Operator overloading with friend function error

I am working on an assignment that introduces me to operator overloading. I have to overload some binary operators as member functions and also as friend functions. My member function that overloads the "+" operator works fine but my friend function that overloads the "-" operator seems to have trouble finding data that the member function is able to use.

class def:

class matrix
{
    friend ostream& operator << (ostream&, const matrix&);
    friend bool operator == (const matrix &, const matrix &);
    friend matrix operator - (const matrix &, const matrix &);

private:
    int size;
    int range;
    int array[10][10];

public:
    matrix(int);
    matrix(int, int);
    bool operator != (const matrix &) const;
    matrix operator + (const matrix &) const;
    const matrix & operator = (const matrix &);
};

"+" overload:

matrix matrix::operator + (const matrix & a) const
{
    matrix temp(size,range);

    f开发者_如何转开发or (int i = 0; i < a.size; i++)
        for (int j = 0; j < a.size; j++)
            temp.array[i][j] = a.array[i][j] + array[i][j];

    return temp;
} 

"-" overload:

matrix operator - (const matrix & a, const matrix & b)
{
    matrix temp(size, range);

    for (int i = 0; i < a.size; i++)
        for (int j = 0; j < a.size; j++)
            temp.array[i][j] = a.array[i][j] - array[i][j];

    return temp;
}

The error I am getting in the friend function is that size, range, and array are all undeclared. I am confused because I thought member and friend functions both had equal access to data in a class and I basically doing the same thing in both functions. Does anyone know what my issue may be?

The friend operator is not part of the class. Thus, it does not know size and range and array. You must use the objects a and b. It should be something like this:

matrix operator - (const matrix & a, const matrix & b)
{
   if(a.size != b.size)
      throw std::exception(...);

   matrix temp(a.size, a.range);

   for (int i = 0; i < a.size; i++)
                for (int j = 0; j < a.size; j++)
                     temp.array[i][j] = a.array[i][j] - b.array[i][j];

    return temp;
}

Though your friend function would be able to access the private data of the Objects, but this doesn't imply that the attributes are in the scope of that function. I mean, it wont act like a member function as you are expecting it to. You would need to provide the size from one of the objects that you are passing.