类内运算符重载报错：参数过多

When operator+ is defined inside class, left operand of operator is current instance. So, to declare a overload of operator+ you have 2 choices
inside class, with only one parameter which is right operand
outside of class, with two parameters, left and right operands.
Choice 1: outside class

class Vector
{
private:
    double i;
    double j;
    double k;
public:
    Vector(double _i, double _j, double _k)
    {
        i = _i;
        j = _j;
        k = _k;
    }

    Vector& operator+=(const Vector& p1)
    {
        i += p1.i;
        j += p1.j;
        k += p1.k;
        return *this;
    }

    //Some other functionality...


};

Vector operator+(const Vector& p1, const Vector& p2)
{
    Vector temp(p1);
    temp += p2;
    return temp;
}

Choice 2: inside class

class Vector
{
private:
    double i;
    double j;
    double k;
public:
    Vector(double _i, double _j, double _k)
    {
        i = _i;
        j = _j;
        k = _k;
    }

    Vector& operator+=(const Vector& p1)
    {
        i += p1.i;
        j += p1.j;
        k += p1.k;
        return *this;
    }



    Vector operator+(consr Vector & p2)
    {
        Vector temp(*this);
        temp += p2;
        return temp;
    }

};

https://stackoverflow.com/questions/35943537/error-c2804-binary-operator-has-too-many-parameters-compiling-with-vc-120