class vs import

Question

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

User · Answer

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  lt UIKit UIKit h gt   class DroneSearchField   protocol DroneSearchFieldDelegate lt UITextFieldDelegate gt   optional -  void DroneTextFieldButtonClicked  DroneSearchField   textField   end  interface DroneSearchField   UITextField  end

User · Answer

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

User · Answer

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  lt UIKit UIKit h gt   class DroneSearchField   protocol DroneSearchFieldDelegate lt UITextFieldDelegate gt   optional -  void DroneTextFieldButtonClicked  DroneSearchField   textField   end  interface DroneSearchField   UITextField  end

User · Answer

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

User · Answer

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    This could be like if you are going to derive your class from it or 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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

for extra info about file dependencies  amp   import  amp   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

User · Answer

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

User · Answer

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

User · Answer

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  lt AppKit AppKit h gt    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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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  lt AppKit AppKit h gt    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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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    This could be like if you are going to derive your class from it or 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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

for extra info about file dependencies  amp   import  amp   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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

When I develop  I have only three things in mind that never cause me any problems    Import super classes Import parent classes  when you have children and parents  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

When I develop  I have only three things in mind that never cause me any problems    Import super classes Import parent classes  when you have children and parents  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

User · Answer

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

[objective-c] @class vs. #import

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