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

I wonder about Objective-C style.

I h开发者_开发百科ave 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.

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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

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#import in .h or .m files

• .h file - public interface of a class
• .m file - private implementation of the class

My rules are:

• Forward declaration^[About] - @class inside .h file
• You should reduce of imports in .h
• The accent should be on import in .m file that prevents dependency issues

When you need something in implementation then you should import it in .m. When you need something in interface then you should import it in .h.

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