Question

c++运算符重载调用问题求解答

有如下代码，其中标识1，2，3的三个地方的if语句在此处调用3个不同类型的函数，它在调用对应的函数是怎么区分的，比如按照什么调用类成员运算符函数，类成员函数，和外部普通函数。程式是怎么区别它们进行调用的。请帮忙解答，非常感谢。

#include<iostream>
using std::cout;
using std::endl;

class CBox
{
public:

explicit CBox(double lv=1.0,double wv=1.0,double hv=1.0):
m_Length(lv),m_Width(wv),m_Height(hv)

{
cout<<endl<<"Constructor called.";
}
double Volume()const
{
return m_Length*m_Width*m_Height;
}

bool operator>(const CBox& aBox) const
{
return this->Volume() >aBox.Volume();
}
bool operator>(const double&value)const
{
return this->Volume()>value;
}

~CBox()
{
cout<<"Destructor called."<<endl;
}
private:
double m_Length;
double m_Width;
double m_Height;
};

bool operator>(const double& value,const CBox&aBox);

int main()
{
CBox smallBox(4.0,2.0,1.0);
CBox mediumBox(10.0,4.0,2.0);

if(mediumBox>smallBox)-------------------------------------------------------1.
cout<<endl<<"mediumBox is bigger than smallBox";
if(mediumBox>50.0)------------------------------------------------------------2.
cout<<endl<<"mediumBox capacity is more than 50";
else cout<<endl<<"mediumBox capacity is not more than 50";
if(10.0>smallBox)----------------------------------------------------------------3.
cout<<endl<<"smallBox capacity is less than 10";
else
cout<<endl<<"smallBox capacity is not less than 10";

cout<<endl;
return 0;
}

bool operator>(const double&value, const CBox& aBox)
{
return value>aBox.Volume();
}

Answer 1

1. mediumBox>smallBox
   

   比较符两边都是CBox，等同于mediumBox.operator>(smallBox)

   因此调用bool operator>(const CBox& aBox) const
   

2. mediumBox>50.0
   左边是CBox，右边是double，等同于mediumBox.operator>(50.0)

   因此调用bool operator>(const double&value)const
   

3. 10.0>smallBox
   左边double，右边CBox，但是不存在10.0.operator>(CBox)

   但是有非成员函数bool operator>(const double&value, const CBox& aBox)
   

   因此调用bool operator>(const double&value, const CBox& aBox)