Detecting superfluous includes in C C

Question

I often find that the headers section of a file get larger and larger all the time but it never gets smaller  Throughout the life of a source file classes may have moved and been refactored and it s very possible that there are quite a few  includes that don t need to be there and anymore  Leaving them there only prolong the compile time and adds unnecessary compilation dependencies  Trying to figure out which are still needed can be quite tedious   Is there some kind of tool that can detect superfluous  include directives and suggest which ones I can safely remove  Does lint do this maybe

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Google s cppclean  links to  download  documentation  can find several categories of C   problems  and it can now find superfluous  includes   There s also a Clang-based tool  include-what-you-use  that can do this  include-what-you-use can even suggest forward declarations  so you don t have to  include so much  and optionally clean up your  includes for you    Current versions of Eclipse CDT also have this functionality built in  going under the Source menu and clicking Organize Includes will alphabetize your  include s  add any headers that Eclipse thinks you re using without directly including them  and comments out any headers that it doesn t think you need   This feature isn t 100  reliable  however

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There s two types of superfluous  include files    A header file actually not needed by the module  c   cpp  at all  A header file is need by the module but being included more than once  directly  or indirectly    There s 2 ways in my experience that works well to detecting it    gcc -H or cl exe  showincludes  resolve problem 2   In real world  you can export CFLAGS -H before make  if all the Makefile s not override CFLAGS options     Or as I used  you can create a cc g   wrapper to add -H options forcibly to each invoke of   CC  and   CXX   and prepend the wrapper s directory to  PATH variable  then your make will all uses you wrapper command instead  Of course your wrapper should invoke the real gcc compiler     This tricks need to change if your Makefile uses gcc directly  instead of   CC  or   CXX  or by implied rules   You can also compile a single file by tweaking with the command line  But if you want to clean headers for the whole project  You can capture all the output by   make clean  make 2  amp 1   tee result txt PC-Lint FlexeLint resolve problem both 1 and 2   make sure add the  e766 options  this warning is about  unused header files   pclint flint  -vf        This will cause pclint output included header files  nested header files will be indented appropriately

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The CScout refactoring browser can detect superfluous include directives in C  unfortunately not C    code   You can find a description of how it works in this journal article

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To end this discussion  the c   preprocessor is turing complete  It is a semantic property  whether an include is superfluous  Hence  it follows from Rice s theorem that it is undecidable whether an include is superfluous or not  There CAN T be a program  that  always correctly  detects whether an include is superfluous

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There is a free tool Include File Dependencies Watcher which can be integrated in the visual studio  It shows superfluous  includes in red

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I ve tried using Flexelint  the unix version of PC-Lint  and had somewhat mixed results   This is likely because I m working on a very large and knotty code base   I recommend carefully examining each file that is reported as unused   The main worry is false positives   Multiple includes of the same header are reported as an unneeded header  This is bad since Flexelint does not tell you what line the header is included on or where it was included before   One of the ways automated tools can get this wrong   In A hpp   class A                  In B hpp    include  A hpp  class B       public          A foo       In C cpp    include  C hpp      include  B hpp       lt -- Unneeded  but lint reports it as needed  include  A hpp       lt -- Needed  but lint reports it as unneeded   If you blindly follow the messages from Flexelint you ll muck up your  include dependencies   There are more pathological cases  but basically you re going to need to inspect the headers yourself for best results   I highly recommend this article on Physical Structure and C   from the blog Games from within   They recommend a comprehensive approach to cleaning up the  include mess      Guidelines      Here   s a distilled set of guidelines from Lakos    book that minimize the number of  physical dependencies between files  I   ve been using them for years and I   ve always been really happy with the results          Every cpp file includes its own header file first   snip    A header file must include all the header files necessary to parse it   snip    A header file should have the bare minimum number of header files necessary to parse it   snip

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The problem with detecting superfluous includes is that it can t be just a type dependency checker   A superfluous include is a file which provides nothing of value to the compilation and does not alter another item which other files depend   There are many ways a header file can alter a compile  say by defining a constant  redefining and or deleting a used macro  adding a namespace which alters the lookup of a name some way down the line   In order to detect items like the namespace you need much more than a preprocessor  you in fact almost need a full compiler     Lint is more of a style checker and certainly won t have this full capability    I think you ll find the only way to detect a superfluous include is to remove  compile and run suites

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Also check out include-what-you-use  which solves a similar problem

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Gimpel Software s PC Lint can report on when an include file has been included more than once in a compilation unit  but it can t find include files which are not needed in the way you are looking for   Edit  It can  See itsmatt s answer

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Here is a simple brute force way of identifying superfluous header includes  It s not perfect but eliminates the  obvious  unnecessary includes  Getting rid of these goes a long way in cleaning up the code   The scripts can be accessed directly on GitHub

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Maybe a little late  but I once found a WebKit perl script that did just what you wanted  It ll need some adapting I believe  I m not well versed in perl   but it should do the trick   http   trac webkit org browser branches old safari-3-2-branch WebKitTools Scripts find-extra-includes   this is an old branch because trunk doesn t have the file anymore

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If you are using Eclipse CDT you can try http   includator com which is free for beta testers  at the time of this writing  and automatically removes superfluous  includes or adds missing ones  For those users who have FlexeLint or PC-Lint and are using Elicpse CDT  http   linticator com might be an option  also free for beta test   While it uses Lint s analysis  it provides quick-fixes for automatically remove the superfluous  include statements

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I thought that PCLint would do this  but it has been a few years since I ve looked at it   You might check it out   I looked at this blog and the author talked a bit about configuring PCLint to find unused includes   Might be worth a look

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Sorry to  re- post here  people often don t expand comments   Check my comment to crashmstr  FlexeLint   PC-Lint will do this for you   Informational message 766   Section 11 8 1 of my manual  version 8 0  discusses this   Also  and this is important  keep iterating until the message goes away   In other words  after removing unused headers  re-run lint  more header files might have become  unneeded  once you remove some unneeded headers    That might sound silly  read it slowly  amp  parse it  it makes sense

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This article explains a technique of  include removing by using the parsing of Doxygen  That s just a perl script  so it s quite easy to use

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CLion  the C C   IDE from JetBrains  detects redundant includes out-of-the-box  These are grayed-out in the editor  but there are also functions to optimise includes in the current file or whole project   I ve found that you pay for this functionality though  CLion takes a while to scan and analyse your project when first loaded

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I ve never found a full-fledged tool that accomplishes what you re asking   The closest thing I ve used is IncludeManager  which graphs your header inclusion tree so you can visually spot things like headers included in only one file and circular header inclusions

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You can write a quick script that erases a single  include directive  compiles the projects  and logs the name in the  include and the file it was removed from in the case that no compilation errors occurred   Let it run during the night  and the next day you will have a 100  correct list of include files you can remove   Sometimes brute-force just works  -     edit  and sometimes it doesn t  -   Here s a bit of information from the comments    Sometimes you can remove two header files separately  but not both together  A solution is to remove the header files during the run and not bring them back  This will find a list of files you can safely remove  although there might a solution with more files to remove which this algorithm won t find   it s a greedy search over the space of include files to remove  It will only find a local maximum  There may be subtle changes in behavior if you have some macros redefined differently depending on some  ifdefs  I think these are very rare cases  and the Unit Tests which are part of the build should catch these changes

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It s not automatic  but doxygen will produce dependency diagrams for  included files  You will have to go through them visually  but they can be very useful for getting a picture of what is using what

[c++] Detecting superfluous #includes in C/C++?

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