#import vs. @class

You #import or #include when there is a physical dependency. Otherwise, you use forward declarations (@class MONClass, struct MONStruct, @protocol MONProtocol).

Here are some common examples of physical dependence:

• Any C or C++ value (a pointer or reference is not a physical dependency). If you have a CGPoint as an ivar or property, the compiler will need to see the declaration of CGPoint.
• Your superclass.
• A method you use.

参考：http://stackoverflow.com/questions/322597/class-vs-import

#import vs. @class

原文：http://www.cnblogs.com/drbbq/p/5242965.html