@class vs. #import

It is to my understanding that one should use a forward-class declaration in the event ClassA needs to include a ClassB header, and ClassB needs to include a ClassA header to avoid any circular inclusions. I also understand that an #import is a simple ifndef so that an include only happens once.

My inquiry is this: When does one use #import and when does one use @class? Sometimes if I use a @class declaration, I see a common compiler warning such as the following:

warning: receiver 'FooController' is a forward class and corresponding @interface may not exist.

Would really love to understand this, versus just removing the @class forward-declaration and throwing an #import in to silence the warnings the compiler is giving me.

Answers


If you see this warning:

warning: receiver 'MyCoolClass' is a forward class and corresponding @interface may not exist

you need to #import the file, but you can do that in your implementation file (.m), and use the @class declaration in your header file.

@class does not (usually) remove the need to #import files, it just moves the requirement down closer to where the information is useful.

For Example

If you say @class MyCoolClass, the compiler knows that it may see something like:

MyCoolClass *myObject;

It doesn't have to worry about anything other than MyCoolClass is a valid class, and it should reserve room for a pointer to it (really, just a pointer). Thus, in your header, @class suffices 90% of the time.

However, if you ever need to create or access myObject's members, you'll need to let the compiler know what those methods are. At this point (presumably in your implementation file), you'll need to #import "MyCoolClass.h", to tell the compiler additional information beyond just "this is a class".


Three simple rules:

  • Only #import the super class, and adopted protocols, in header files (.h files).
  • #import all classes, and protocols, you send messages to in implementation (.m files).
  • Forward declarations for everything else.

If you do forward declaration in the implementation files, then you probably do something wrong.


Look at the Objective-C Programming Language documentation on ADC

Under the section on Defining a Class | Class Interface it describes why this is done:

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.

I hope this helps.


Use a forward declaration in the header file if needed, and #import the header files for any classes you're using in the implementation. In other words, you always #import the files you're using in your implementation, and if you need to reference a class in your header file use a forward declaration as well.

The exception to this is that you should #import a class or formal protocol you're inheriting from in your header file (in which case you wouldn't need to import it in the implementation).


The common practice is using @class in header files (but you still need to #import the superclass), and #import in implementation files. This will avoid any circular inclusions, and it just works.


Another advantage: Quick compilation

If you include a header file, any change in it causes the current file also to compile but this is not the case if the class name is included as @class name. Of course you will need to include the header in source file


My inquiry is this. When does one use #import and when does one use @class?

Simple answer: You #import or #include when there is a physical dependency. Otherwise, you use forward declarations (@class MONClass, struct MONStruct, @protocol MONProtocol).

Here are some common examples of physical dependence:

  • Any C or C++ value (a pointer or reference is not a physical dependency). If you have a CGPoint as an ivar or property, the compiler will need to see the declaration of CGPoint.
  • Your superclass.
  • A method you use.

Sometimes if I use a @class declaration, I see a common compiler warning such as the following: "warning: receiver 'FooController' is a forward class and corresponding @interface may not exist."

The compiler's actually very lenient in this regard. It will drop hints (such as the one above), but you can trash your stack easily if you ignore them and don't #import properly. Although it should (IMO), the compiler does not enforce this. In ARC, the compiler is more strict because it is responsible for reference counting. What happens is the compiler falls back on a default when it encounters an unknown method which you call. Every return value and parameter is assumed to be id. Thus, you ought to eradicate every warning from your codebases because this should be considered physical dependence. This is analogous to calling a C function which is not declared. With C, parameters are assumed to be int.

The reason you would favor forward declarations is that you can reduce your build times by factors because there is minimal dependence. With forward declarations, the compiler sees there is a name, and can correctly parse and compile the program without seeing the class declaration or all of its dependencies when there is no physical dependency. Clean builds take less time. Incremental builds take less time. Sure, you will end up spending a little more time making sure the all the headers you need are visible to every translation as a consequence, but this pays off in reduced build times quickly (assuming your project is not tiny).

If you use #import or #include instead, you're throwing a lot more work at the compiler than is necessary. You're also introducing complex header dependencies. You can liken this to a brute-force algorithm. When you #import, you're dragging in tons of unnecessary information, which requires a lot of memory, disk I/O, and CPU to parse and compile the sources.

ObjC is pretty close to ideal for a C based language with regards to dependency because NSObject types are never values -- NSObject types are always reference counted pointers. So you can get away with incredibly fast compile times if you structure your program's dependencies appropriately and forward where possible because there is very little physical dependence required. You can also declare properties in the class extensions to further minimize dependence. That's a huge bonus for large systems -- you would know the difference it makes if you have ever developed a large C++ codebase.

Therefore, my recommendation is to use forwards where possible, and then to #import where there is physical dependence. If you see the warning or another which implies physical dependence -- fix them all. The fix is to #import in your implementation file.

As you build libraries, you will likely classify some interfaces as a group, in which case you would #import that library where physical dependence is introduced (e.g. #import <AppKit/AppKit.h>). This can introduce dependence, but the library maintainers can often handle the physical dependencies for you as needed -- if they introduce a feature, they can minimize the impact it has on your builds.


I see a lot of "Do it this way" but I don't see any answers to "Why?"

So: Why should you @class in your header and #import only in your implementation? You're doubling your work by having to @class and #import all the time. Unless you make use of inheritance. In which case you'll be #importing multiple times for a single @class. Then you have to remember to remove from multiple different files if you suddenly decide you don't need access to a declaration anymore.

Importing the same file multiple times isn't an issue because of the nature of #import. Compiling performance isn't really an issue either. If it were, we wouldn't be #importing Cocoa/Cocoa.h or the like in pretty much every header file we have.


if we do this

@interface Class_B : Class_A

mean we are inheriting the Class_A into Class_B, in Class_B we can access all the variables of class_A.

if we are doing this

#import ....
@class Class_A
@interface Class_B

here we saying that we are using the Class_A in our program, but if we want to use the Class_A variables in Class_B we have to #import Class_A in .m file(make a object and use it's function and variables).


for extra info about file dependencies & #import & @class check this out:

http://qualitycoding.org/file-dependencies/ itis good article

summary of the article

imports in header files:

  • #import the superclass you’re inheriting, and the protocols you’re implementing.
  • Forward-declare everything else (unless it comes from a framework with a master header).
  • Try to eliminate all other #imports.
  • Declare protocols in their own headers to reduce dependencies.
  • Too many forward declarations? You have a Large Class.

imports in implementation files:

  • Eliminate cruft #imports that aren’t used.
  • If a method delegates to another object and returns what it gets back, try to forward-declare that object instead of #importing it.
  • If including a module forces you to include level after level of successive dependencies, you may have a set of classes that wants to become a library. Build it as a separate library with a master header, so everything can be brought in as a single prebuilt chunk.
  • Too many #imports? You have a Large Class.

When I develop, I have only three things in mind that never cause me any problems.

  1. Import super classes
  2. Import parent classes (when you have children and parents)
  3. Import classes outside your project (like in frameworks and libraries)

For all other classes (subclasses and child classes in my project self), I declare them via forward-class.


If you try to declare a variable, or a property in your header file, which you didn't import yet, your gonna get an error saying that the compiler doesn't know this class.

Your first thought is probably #import it. This may cause problems in some cases.

For example if you implement a bunch of C-methods in the header file, or structs, or something similar, because they shouldn't be imported multiple times.

Therefore you can tell the compiler with @class:

I know you don't know that class, but it exists. It's going to be imported or implemented elsewhere

It basically tells the compiler to shut up and compile, even though it's not sure if this class is ever going to be implemented.

You will usually use #import in the .m and @class in the .h files.


Forward declaration just to the prevent compiler from showing error.

the compiler will know that there is class with the name you've used in your header file to declare.


Compiler will complain only if you are going to use that class in such a way that the compiler needs to know its implementation.

Ex:

  1. This could be like if you are going to derive your class from it or
  2. If you are going to have an object of that class as a member variable (though rare).

It will not complain if you are just going to use it as a pointer. Of course, you will have to #import it in the implementation file (if you are instantiating an object of that class) since it needs to know the class contents to instantiate an object.

NOTE: #import is not same as #include. This means there is nothing called circular import. import is kind of a request for the compiler to look into a particular file for some information. If that information is already available, compiler ignores it.

Just try this, import A.h in B.h and B.h in A.h. There will be no problems or complaints and it will work fine too.

When to use @class

You use @class only if you don't even want to import a header in your header. This could be a case where you don't even care to know what that class will be. Cases where you may not even have a header for that class yet.

An example of this could be that you are writing two libraries. One class, lets call it A, exists in one library. This library includes a header from the second library. That header might have a pointer of A but again might not need to use it. If library 1 is not yet available, library B will not be blocked if you use @class. But if you are looking to import A.h, then library 2's progress is blocked.


Think of @class as telling the compiler "trust me, this exists".

Think of #import as copy-paste.

You want to minimize the number of imports you have for a number of reasons. Without any research, the first thing that comes to mind is it reduces compile time.

Notice that when you inherit from a class, you can't simply use a forward declaration. You need to import the file, so that the class you're declaring knows how it's defined.


This is an example scenario, where we need @class.

Consider if you wish to create a protocol within header file, which has a parameter with data type of the same class, then you can use @class. Please do remember that you can also declare protocols separately, this is just an example.

// DroneSearchField.h

#import <UIKit/UIKit.h>
@class DroneSearchField;
@protocol DroneSearchFieldDelegate<UITextFieldDelegate>
@optional
- (void)DroneTextFieldButtonClicked:(DroneSearchField *)textField;
@end
@interface DroneSearchField : UITextField
@end

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