Since the catapult is a physics body constrained by a revolute joint with a motor and limits enabled, it would be rather complicated to make it move in a non-physics way. Thus, we use the b2MouseJoint class provided by Box2D, which is specifically designed for a situation like ours. So, let's define the creation of the b2MouseJoint class in the ccTouchesBegan function in GameWorld.cpp:
void GameWorld::ccTouchesBegan(CCSet* set, CCEvent* event)
{
if(is_catapult_ready_ && !mouse_joint_)
{
CCTouch* touch = (CCTouch*)(*set->begin());
CCPoint touch_point = touch->getLocationInView();
touch_point = CCDirector::sharedDirector()->
convertToGL(touch_point);
// convert from screen to physics co-ordinates
b2Vec2 touch_world_point = b2Vec2(SCREEN_TO_WORLD(touch_point.x),
SCREEN_TO_WORLD(touch_point.y));
// only accept touches to the left of the catapult
if(touch_world_point.x < catapult_body_->GetPosition().x)
...