h or hpp for your class definitions

Question

I ve always used a   h file for my class definitions  but after reading some boost library code  I realised they all use   hpp  I ve always had an aversion to that file extension  I think mainly because I m not used to it   What are the advantages and disadvantages of using   hpp over   h

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In one of my jobs in the early 90 s  we used  cc and  hh for source and header files respectively  I still prefer it over all the alternatives  probably because it s easiest to type

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I always considered the  hpp header to be a sort of portmanteau of  h and  cpp files   a header which contains implementation details as well     Typically when I ve seen  and use   hpp as an extension  there is no corresponding  cpp file   As others have said  this isn t a hard and fast rule  just how I tend to use  hpp files

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As many here have already mentioned  I also prefer using  hpp for header-only libraries that use template classes functions   I prefer to use  h for header files accompanied by  cpp source files or shared or static libraries   Most of the libraries I develop are template based and thus need to be header only  but when writing applications I tend to separate declaration from implementation and end up with  h and  cpp files

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In  The C   Programming Language  Third Edition by Bjarne Stroustrup   the n  1 must-read C   book  he uses   h  So I assume the best practice is to use   h    However    hpp is fine as well

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It is easy for tools and humans to differentiate something  That s it   In conventional use  by boost  etc    hpp is specifically C   headers  On the other hand   h is for non-C  -only headers  mainly C   To precisely detect the language of the content is generally hard since there are many non-trivial cases  so this difference often makes a ready-to-use tool easy to write  For humans  once get the convention  it is also easy to remember and easy to use   However  I d point out the convention itself does not always work  as expected    It is not forced by the specification of languages  neither C nor C    There exist many projects which do not follow the convention  Once you need to merge  to mix  them  it can be troublesome   hpp itself is not the only choice  Why not  hh or  hxx   Though anyway  you usually need at least one conventional rule about filenames and paths     I personally use both  h and  hpp in my C   projects  I don t follow the convention above because    The languages used by each part of the projects are explicitly documented  No chance to mix C and C   in same module  directory   Every 3rdparty library is required to conforming to this rule  The conformed language specifications and allowed language dialects used by the projects are also documented   In fact  I even document the source of the standard features and bug fix  on the language standard  being used   This is somewhat more important than distinguishing the used languages since it is too error-prone and the cost of test  e g  compiler compatibility  may be significant  complicated and time-consuming   especially in a project which is already in almost pure C    Filenames are too weak to handle this  Even for the same C   dialect  there may be more important properties suitable to the difference  For example  see the convention below  Filenames are essentially pieces of fragile metadata  The violation of convention is not so easy to detect  To be stable dealing the content  a tool should eventually not only depend on names  The difference between extensions is only a hint  Tools using it should also not be expected behave same all the time  e g  language-detecting of  h files on github com   There may be something in comments like shebang for these source files to be better metadata  but it is even not conventional like filenames  so also not reliable in general     I usually use  hpp on C   headers and the headers should be used  maintained  in a header-only manner  e g  as template libraries  For other headers in  h  either there is a corresponding  cpp file as implementation  or it is a non-C   header  The latter is trivial to differentiate through the contents of the header by humans  or by tools with explicit embedded metadata  if needed

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You can call your includes whatever you like   Just need to specify that full name in the  include   I suggest that if you work with C to use  h and when with C   to use  hpp   It is in the end just a convention

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I use  hpp because I want the user to differentiate what headers are C   headers  and what headers are C headers   This can be important when your project is using both C and C   modules  Like someone else explained before me  you should do it very carefully  and its starts by the  contract  you offer through the extension   hpp   C   Headers   Or  hxx  or  hh  or whatever   This header is for C   only   If you re in a C module  don t even try to include it  You won t like it  because no effort is done to make it C-friendly  too much would be lost  like function overloading  namespaces  etc  etc      h   C C   compatible or pure C Headers  This header can be included by both a C source  and a C   source  directly or indirectly   It can included directly  being protected by the   cplusplus macro    Which mean that  from a C   viewpoint  the C-compatible code will be defined as extern  C   From a C viewpoint  all the C code will be plainly visible  but the C   code will be hidden  because it won t compile in a C compiler     For example    ifndef MY HEADER H  define MY HEADER H      ifdef   cplusplus       extern  C              endif     void myCFunction          ifdef   cplusplus            extern  C      endif   endif    MY HEADER H   Or it could be included indirectly by the corresponding  hpp header enclosing it with the extern  C  declaration   For example    ifndef MY HEADER HPP  define MY HEADER HPP  extern  C     include  my header h      endif    MY HEADER HPP   and    ifndef MY HEADER H  define MY HEADER H  void myCFunction       endif    MY HEADER H

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I use  h because that s what Microsoft uses  and what their code generator creates   No need to go against the grain

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C     C Plus Plus   makes sense as  cpp  Having header files with a  hpp extension doesn t have the same logical flow

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Bjarne Stroustrup and Herb Sutter have a statement to this question in their C   Core guidelines found on  https   github com isocpp CppCoreGuidelines blob master CppCoreGuidelines md S-source which is also refering to the latest changes in the standard extension  C  11  C  14  etc    SF 1  Use a  cpp suffix for code files and  h for interface files if your Y project doesn t already follow another convention Reason It s a longstanding convention  But consistency is more important  so if your project uses something else  follow that  Note This convention reflects a common use pattern  Headers are more often shared with C to compile as both C   and C  which typically uses  h  and it s easier to name all headers  h instead of having different extensions for just those headers that are intended to be shared with C  On the other hand  implementation files are rarely shared with C and so should typically be distinguished from  c files  so it s normally best to name all C   implementation files something else  such as  cpp   The specific names  h and  cpp are not required  just recommended as a default  and other names are in widespread use  Examples are  hh   C  and  cxx  Use such names equivalently  In this document  we refer to  h and  cpp  gt  as a shorthand for header and implementation files  even though the actual extension may be different  Your IDE  if you use one  may have strong opinions about suffices   I m not a big fan of this convention because if you are using a popular library like boost  your consistency is already broken and you should better use  hpp

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The extension of the source file may have meaning to your build system  for example  you might have a rule in your makefile for  cpp or  c files  or your compiler  e g  Microsoft cl exe  might compile the file as C or C   depending on the extension    Because you have to provide the whole filename to the  include directive  the header file extension is irrelevant  You can include a  c file in another source file if you like  because it s just a textual include  Your compiler might have an option to dump the preprocessed output which will make this clear  Microsoft   P to preprocess to file   E to preprocess to stdout   EP to omit  line directives   C to retain comments   You might choose to use  hpp for files that are only relevant to the C   environment  i e  they use features that won t compile in C

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Fortunately  it is simple   You should use the  hpp extension if you re working with C   and you should use  h for C or mixing C and C

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I prefer  hpp for C   to make it clear to both editors and to other programmers that it is a C   header rather than a C header file

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There is no advantage to any particular extension  other than that one may have a different meaning to you  the compiler  and or your tools   header h is a valid header   header hpp is a valid header   header hh is a valid header   header hx is a valid header   h header is a valid header   this is not a valid header is a valid header in denial   ihjkflajfajfklaf is a valid header   As long as the name can be parsed properly by the compiler  and the file system supports it  it s a valid header  and the only advantage to its extension is what one reads into it   That being said  being able to accurately make assumptions based on the extension is very useful  so it would be wise to use an easily-understandable set of rules for your header files   Personally  I prefer to do something like this    If there are already any established guidelines  follow them to prevent confusion  If all source files in the project are for the same language  use  h   There s no ambiguity  If some headers are compatible with multiple languages  while others are only compatible with a single language  extensions are based on the most restrictive language that a header is compatible with   A header compatible with C  or with both C  amp  C    gets  h  while a header compatible with C   but not C gets  hpp or  hh or something of the sort    This  of course  is but one of many ways to handle extensions  and you can t necessarily trust your first impression even if things seem straightforward   For example  I ve seen mention of using  h for normal headers  and  tpp for headers that only contain definitions for templated class member functions  with  h files that define templated classes including the  tpp files that define their member functions  instead of the  h header directly containing both the function declaration and the definition    For another example  a good many people always reflect the header s language in its extension  even when there s no chance of ambiguity  to them   h is always a C header and  hpp  or  hh  or  hxx  etc   is always a C   header   And yet again  some people use  h for  header associated with a source file  and  hpp for  header with all functions defined inline    Considering this  the main advantage would come in consistently naming your headers in the same style  and making that style readily apparent to anyone examining your code   This way  anyone familiar with your usual coding style will be able to determine what you mean with any given extension with just a cursory glance

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It does not matter which extension you use  Either one is OK   I use   h for C and   hpp for C

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Here are a couple of reasons for having different naming of C vs C   headers    Automatic code formatting  you might have different guidelines for formatting C and C   code  If the headers are separated by extension you can set your editor to apply the appropriate formatting automatically Naming  I ve been on projects where there were libraries written in C and then wrappers had been implemented in C    Since the headers usually had similar names  i e  Feature h vs Feature hpp  they were easy to tell apart  Inclusion  maybe your project has more appropriate versions available written in C   but you are using the C version  see above point   If headers are named after the language they are implemented in you can easily spot all the C-headers and check for C   versions    Remember  C is not C   and it can be very dangerous to mix and match unless you know what you are doing  Naming your sources appropriately helps you tell the languages apart

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Codegear C  Builder uses  hpp for header files automagically generated from Delphi source files  and  h files for your  own  header files    So  when I m writing a C   header file I always use  h

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I m answering this as an reminder  to give point to my comment s  on  user1949346  answer to this same OP     So as many already answered  either way is fine  Followed by emphasizes of their own impressions   Introductory  as also in the previous named comments stated  my opinion is C   header extensions are proposed to be  h if there is actually no reason against it   Since the ISO IEC documents use this notation of header files and no string matching to  hpp even occurs in their language documentations about C     But I m now aiming for an approvable reason WHY either way is ok  and especially why it s not subject of the language it self   So here we go   The C   documentation  I m actually taking reference from the version N3690  defines that a header has to conform to the following syntax      2 9 Header names  header-name       lt  h-char-sequence  gt        q-char-sequence   h-char-sequence      h-char     h-char-sequence h-char h-char      any member of the source character set except new-line and  gt  q-char-sequence      q-char     q-char-sequence q-char q-char      any member of the source character set except new-line and      So as we can extract from this part  the header file name may be anything that is valid in the source code  too  Except containing   n  characters and depending on if it is to be included by  lt  gt  it is not allowed to contain a  gt   Or the other way if it is included by   -include it is not allowed to contain a     In other words  if you had a environment supporting filenames like prettyStupidIdea  gt   an include like    include  prettyStupidIdea  gt     would be valid  but    include  lt prettyStupidIdea  gt  gt    would be invalid  The other way around the same   And even   include  lt  lt   lt  gt    would be a valid includable header file name   Even this would conform to C    it would be a pretty pretty stupid idea  tho   And that s why  hpp is valid  too   But it s not an outcome of the committees designing decisions for the language   So discussing about to use  hpp is same as doing it about  cc   mm or what ever else I read in other posts on this topic   I have to admit I have no clue where  hpp came from1  but I would bet an inventor of some parsing tool  IDE or something else concerned with C   came to this idea to optimize some internal processes or just to invent some  probably even for them necessarily  new naming conventions   But it is not part of the language   And whenever one decides to use it this way  May it be because he likes it most or because some applications of the workflow require it  it never2 is a requirement of the language  So whoever says  the pp is because it is used with C     simply is wrong in regards of the languages definition   C   allows anything respecting the previous paragraph  And if there is anything the committee proposed to use  then it is using  h since this is the extension sued in all examples of the ISO document   Conclusion   As long you don t see feel any need of using  h over  hpp or vise versa  you shouldn t bother  Because both would be form a valid header name of same quality in respect to the standard  And therefore anything that REQUIRES you to use  h or  hpp is an additional restriction of the standard which could even be contradicting with other additional restrictions not conform with each other  But as OP doesn t mention any additional language restriction  this is the only correct and approvable answer to the question     h or   hpp for your class definitions  is   Both are equally correct and applicable as long as no external restrictions are present     1From what I know  apparently  it is the boost framework that came up with that  hpp extension   2Of course I can t say what some future versions will bring with it

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EDIT  Added suggestion from Dan Nissenbaum   By one convention   hpp files are used when the prototypes are defined in the header itself  Such definitions in headers are useful in case of templates  since the compiler generates the code for each type only on template instantiation  Hence  if they are not defined in header files  their definitions will not be resolved at link time from other compilation units  If your project is a C   only project that makes heavy use of templates  this convention will be useful  Certain template libraries that adhere to this convention provide headers with  hpp extensions to indicate that they do not have corresponding  cpp files  another convention is to use  h for C headers and  hpp for C    a good example would be the boost library   Quote from Boost FAQ  File extensions communicate the  quot type quot  of the file  both to humans and to computer programs  The   h  extension is used for C header files  and therefore communicates the wrong thing about C   header files  Using no extension communicates nothing and forces inspection of file contents to determine type  Using   hpp  unambiguously identifies it as C   header file  and works well in actual practice   Rainer Deyke

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I ve recently started using   hpp for c   headers   The reason is that I use emacs as my primary editor and it enters automatically into c-mode when you load a   h file and into c  -mode when you load a   hpp file    Apart that fact I see no good reasons for choosing   h over   hpp or vice-versa

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