Operator Overloading, in General

[Salbrismind]

I'm having trouble overriding the + and - operators.


My struct definition with overloaded methods:

struct vectorf
{
	float x;
	float y;
	float z;

	const vectorf operator+(const vectorf other);
	const vectorf operator-(const vectorf other);
};

const vectorf vectorf::operator+(vectorf other)
{
	vectorf ans;

	ans.x = x + other.x;
	ans.y = y + other.y;
	ans.z = z + other.z;

	return ans;
}

const vectorf vectorf::operator-(const vectorf other)
{
	vectorf ans;

	ans.x = x - other.x;
	ans.y = y - other.y;
	ans.z = z - other.z;

	return ans;
}

And this is the line of code giving me trouble:

glTranslateVector(MAIN_POSITION - BACKGROUND_POSITION);

The error says:
" error C2678: binary '-' : no operator found which takes a left-hand operand of type 'const vectorf' (or there is no acceptable conversion) "

And:
" could be 'const vectorf vectorf:: operator -(const vectorf)' "


Any help would be greatly appreciated.
-Salbris