copy Curve2D getCenterPoint getLength isClosed

isClosed() virtual bool isClosed ( ) pure virtual 在 ArcofCircle2D, ArcofEllipse2D, BezierCurve2D, BSplineCurve2D, Circle2D, Ellipse2D, Hyperbola2D, Line2D, LineSegment2D, NURBSCurve2D, Parabola2D , 以及 PolyLine2D 内被实现.