What is the difference between import and include in Objective-C

Question

What are the differences between  import and  include in Objective-C and are there times where you should use one over the other  Is one deprecated   I was reading the following tutorial  http   www otierney net objective-c html preamble and its paragraph about  import and  include seems to contradict itself or at least is unclear

User · Answer

If you are familiar with C   and macros  then   import  Class h     is similar to     pragma once   include  class h      which means that your Class will be loaded only once when your app runs

User · Answer

include it used to get  things  from another file to the one the  include is used in  Ex   in file  main cpp   include  otherfile h      some stuff here using otherfile h objects     functions or classes declared inside   Header guard is used on the top of each header file    h  to prevent including the same file more then once  if it happens you will get compile errors    in file  otherfile h   ifndef OTHERFILE  define OTHERFILE     declare functions  classes or objects here   endif   even if you put  include  otherfile h  n time in your code  this inside it will not be redeclared

User · Answer

include works just like the C  include    import keeps track of which headers have already been included and is ignored if a header is imported more than once in a compilation unit  This makes it unnecessary to use header guards   The bottom line is just use  import in Objective-C and don t worry if your headers wind up importing something more than once

User · Answer

In may case I had a global variable in one of my  h files that was causing the problem  and I solved it by adding extern in front of it

User · Answer

If you are familiar with C   and macros  then   import  Class h     is similar to     pragma once   include  class h      which means that your Class will be loaded only once when your app runs

User · Answer

include works just like the C  include    import keeps track of which headers have already been included and is ignored if a header is imported more than once in a compilation unit  This makes it unnecessary to use header guards   The bottom line is just use  import in Objective-C and don t worry if your headers wind up importing something more than once

User · Answer

The  import directive was added to Objective-C as an improved version of  include  Whether or not it s improved  however  is still a matter of debate   import ensures that a file is only ever included once so that you never have a problem with recursive includes  However  most decent header files protect themselves against this anyway  so it s not really that much of a benefit   Basically  it s up to you to decide which you want to use  I tend to  import headers for Objective-C things  like class definitions and such  and  include standard C stuff that I need  For example  one of my source files might look like this    import  lt Foundation Foundation h gt    include  lt asl h gt   include  lt mach mach h gt

User · Answer

I agree with Jason   I got caught out doing this    import  lt sys time h gt      to use gettimeofday   function  import  lt time h gt          to use time   function   For GNU gcc  it kept complaining that time   function was not defined   So then I changed  import to  include and all went ok   Reason    You  import  lt sys time h gt    nbsp  nbsp  nbsp  nbsp  lt sys time h gt  includes only a part of  lt time h gt  by using  defines  You  import  lt time h gt    nbsp  nbsp  nbsp  nbsp No go  Even though only part of  lt time h gt  was already included  as  nbsp  nbsp  nbsp  nbsp far as  import is concerned  that file is now already completely included   Bottom line   C C   headers traditionally includes parts of other include files  So for C C   headers  use  include  For objc objc   headers  use  import

User · Answer

include works just like the C  include    import keeps track of which headers have already been included and is ignored if a header is imported more than once in a compilation unit  This makes it unnecessary to use header guards   The bottom line is just use  import in Objective-C and don t worry if your headers wind up importing something more than once

User · Answer

There seems to be a lot of confusion regarding the preprocessor   What the compiler does when it sees a  include that it replaces that line with the contents of the included files  no questions asked   So if you have a file a h with this contents   typedef int my number    and a file b c with this content    include  a h   include  a h    the file b c will be translated by the preprocessor before compilation to  typedef int my number  typedef int my number    which will result in a compiler error  since the type my number is defined twice  Even though the definition is the same this is not allowed by the C language   Since a header often is used in more than one place include guards usually are used in C  This looks like this     ifndef  a h included    define  a h included    typedef int my number     endif   The file b c still would have the whole contents of the header in it twice after being preprocessed  But the second instance would be ignored since the macro  a h included  would already have been defined   This works really well  but has two drawbacks  First of all the include guards have to be written  and the macro name has to be different in every header  And secondly the compiler has still to look for the header file and read it as often as it is included   Objective-C has the  import preprocessor instruction  it also can be used for C and C   code with some compilers and options   This does almost the same as  include  but it also notes internally which file has already been included  The  import line is only replaced by the contents of the named file for the first time it is encountered  Every time after that it is just ignored

User · Answer

The  import directive was added to Objective-C as an improved version of  include  Whether or not it s improved  however  is still a matter of debate   import ensures that a file is only ever included once so that you never have a problem with recursive includes  However  most decent header files protect themselves against this anyway  so it s not really that much of a benefit   Basically  it s up to you to decide which you want to use  I tend to  import headers for Objective-C things  like class definitions and such  and  include standard C stuff that I need  For example  one of my source files might look like this    import  lt Foundation Foundation h gt    include  lt asl h gt   include  lt mach mach h gt

User · Answer

I know this thread is old    but in  modern times    there is a far superior  include strategy  via clang s  import modules - that is oft-overlooked       Modules improve access to the API of software libraries by replacing the textual preprocessor inclusion model with a more robust  more efficient semantic model  From the user   s perspective  the code looks only slightly different  because one uses an import declaration rather than a  include preprocessor directive     import Darwin     Like including all of  usr include   see  usr include module map   or    import Foundation       Like  import  lt Foundation Foundation h gt   import ObjectiveC       Like  import  lt objc runtime h gt       However  this module import behaves quite differently from the corresponding  include  when the compiler sees the module import above  it loads a binary representation of the module and makes its API available to the application directly  Preprocessor definitions that precede the import declaration have no impact on the API provided    because the module itself was compiled as a separate  standalone module  Additionally  any linker flags required to use the module will automatically be provided when the module is imported  This semantic import model addresses many of the problems of the preprocessor inclusion model    To enable modules  pass the command-line flag -fmodules aka CLANG ENABLE MODULES in Xcode- at compile time   As mentioned above   this strategy obviates ANY and ALL LDFLAGS   As in  you can REMOVE any  OTHER LDFLAGS  settings  as well as any  Linking  phases      I find compile   launch times to  feel  much snappier  or possibly  there s just less of a lag while  linking      and also  provides a great opportunity to purge the now extraneous Project-Prefix pch file  and corresponding build settings  GCC INCREASE PRECOMPILED HEADER SHARING  GCC PRECOMPILE PREFIX HEADER  and GCC PREFIX HEADER  etc   Also  while not well-documented    You can create module maps for your own frameworks and include them in the same convenient fashion   You can take a look at my ObjC-Clang-Modules github repo for some examples of how to implement such miracles

User · Answer

I know this thread is old    but in  modern times    there is a far superior  include strategy  via clang s  import modules - that is oft-overlooked       Modules improve access to the API of software libraries by replacing the textual preprocessor inclusion model with a more robust  more efficient semantic model  From the user   s perspective  the code looks only slightly different  because one uses an import declaration rather than a  include preprocessor directive     import Darwin     Like including all of  usr include   see  usr include module map   or    import Foundation       Like  import  lt Foundation Foundation h gt   import ObjectiveC       Like  import  lt objc runtime h gt       However  this module import behaves quite differently from the corresponding  include  when the compiler sees the module import above  it loads a binary representation of the module and makes its API available to the application directly  Preprocessor definitions that precede the import declaration have no impact on the API provided    because the module itself was compiled as a separate  standalone module  Additionally  any linker flags required to use the module will automatically be provided when the module is imported  This semantic import model addresses many of the problems of the preprocessor inclusion model    To enable modules  pass the command-line flag -fmodules aka CLANG ENABLE MODULES in Xcode- at compile time   As mentioned above   this strategy obviates ANY and ALL LDFLAGS   As in  you can REMOVE any  OTHER LDFLAGS  settings  as well as any  Linking  phases      I find compile   launch times to  feel  much snappier  or possibly  there s just less of a lag while  linking      and also  provides a great opportunity to purge the now extraneous Project-Prefix pch file  and corresponding build settings  GCC INCREASE PRECOMPILED HEADER SHARING  GCC PRECOMPILE PREFIX HEADER  and GCC PREFIX HEADER  etc   Also  while not well-documented    You can create module maps for your own frameworks and include them in the same convenient fashion   You can take a look at my ObjC-Clang-Modules github repo for some examples of how to implement such miracles

User · Answer

The  import directive was added to Objective-C as an improved version of  include  Whether or not it s improved  however  is still a matter of debate   import ensures that a file is only ever included once so that you never have a problem with recursive includes  However  most decent header files protect themselves against this anyway  so it s not really that much of a benefit   Basically  it s up to you to decide which you want to use  I tend to  import headers for Objective-C things  like class definitions and such  and  include standard C stuff that I need  For example  one of my source files might look like this    import  lt Foundation Foundation h gt    include  lt asl h gt   include  lt mach mach h gt

User · Answer

include works just like the C  include    import keeps track of which headers have already been included and is ignored if a header is imported more than once in a compilation unit  This makes it unnecessary to use header guards   The bottom line is just use  import in Objective-C and don t worry if your headers wind up importing something more than once

User · Answer

IF you  include a file two times in  h files  than compiler will give error  But if you  import a file more than once compiler will ignore it

User · Answer

The  import directive was added to Objective-C as an improved version of  include  Whether or not it s improved  however  is still a matter of debate   import ensures that a file is only ever included once so that you never have a problem with recursive includes  However  most decent header files protect themselves against this anyway  so it s not really that much of a benefit   Basically  it s up to you to decide which you want to use  I tend to  import headers for Objective-C things  like class definitions and such  and  include standard C stuff that I need  For example  one of my source files might look like this    import  lt Foundation Foundation h gt    include  lt asl h gt   include  lt mach mach h gt

User · Answer

In may case I had a global variable in one of my  h files that was causing the problem  and I solved it by adding extern in front of it

User · Answer

IF you  include a file two times in  h files  than compiler will give error  But if you  import a file more than once compiler will ignore it

User · Answer

There seems to be a lot of confusion regarding the preprocessor   What the compiler does when it sees a  include that it replaces that line with the contents of the included files  no questions asked   So if you have a file a h with this contents   typedef int my number    and a file b c with this content    include  a h   include  a h    the file b c will be translated by the preprocessor before compilation to  typedef int my number  typedef int my number    which will result in a compiler error  since the type my number is defined twice  Even though the definition is the same this is not allowed by the C language   Since a header often is used in more than one place include guards usually are used in C  This looks like this     ifndef  a h included    define  a h included    typedef int my number     endif   The file b c still would have the whole contents of the header in it twice after being preprocessed  But the second instance would be ignored since the macro  a h included  would already have been defined   This works really well  but has two drawbacks  First of all the include guards have to be written  and the macro name has to be different in every header  And secondly the compiler has still to look for the header file and read it as often as it is included   Objective-C has the  import preprocessor instruction  it also can be used for C and C   code with some compilers and options   This does almost the same as  include  but it also notes internally which file has already been included  The  import line is only replaced by the contents of the named file for the first time it is encountered  Every time after that it is just ignored

User · Answer

include   guard     import    include guardWiki - macro guard  header guard or file guard prevents to double include a header by a preprocessor that can slow down a build time  The next step is    pch About      import About     import in  h or  m

User · Answer

include   guard     import    include guardWiki - macro guard  header guard or file guard prevents to double include a header by a preprocessor that can slow down a build time  The next step is    pch About      import About     import in  h or  m

User · Answer

I agree with Jason   I got caught out doing this    import  lt sys time h gt      to use gettimeofday   function  import  lt time h gt          to use time   function   For GNU gcc  it kept complaining that time   function was not defined   So then I changed  import to  include and all went ok   Reason    You  import  lt sys time h gt    nbsp  nbsp  nbsp  nbsp  lt sys time h gt  includes only a part of  lt time h gt  by using  defines  You  import  lt time h gt    nbsp  nbsp  nbsp  nbsp No go  Even though only part of  lt time h gt  was already included  as  nbsp  nbsp  nbsp  nbsp far as  import is concerned  that file is now already completely included   Bottom line   C C   headers traditionally includes parts of other include files  So for C C   headers  use  include  For objc objc   headers  use  import

User · Answer

include it used to get  things  from another file to the one the  include is used in  Ex   in file  main cpp   include  otherfile h      some stuff here using otherfile h objects     functions or classes declared inside   Header guard is used on the top of each header file    h  to prevent including the same file more then once  if it happens you will get compile errors    in file  otherfile h   ifndef OTHERFILE  define OTHERFILE     declare functions  classes or objects here   endif   even if you put  include  otherfile h  n time in your code  this inside it will not be redeclared

[objective-c] What is the difference between #import and #include in Objective-C?

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