班级内部的操作员如何工作？

class A {
public:
    string operator+( const A& rhs ) {
        return "this and A&";
    }
};

string operator+( const A& lhs, const A& rhs ) {
    return "A& and A&";
}

string operator-( const A& lhs, const A& rhs ) {
    return "A& and A&";
}

int main() {
    A a;
    cout << "a+a = " << a + a << endl;
    cout << "a-a = " << a - a << endl;
    return 0;
}

//output
a+a = this and A&
a-a = A& and A&

我很好奇为什么为什么要调用类内部的运算符而不是外部的运算符。运营商之间是否有某种优先级？