Error in Cocos2dxSimpleGame Chapter 5 - How to Detect the Collisions

[[http://www.cocos2d-x.org/wiki/Chapter_5__How_to_Detect_the_Collisions]]
I’am new to Cocos2dx, so I follow the tourial Cocos2dxSimpleGame.It is great in Chapter 1
4 ,except Chapter 5.
In Chapter 5, function *void HelloWorld::update(float dt)* would through “error C2065: ‘_target’ : undeclared” and “error C2065: ‘_bullet’ : undeclared”.
But I don’t know where the two object should be quoted from or instantiate in some place.
How can I solve the errors?