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.

Answer 2

I decided to refer to the documentation because I was still confused:

#import

This directive is identical to #include, except that it makes sure that the same file is never included more than once. It’s therefore preferred and is used in place of #include in code examples throughout Objective-C–based documentation.

This convention means that every interface file includes, indirectly, the interface files for all inherited classes. When a source module imports a class interface, it gets interfaces for the entire inheritance hierarchy that the class is built upon.

@class

Declarations like this simply use the class name as a type and don’t depend on any details of the class interface (its methods and instance variables), the @class directive gives the compiler sufficient forewarning of what to expect. However, where the interface to a class is actually used (instances created, messages sent), the class interface must be imported.

http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/ObjectiveC/Articles/ocDefiningClasses.html#//apple_ref/doc/uid/TP30001163-CH12-TPXREF123

Answer 3

Basic rule: use @class in you header file and #import in your implementation file. (However, you need to #import your class' superclass. And in some other circumstances you also need to use `#import" in the header.)

#import is not equivalent to #include. If a file is included many times, it will be loaded each time, but with many #imports of the same file, it will still only be loaded once.

Therefore, the main reason to use @class is not to avoid circular dependencies, but to make compilation faster.

Here's an example of when you must use @class

//MYControl.h

@class MYControl;  // Must use class

@protocol MYControlDelegate
-(void)control:(MYControl *)control didChangeToState:(UIControlState)state;
@end

@interface MYControl : UIControl
{
   id<MYControlDelegate> delegate_;
}
@property (nonatomic, assign) id<MYControlDelegate> delegate;
@end

//MYControl.m

@implementation MYControl
@synthesize delegate = delegate_;
. . .

In this case, there is nothing to import, because the delegate protocol is declared above the main class in the header file. But you still need to be able to refer to the main class which has not yet been declared. So what @class does is to just let the compiler know that there is some class that is called MYControl and will be defined at some point. (Not at runtime however. The class will be defined in the course of the compilation.)

EDIT: From the Objective-C manual:

Since declarations like this simply use the class name as a type and don’t depend on any details of the class interface (its methods and instance variables), the @class directive gives the compiler sufficient forewarning of what to expect. However, where the interface to a class is actually used (instances created, messages sent), the class interface must be imported. Typically, an interface file uses @class to declare classes, and the corresponding implementation file imports their interfaces (since it will need to create instances of those classes or send them messages).

The @class directive minimizes the amount of code seen by the compiler and linker, and is therefore the simplest way to give a forward declaration of a class name. Being simple, it avoids potential problems that may come with importing files that import still other files. For example, if one class declares a statically typed instance variable of another class, and their two interface files import each other, neither class may compile correctly.

Note that circularity is mentioned in the last sentence as one in a general class of issues dealt with by using @class.

Answer 4

@class is used to avoid circular dependency... This prevents circular references where in one header A imports a second header B which(B) imports the first(A) which imports the second (B)and so on in an endless cycle....@class is generally used to ask compiler to look for its definition at runtime... especially when it resides in some static library..

Other than that #import works

See this question