What is the importance of .pch file and what is the significance of"#ifdef OBJC"? Also, where do we define parameters like "#ifdef IS_PRODUCTION" which are checked in .pch file.

I want to use the PCH file to speed up the build, so i store the PCH file in a separate folder and reuse it when build the project later. but I have some #import *.dll in my stdafx.h. so if someth...