C++ why include related header first?

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Related to this question: C/C++ include file order/best practices



The best answer recommends to include the related header first.



Same for Google and Mozilla style guides.



However, in both cases, I couldn't find a good reason why you would do this.



Google and Mozilla coding rules look best to me, because they enforce you to include the most "standard" headers first.



This way, I think you are less likely to mess up the included files (for examples by undefining some macros used in the other headers etc.) so that looks like the best way to go for me.



But following that rationale, why would you include the related header first? Since any syntax error in it might mess all the following headers?



I would think including the related header last would be best instead.






c++ header-files







share|improve this question



















  • 2




    I fail to see why it's downvoted. The OP presents research, and asks about something they did not understand from conducting it. There is a well defined answer as well. All in all, it fits the criteria of a decent StackOverflow question.
    – StoryTeller
    50 mins ago







  • 1




    @StoryTeller didnt downvote, though the google guidelines give more rationale than what the question suggests: "With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages." this is more or less the same what the answers are repeating here
    – user463035818
    45 mins ago










  • @user463035818 - Updated my answer accordingly. Google prohibits forward declarations IIRC, so that's a bit of rationale they won't post about.
    – StoryTeller
    41 mins ago














up vote
6
down vote

favorite












Related to this question: C/C++ include file order/best practices



The best answer recommends to include the related header first.



Same for Google and Mozilla style guides.



However, in both cases, I couldn't find a good reason why you would do this.



Google and Mozilla coding rules look best to me, because they enforce you to include the most "standard" headers first.



This way, I think you are less likely to mess up the included files (for examples by undefining some macros used in the other headers etc.) so that looks like the best way to go for me.



But following that rationale, why would you include the related header first? Since any syntax error in it might mess all the following headers?



I would think including the related header last would be best instead.






c++ header-files







share|improve this question



















  • 2




    I fail to see why it's downvoted. The OP presents research, and asks about something they did not understand from conducting it. There is a well defined answer as well. All in all, it fits the criteria of a decent StackOverflow question.
    – StoryTeller
    50 mins ago







  • 1




    @StoryTeller didnt downvote, though the google guidelines give more rationale than what the question suggests: "With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages." this is more or less the same what the answers are repeating here
    – user463035818
    45 mins ago










  • @user463035818 - Updated my answer accordingly. Google prohibits forward declarations IIRC, so that's a bit of rationale they won't post about.
    – StoryTeller
    41 mins ago












up vote
6
down vote

favorite









up vote
6
down vote

favorite











Related to this question: C/C++ include file order/best practices



The best answer recommends to include the related header first.



Same for Google and Mozilla style guides.



However, in both cases, I couldn't find a good reason why you would do this.



Google and Mozilla coding rules look best to me, because they enforce you to include the most "standard" headers first.



This way, I think you are less likely to mess up the included files (for examples by undefining some macros used in the other headers etc.) so that looks like the best way to go for me.



But following that rationale, why would you include the related header first? Since any syntax error in it might mess all the following headers?



I would think including the related header last would be best instead.






c++ header-files







share|improve this question















Related to this question: C/C++ include file order/best practices



The best answer recommends to include the related header first.



Same for Google and Mozilla style guides.



However, in both cases, I couldn't find a good reason why you would do this.



Google and Mozilla coding rules look best to me, because they enforce you to include the most "standard" headers first.



This way, I think you are less likely to mess up the included files (for examples by undefining some macros used in the other headers etc.) so that looks like the best way to go for me.



But following that rationale, why would you include the related header first? Since any syntax error in it might mess all the following headers?



I would think including the related header last would be best instead.






c++ header-files




c++ header-files






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 10 mins ago









StoryTeller

84.5k12169236




84.5k12169236










asked 58 mins ago









Arnaud

1205




1205







  • 2




    I fail to see why it's downvoted. The OP presents research, and asks about something they did not understand from conducting it. There is a well defined answer as well. All in all, it fits the criteria of a decent StackOverflow question.
    – StoryTeller
    50 mins ago







  • 1




    @StoryTeller didnt downvote, though the google guidelines give more rationale than what the question suggests: "With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages." this is more or less the same what the answers are repeating here
    – user463035818
    45 mins ago










  • @user463035818 - Updated my answer accordingly. Google prohibits forward declarations IIRC, so that's a bit of rationale they won't post about.
    – StoryTeller
    41 mins ago












  • 2




    I fail to see why it's downvoted. The OP presents research, and asks about something they did not understand from conducting it. There is a well defined answer as well. All in all, it fits the criteria of a decent StackOverflow question.
    – StoryTeller
    50 mins ago







  • 1




    @StoryTeller didnt downvote, though the google guidelines give more rationale than what the question suggests: "With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages." this is more or less the same what the answers are repeating here
    – user463035818
    45 mins ago










  • @user463035818 - Updated my answer accordingly. Google prohibits forward declarations IIRC, so that's a bit of rationale they won't post about.
    – StoryTeller
    41 mins ago







2




2




I fail to see why it's downvoted. The OP presents research, and asks about something they did not understand from conducting it. There is a well defined answer as well. All in all, it fits the criteria of a decent StackOverflow question.
– StoryTeller
50 mins ago





I fail to see why it's downvoted. The OP presents research, and asks about something they did not understand from conducting it. There is a well defined answer as well. All in all, it fits the criteria of a decent StackOverflow question.
– StoryTeller
50 mins ago





1




1




@StoryTeller didnt downvote, though the google guidelines give more rationale than what the question suggests: "With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages." this is more or less the same what the answers are repeating here
– user463035818
45 mins ago




@StoryTeller didnt downvote, though the google guidelines give more rationale than what the question suggests: "With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages." this is more or less the same what the answers are repeating here
– user463035818
45 mins ago












@user463035818 - Updated my answer accordingly. Google prohibits forward declarations IIRC, so that's a bit of rationale they won't post about.
– StoryTeller
41 mins ago




@user463035818 - Updated my answer accordingly. Google prohibits forward declarations IIRC, so that's a bit of rationale they won't post about.
– StoryTeller
41 mins ago












2 Answers
2






active

oldest

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up vote
9
down vote



accepted










It's to make sure your clients don't hate you when they include your library header.



If the header is brittle and subject to break on wrong inclusion order, it may appear to work in your environment when it isn't first - since you include the headers you need - but fail to compile for client code. Because it may not at all be obvious what other headers need to be pulled in for it to work.



Including first the header which corresponds to the current implementation file goes toward checking that the header is self-contained. Self-containment goes beyond just including the necessary headers. It also entails adding the required forward declarations for types you use in your API. Naturally your header will compile if you include the header for the type before it, but you may not wish to pull it in since you only depend on the type name in your API.



Some style guides prohibit forward declarations, so those may not be part of the rationale they pose.






share|improve this answer





























    up vote
    3
    down vote













    Including the header related to the cpp file first ensures that the header is self contained and doesn't require other includes to compile






    share|improve this answer
















    • 2




      Why the downvote? May be a bit short, but is correct.
      – Rene
      53 mins ago






    • 1




      "ensures that the header is self contained" - ensuring this is not possible in general. Situation when header file may be not self contained but works on current platform because on that platform required header gets included indirectly is quite typical and yet difficult to detect until you actually get an error. Including project headers prior to library headers may indeed help to detect whether related header is not self contained, but may not ensure it.
      – VTT
      41 mins ago






    • 1




      @VTT - Ensure? No. Provide some checks which are better than none? Yes.
      – StoryTeller
      40 mins ago










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    2 Answers
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    2 Answers
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    up vote
    9
    down vote



    accepted










    It's to make sure your clients don't hate you when they include your library header.



    If the header is brittle and subject to break on wrong inclusion order, it may appear to work in your environment when it isn't first - since you include the headers you need - but fail to compile for client code. Because it may not at all be obvious what other headers need to be pulled in for it to work.



    Including first the header which corresponds to the current implementation file goes toward checking that the header is self-contained. Self-containment goes beyond just including the necessary headers. It also entails adding the required forward declarations for types you use in your API. Naturally your header will compile if you include the header for the type before it, but you may not wish to pull it in since you only depend on the type name in your API.



    Some style guides prohibit forward declarations, so those may not be part of the rationale they pose.






    share|improve this answer


























      up vote
      9
      down vote



      accepted










      It's to make sure your clients don't hate you when they include your library header.



      If the header is brittle and subject to break on wrong inclusion order, it may appear to work in your environment when it isn't first - since you include the headers you need - but fail to compile for client code. Because it may not at all be obvious what other headers need to be pulled in for it to work.



      Including first the header which corresponds to the current implementation file goes toward checking that the header is self-contained. Self-containment goes beyond just including the necessary headers. It also entails adding the required forward declarations for types you use in your API. Naturally your header will compile if you include the header for the type before it, but you may not wish to pull it in since you only depend on the type name in your API.



      Some style guides prohibit forward declarations, so those may not be part of the rationale they pose.






      share|improve this answer
























        up vote
        9
        down vote



        accepted







        up vote
        9
        down vote



        accepted






        It's to make sure your clients don't hate you when they include your library header.



        If the header is brittle and subject to break on wrong inclusion order, it may appear to work in your environment when it isn't first - since you include the headers you need - but fail to compile for client code. Because it may not at all be obvious what other headers need to be pulled in for it to work.



        Including first the header which corresponds to the current implementation file goes toward checking that the header is self-contained. Self-containment goes beyond just including the necessary headers. It also entails adding the required forward declarations for types you use in your API. Naturally your header will compile if you include the header for the type before it, but you may not wish to pull it in since you only depend on the type name in your API.



        Some style guides prohibit forward declarations, so those may not be part of the rationale they pose.






        share|improve this answer














        It's to make sure your clients don't hate you when they include your library header.



        If the header is brittle and subject to break on wrong inclusion order, it may appear to work in your environment when it isn't first - since you include the headers you need - but fail to compile for client code. Because it may not at all be obvious what other headers need to be pulled in for it to work.



        Including first the header which corresponds to the current implementation file goes toward checking that the header is self-contained. Self-containment goes beyond just including the necessary headers. It also entails adding the required forward declarations for types you use in your API. Naturally your header will compile if you include the header for the type before it, but you may not wish to pull it in since you only depend on the type name in your API.



        Some style guides prohibit forward declarations, so those may not be part of the rationale they pose.







        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited 42 mins ago

























        answered 55 mins ago









        StoryTeller

        84.5k12169236




        84.5k12169236






















            up vote
            3
            down vote













            Including the header related to the cpp file first ensures that the header is self contained and doesn't require other includes to compile






            share|improve this answer
















            • 2




              Why the downvote? May be a bit short, but is correct.
              – Rene
              53 mins ago






            • 1




              "ensures that the header is self contained" - ensuring this is not possible in general. Situation when header file may be not self contained but works on current platform because on that platform required header gets included indirectly is quite typical and yet difficult to detect until you actually get an error. Including project headers prior to library headers may indeed help to detect whether related header is not self contained, but may not ensure it.
              – VTT
              41 mins ago






            • 1




              @VTT - Ensure? No. Provide some checks which are better than none? Yes.
              – StoryTeller
              40 mins ago














            up vote
            3
            down vote













            Including the header related to the cpp file first ensures that the header is self contained and doesn't require other includes to compile






            share|improve this answer
















            • 2




              Why the downvote? May be a bit short, but is correct.
              – Rene
              53 mins ago






            • 1




              "ensures that the header is self contained" - ensuring this is not possible in general. Situation when header file may be not self contained but works on current platform because on that platform required header gets included indirectly is quite typical and yet difficult to detect until you actually get an error. Including project headers prior to library headers may indeed help to detect whether related header is not self contained, but may not ensure it.
              – VTT
              41 mins ago






            • 1




              @VTT - Ensure? No. Provide some checks which are better than none? Yes.
              – StoryTeller
              40 mins ago












            up vote
            3
            down vote










            up vote
            3
            down vote









            Including the header related to the cpp file first ensures that the header is self contained and doesn't require other includes to compile






            share|improve this answer












            Including the header related to the cpp file first ensures that the header is self contained and doesn't require other includes to compile







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 55 mins ago









            Alan Birtles

            5,964531




            5,964531







            • 2




              Why the downvote? May be a bit short, but is correct.
              – Rene
              53 mins ago






            • 1




              "ensures that the header is self contained" - ensuring this is not possible in general. Situation when header file may be not self contained but works on current platform because on that platform required header gets included indirectly is quite typical and yet difficult to detect until you actually get an error. Including project headers prior to library headers may indeed help to detect whether related header is not self contained, but may not ensure it.
              – VTT
              41 mins ago






            • 1




              @VTT - Ensure? No. Provide some checks which are better than none? Yes.
              – StoryTeller
              40 mins ago












            • 2




              Why the downvote? May be a bit short, but is correct.
              – Rene
              53 mins ago






            • 1




              "ensures that the header is self contained" - ensuring this is not possible in general. Situation when header file may be not self contained but works on current platform because on that platform required header gets included indirectly is quite typical and yet difficult to detect until you actually get an error. Including project headers prior to library headers may indeed help to detect whether related header is not self contained, but may not ensure it.
              – VTT
              41 mins ago






            • 1




              @VTT - Ensure? No. Provide some checks which are better than none? Yes.
              – StoryTeller
              40 mins ago







            2




            2




            Why the downvote? May be a bit short, but is correct.
            – Rene
            53 mins ago




            Why the downvote? May be a bit short, but is correct.
            – Rene
            53 mins ago




            1




            1




            "ensures that the header is self contained" - ensuring this is not possible in general. Situation when header file may be not self contained but works on current platform because on that platform required header gets included indirectly is quite typical and yet difficult to detect until you actually get an error. Including project headers prior to library headers may indeed help to detect whether related header is not self contained, but may not ensure it.
            – VTT
            41 mins ago




            "ensures that the header is self contained" - ensuring this is not possible in general. Situation when header file may be not self contained but works on current platform because on that platform required header gets included indirectly is quite typical and yet difficult to detect until you actually get an error. Including project headers prior to library headers may indeed help to detect whether related header is not self contained, but may not ensure it.
            – VTT
            41 mins ago




            1




            1




            @VTT - Ensure? No. Provide some checks which are better than none? Yes.
            – StoryTeller
            40 mins ago




            @VTT - Ensure? No. Provide some checks which are better than none? Yes.
            – StoryTeller
            40 mins ago

















             

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