Sort points by angle from given axis?

Question

How can I sort an array of points/vectors by counter-clockwise increasing angle from a given axis vector?

For example:

Answer 1

This is an example of how I went about solving this. It converts to polar to get the angle and then is used to compare them. You should be able to use this in a sort function like so:

std::sort(vectors.begin(), vectors.end(), VectorComp(centerPoint));

Below is the code for comparing

struct VectorComp : std::binary_function
{

    sf::Vector2f M;
    IntersectComp(sf::Vector2f v) : M(v) {}

    bool operator() ( sf::Vector2f o1,  sf::Vector2f o2)
    {
        float ang1     = atan( ((o1.y - M.y)/(o1.x - M.x) ) * M_PI / 180);
        float ang2     = atan( (o2.y - M.y)/(o2.x - M.x) * M_PI / 180);
        if(ang1 < ang2) return true;
        else if (ang1 > ang2) return false;
        return true;
    }
};

It uses sfml library but you can switch any vector/point class instead of sf::Vector2f. M would be the center point. It works great if your looking to draw a triangle fan of some sort.