In iphone, some elements prompted undefined, but I added the corresponding header files, compile or prompt again undefined

In iphone, some elements prompted undefined, but I added the corresponding header files, compile again or tips undefined
What is the reason?
Reply:
I encountered this problem solution is usually
Close compiler restart
Reply:
There is such a thing? Would not be modifying the code does not save it?
Reply:
In this case you have to add an @ class up.

This thing is a bit like C + + forward declaration inside the class.

Let you know that this class is a class of unknown type on it.