Curve is connected line.
so you can draw curve using connected line.
Accumulate the points in cctouchesmove/cctouchemove
event function.
Make line sprite, add and draw them.
and also cocos2d-x have curve classes. if you use them, you can run action easily.
CCCardinalSplineTo
CCCardinalSplineBy
CCCatmullRomTo
CCCatmullRomBy
CCBezierBy
CCBezierTo
And for drawing, this is sample code.
void HelloWorld::draw()
{
// move to 50,50 since the "by" path will start at 50,50
kmGLPushMatrix();
kmGLTranslatef(50, 50, 0);
ccDrawCardinalSpline(m_pArray, 7, 100);
kmGLPopMatrix();
CCSize s = CCDirector::sharedDirector()->getWinSize();
kmGLPushMatrix();
kmGLTranslatef(s.width/2, 50, 0);
ccDrawCardinalSpline(m_pArray, 1, 100);
kmGLPopMatrix();
}