Difference between @interface definition in .h and .m file

Question

Normally we use

@interface interface_name : parent_class <delegates>
{
......
}
@end 

method in .h file and in .m file we synthesis the properties of variables declared in .h file.

But in some code, this @interface.....@end method is kept in the .m file also. What does it mean? What is the difference between them?

Also give some words about getters and setters for the interface file that is defined in .m file...

Answer 1

It's common to put an additional @interface that defines a category containing private methods:

Person.h:

@interface Person
{
    NSString *_name;
}

@property(readwrite, copy) NSString *name;
-(NSString*)makeSmallTalkWith:(Person*)person;
@end

Person.m:

@interface Person () //Not specifying a name for the category makes compiler checks that these methods are implemented.

-(void)startThinkOfWhatToHaveForDinner;
@end


@implementation Person

@synthesize name = _name;

-(NSString*)makeSmallTalkWith:(Person*)person
{
    [self startThinkOfWhatToHaveForDinner];
    return @"How's your day?";
}


-(void)startThinkOfWhatToHaveForDinner
{

}

@end

The 'private category' (the proper name for a nameless category is not 'private category', it's 'class extension') .m prevents the compiler from warning that the methods are defined. However, because the @interface in the .m file is a category you can't define ivars in it.

Update 6th Aug '12: Objective-C has evolved since this answer was written:

• ivars can be declared in a class extension (and always could be - the answer was incorrect)
• @synthesize is not required
• ivars can now be declared in braces at the top of @implementation:

that is,

@implementation { 
     id _ivarInImplmentation;
}
//methods
@end