To draw a circle you need a radius(r) and a position(pos). The radius is a float, the position a Vec2. There is more than one way to pass these to a circle:
// using the constructor, with r(float), pos.x(float), pos.y(float)
Circle c(0.1, 0, 0);
// using the constructor, with r(float), pos(Vec2)
5Circle c(0.1, Vec2(0, 0));
// during the coarse of the application
c.set(0.1, 0, 0);
c.set(0.1, Vec2(0, 0));
10
// directly changing the variables
c.r = 0.1;
c.pos = Vec2(0, 0);