8,a6,a7;
    a5 = a1 + a2;    //先运行operator+,后运行operator=    ，a1的mValue = 3 
    a5.show();
    
    a6 = a1 + 3;    // 先运行operator+,后运行operator=    , a1的mValue = 6
    a6.show();
    
    a7 = 5 + a1;
    a7.show();
    
    a8 = a1 + a2;
    if(a8 == a5)
    {
        std::cout << "==运算符运行！" << std::endl; 
    }else
    {
        std::cout << "==不能运算符运行！" << std::endl;
    }
    return 0;
}

运算的结果：

AAA
EEE
value : 3
AAA
EEE
value : 4
AAA
EEE
value : 6
AAA
EEE
QQQ
==运算符运行！

这样子是我们期望的结果，但是当我们把类内注释的部分取消注释时，会发现是这样的结果！

!!!!!
EEE
value : 3
!!!!!
EEE
value : 6
AAA
EEE
value : 11
!!!!!
EEE
QQQ
==不能运算符运行！

这是为什么呢？

原因是：当实例中编译器碰到+号的时候，首先在类内的成员函数寻找，这样就找到了A& operator = (const A& rhs);

如果是类内成员函数美誉匹配的operator+时，编译器才会接着往下找全局函数，就找到了friend A operator +(const A& lhs,const A& rhs);

首先他是友函数,是全局函数，