c++ - Operator overloading outside class

There are two ways to overload operators for a C++ class:

Inside class

class Vector2
{
public:
    float x, y ;

    Vector2 operator+( const Vector2 & other )
    {

        Vector2 ans ;
        ans.x = x + other.x ;
        ans.y = y + other.y ;
        return ans ;
    }
} ;

Outside class

class Vector2
{
public:
    float x, y ;
} ;

Vector2 operator+( const Vector2& v1, const Vector2& v2 )
{
    Vector2 ans ;
    ans.x = v1.x + v2.x ;

    ans.y = v1.y + v2.y ;
    return ans ;
}

(Apparently in C# you can only use the "outside class" method.)

In C++, which way is more correct? Which is preferable?

Answer

The basic question is "Do you want conversions to be performed on the left-hand side parameter of an operator?". If yes, use a free function. If no, use a class member.

For example, for operator+() for strings, we want conversions to be performed so we can say things like:

string a = "bar";
string b = "foo" + a;

where a conversion is performed to turn the char * "foo" into an std::string. So, we make operator+() for strings into a free function.