This guide discusses how to add curve objects to Rhino using the Rhino C/C++ SDK.

## Overview

Curve objects can be added to Rhino by using the following functions found on CRhinoDoc:

1. CRhinoDoc::AddCurveObject
2. CRhinoDoc::AddObject

For CRhinoDoc::AddCurveObject, there are six overridden versions that will make curve objects from a variety of inputs, including:

1. ON_Line: line definition objects
2. ON_Polyline: polyline definition objects
3. ON_Arc: arc definition objects
4. ON_Circle: circle definition objects
5. ON_BezierCurve: bezier curve objects
6. ON_Curve: ON_Curve-derived curve objects

## Examples

The following code samples will demonstrate three different ways of adding curves to Rhino. In these examples, we will create circle curves; but, there is no reason that we could create lines, polylines, arcs or any other type of curve.

### Example 1

In this example, we define a circle, using ON_Circle, and pass the definition off to CRhinoDoc::AddCurveObject.

ON_3dPoint center(0.0, 0.0, 0.0);

CRhinoCurveObject* curve_object = context.m_doc.AddCurveObject( circle );
context.m_doc.Redraw();


### Example 2

In this example, we first define a circle. Then we create an ON_ArcCurve object from the circle definition. ON_ArcCurve is one of the many curve classes this is derived from ON_Curve. We then pass the ON_ArcCurve object off to CRhinoDoc::AddCurveObject.

ON_3dPoint center(0, 0, 0);

ON_ArcCurve arc_curve( circle );

CRhinoCurveObject* curve_object = context.m_doc.AddCurveObject( arc_curve );
context.m_doc.Redraw();


### Example 3

In this example, we will add a circle curve the brute force way. We first define a circle. Then we allocate a new ON_ArcCurve and pass the circle definition to its constructor. We then allocate a new CRhinoCurveObject and assign our ON_ArcCurve object pointer to it. Finally, we pass the pointer to the CRhinoCurveObject to CRhinoDoc::AddObject.

ON_3dPoint center(0.0, 0.0, 0.0);

ON_ArcCurve* arc_curve = new ON_ArcCurve( circle );
if( arc_curve )
{
CRhinoCurveObject* curve_object = new CRhinoCurveObject();
if( curve_object )
{
// Set the curve to the curve object. Note,
// curve_object will delete arc_curve.
curve_object->SetCurve( arc_curve );

What is interesting to note is that the code found in Example 3 is essentially what Rhino does inside of CRhinoDoc::AddCurveObject. All of the CRhinoDoc::AddCurveObject overrides are simply provided to make it easier for SDK developers to add curves to Rhino.