objective c - Do you put #import s in .h or .m files?

Question

I wonder about Objective-C style.

I have FooClass.[hm] that depends on BarClass.[hm] for its implementation (though not for its interface). I can #import "BarClass.h" either directly in FooClass.m or indirectly through FooClass.h. I wonder about the common choice for this.

Answer 1

You should ALWAYS #import other classes in your .m file.

If they happen to also be members of your class, you can forward declare them (by using the @class directive) in your .h file.

The reason for doing this is because when you #import a .h file, you only want to import declarations, not definitions. By using @class and only #importing in .m files, you are a) reducing overhead and b) makes for cleaner code.

Another reason you should do it this way was pointed out by Matt Gallagher:

The reasoning behind forward declarations in header files is that it avoids unnecessary dependencies. i.e. Imagine B.h forward declares A and B.m imports A.h. Then imagine C.m, D.m, E.m and F.m import B.h. After all this is done, A.h changes. Since A is only forward declared in B.h, only B.m needs to rebuild. Without forward declarations, C.m, D.m, E.m and F.m would all need to be rebuild if A changes

Example:

.h file:

@class BarClass;

@interface FooClass : NSObject

...

@end

.m file

#import "BarClass.h"

@implementation FooClass

...

@end