C C include header file order

Question

What order should include files be specified  i e  what are the reasons for including one header before another   For example  do the system files  STL  and Boost go before or after the local include files

User · Answer

I follow two simple rules that avoid the vast majority of problems:

All headers (and indeed any source files) should include what they need. They should not rely on their users including things.
As an adjunct, all headers should have include guards so that they don't get included multiple times by over-ambitious application of rule 1 above.

I also follow the guidelines of:

Include system headers first (stdio.h, etc) with a dividing line.
Group them logically.

In other words:

#include <stdio.h>
#include <string.h>

#include "btree.h"
#include "collect_hash.h"
#include "collect_arraylist.h"
#include "globals.h"

Although, being guidelines, that's a subjective thing. The rules on the other hand, I enforce rigidly, even to the point of providing 'wrapper' header files with include guards and grouped includes if some obnoxious third-party developer doesn't subscribe to my vision :-)

User · Answer

To add my own brick to the wall    Each header needs to be self-sufficient  which can only be tested if it s included first at least once One should not mistakenly modify the meaning of a third-party header by introducing symbols  macro  types  etc     So I usually go like this      myproject src example cpp  include  myproject example h    include  lt algorithm gt   include  lt set gt   include  lt vector gt    include  lt 3rdparty foo h gt   include  lt 3rdparty bar h gt    include  myproject another h   include  myproject specific bla h    include  detail impl h    Each group separated by a blank line from the next one    Header corresponding to this cpp file first  sanity check  System headers Third-party headers  organized by dependency order Project headers Project private headers   Also note that  apart from system headers  each file is in a folder with the name of its namespace  just because it s easier to track them down this way

User · Answer

This is not subjective  Make sure your headers don t rely on being  included in specific order  You can be sure it doesn t matter what order you include STL or Boost headers

User · Answer

The big thing to keep in mind is that your headers should not be dependent upon other headers being included first  One way to insure this is to include your headers before any other headers    Thinking in C    in particular mentions this  referencing Lakos   Large Scale C   Software Design    Latent usage errors can be avoided by ensuring that the  h file of a component parses by itself     without externally-provided declarations or definitions    Including the  h file as the very first line of the  c file ensures that no critical piece of information intrinsic to the physical interface of the component is missing from the  h file  or  if there is  that you will find out about it as soon as you try to compile the  c file    That is to say  include in the following order    The prototype interface header for this implementation  ie  the  h  hh file that corresponds to this  cpp  cc file   Other headers from the same project  as needed  Headers from other non-standard  non-system libraries  for example  Qt  Eigen  etc   Headers from other  almost-standard  libraries  for example  Boost  Standard C   headers  for example  iostream  functional  etc   Standard C headers  for example  cstdint  dirent h  etc     If any of the headers have an issue with being included in this order  either fix them  if yours  or don t use them  Boycott libraries that don t write clean headers   Google s C   style guide argues almost the reverse  with really no justification at all  I personally tend to favor the Lakos approach

User · Answer

Include from the most specific to the least specific  starting with the corresponding  hpp for the  cpp  if one such exists  That way  any hidden dependencies in header files that are not self-sufficient will be revealed   This is complicated by the use of pre-compiled headers  One way around this is  without making your project compiler-specific  is to use one of the project headers as the precompiled header include file

User · Answer

I recommend    The header for the  cc module you re building    Helps ensure each header in your project doesn t have implicit dependencies on other headers in your project    C system files  C   system files  Platform   OS   other header files  e g  win32  gtk  openGL   Other header files from your project    And of course  alphabetical order within each section  where possible   Always use forward declarations to avoid unnecessary  includes in your header files

User · Answer

It is a hard question in the C C   world  with so many elements beyond the standard   I think header file order is not a serious problem as long as it compiles  like squelart said   My ideas is  If there is no conflict of symbols in all those headers  any order is OK  and the header dependency issue can be fixed later by adding  include lines to the flawed  h   The real hassle arises when some header changes its action  by checking  if conditions  according to what headers are above   For example  in stddef h in VS2005  there is    ifdef   WIN64  define offsetof s m     size t    ptrdiff t  amp    s   0 - gt m     else  define offsetof s m     size t  amp    s   0 - gt m   endif   Now the problem  If I have a custom header   custom h   that needs to be used with many compilers  including some older ones that don t provide offsetof in their system headers  I should write in my header    ifndef offsetof  define offsetof s m     size t  amp    s   0 - gt m   endif   And be sure to tell the user to  include  custom h  after all system headers  otherwise  the line of offsetof in stddef h will assert a macro redefinition error   We pray not to meet any more of such cases in our career

User · Answer

First include the header corresponding to the  cpp    in other words  source1 cpp should include source1 h before including anything else  The only exception I can think of is when using MSVC with pre-compiled headers in which case  you are forced to include stdafx h before anything else   Reasoning  Including the source1 h before any other files ensures that it can stand alone without it s dependencies  If source1 h takes on a dependency on a later date  the compiler will immediately alert you to add the required forward declarations to source1 h  This in turn ensures that headers can be included in any order by their dependants   Example   source1 h  class Class1       Class2 c2        a dependency which has not been forward declared      source1 cpp   include  source1 h        now compiler will alert you saying that Class2 is undefined                        so you can forward declare Class2 within source1 h       MSVC users  I strongly recommend using pre-compiled headers  So  move all  include directives for standard headers  and other headers which are never going to change  to stdafx h

User · Answer

I m pretty sure this isn t a recommended practice anywhere in the sane world  but I like to line system includes up by filename length  sorted lexically within the same length   Like so    include  lt set gt   include  lt vector gt   include  lt algorithm gt   include  lt functional gt    I think it s a good idea to include your own headers before other peoples  to avoid the shame of include-order dependency

User · Answer

I don t think there s a recommended order  as long as it compiles  What s annoying is when some headers require other headers to be included first    That s a problem with the headers themselves  not with the order of includes   My personal preference is to go from local to global  each subsection in alphabetical order  i e     h file corresponding to this cpp file  if applicable  headers from the same component  headers from other components  system headers    My rationale for 1  is that it should prove that each header  for which there is a cpp  can be  included without prerequisites  terminus technicus  header is  self-contained    And the rest just seems to flow logically from there

[c++] C/C++ include header file order

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