c++ - 函数比较器可以是静态函数吗？

Question

我已经为 Point2D 类实现了两个运算符重载

1. operator<
2. operator>

在头文件中，我需要声明为友元函数，否则会出现编译错误。
问题:运算符重载函数总是友元函数吗？可以是静态的吗？我尝试使用静态函数，但出现错误:

Point2D.h:19:58: error: ‘static bool Point2D::operator<(const Point2D&, const Point2D&)’ must be either a non-static member function or a non-member function
In file included from Point2D.cpp:3:0:
Point2D.h:19:58: error: ‘static bool Point2D::operator<(const Point2D&, const Point2D&)’ must be either a non-static member function or a non-member function
Point2D.h: In function ‘bool operator<(const Point2D&, const Point2D&)’:
Point2D.h:23:7: error: ‘int Point2D::x’ is protected
Point2D.cpp:36:11: error: within this context
Point2D.h:24:7: error: ‘int Point2D::y’ is protected
Point2D.cpp:36:17: error: within this context

---

class Point2D
{

    friend ostream &operator<<( ostream &output, const Point2D &P);
    friend bool operator<(const Point2D& A, const Point2D& B);
    friend bool operator>(const Point2D& A, const Point2D& B);      

    protected:  
        int x;
        int y;
    public:
        //Constructor
        Point2D();
        Point2D (int x, int y);

        //Accessors
        int getX();
        int getY();

        //Mutators
        void setX (int x);
        void setY (int y);
};


ostream &operator<<( ostream &output, const Point2D &P)
              { 
             output << "X : " << P.x << "Y : " << P.y;
             return output;            
              }

    bool operator<(const Point2D& A, const Point2D& B ) 
    {
        return A.x < B.y;
    };


    bool operator>(const Point2D& A, const Point2D& B ) 
    {
        return A.x > B.y;
    };

Answer 1

二元运算符可以作为全局二元函数或一元成员函数来实现。在后者中，运算符的第一个操作数被隐式传递(是 *this，在所有成员函数中):

选项1:全局函数

struct foo
{
    int a;
};

bool operator==( const foo& lhs , const foo& rhs )
{
    return lhs.a == rhs.b;
}

方案二:成员函数

struct foo
{
    int a;

    bool operator==( const foo& rhs )
    {
//             +---- this->a
//             |
//             v
        return a == rhs.a;
    }
};

选项3:类内部的全局函数，声明为友元(我的最爱)

struct foo
{
    int a;

    friend bool operator==( const foo& lhs , const foo& rhs )
    {
        return lhs.a == rhs.b;
    }
};

有关运算符重载的完整而深入的指南，请查看此 C++ wiki 线程:Operator overloading