objective c - What is the difference between @class and #import

Question

When I compile with the following code there are no errors:

@class RootViewController;
//#import "RootViewController.h"

When I compile with the following code I get an error:

//@class RootViewController;
#import "RootViewController.h"

"error: expected specifier-qualifier-list before 'RootViewController'"

I don't understand what the difference is between the two because I used #import in a similar class and it compiled without errors!

Answer 1

@class is used when you need to know the name of a class in a particular file, but you don't need to know any details about the class (its methods, for example). #import is used when you actually need to use the class (i.e., send it a message).

For example, if you're declaring instance variables in a header file, you can use @class to declare an instance variable of a certain type:

@class MyOtherClass;

@interface MyClass : NSObject
{
    MyOtherClass *myIvar;
}
@end

Since you're not using myIvar yet, you don't need to know anything about it except that the type MyOtherClass exists.

However:

#import "MyOtherClass.h"

- (void)doSomething
{
    [myIvar doSomethingElse];
}

In this case, you're sending the doSomethingElse message to myIvar; the compiler needs to know that instances of MyOtherClass define this method, so you have to import the header file or the compiler will complain.

Why worry about this?

It mostly has to do with dependencies. When you #import file A into file B, file B becomes dependent upon file A -- that is, if file A changes, you'll have to recompile file B. If you use @class in file B, file B is not dependent on file A, and thus doesn't need to be recompiled when file A changes -- so if you're just declaring a type and not actually dependent upon the implementation of file A, you can save yourself compilation time by not #importing file A.