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When coding in either C or C++, where should I have the #include's?

callback.h:

#ifndef _CALLBACK_H_ #define _CALLBACK_H_ #include <sndfile.h> #include "main.h" void on_button_apply_clicked(GtkButton* button, struct user_data_s* data); void on_button_cancel_clicked(GtkButton* button, struct user_data_s* data); #endif

callback.c:

#include <stdlib.h> #include <math.h> #include "config.h" #include "callback.h" #include "play.h" void on_button_apply_clicked(GtkButton* button, struct user_data_s* data) { gint page; page = gtk_notebook_get_current_page(GTK_NOTEBOOK(data->notebook)); ...

Should all includes be in either the .h or .c / .cpp, or both like I have done here?

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  • 2
    Let me turn this around and ask: what was your criteria for deciding to put sndfile.h and main.h in callback.h? Commented Jun 8, 2010 at 23:38
  • 1
    for anyone else interested in more details there's this. Commented Mar 29, 2021 at 21:04

5 Answers 5

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Put as much as you can in the .c and as little as possible in the .h. The includes in the .c are only included when that one file is compiled, but the includes for the .h have to be included by every file that uses it.

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6 Comments

True, but doesn't the #ifndef _CALLBACK_H_, on the top of it, prevent the compiler in processing it more than once?
@user9379 That will prevent it from being included more than once per .c or .cpp file. Each .c or .cpp file is generally build individually though, which means a .h will be re-parsed for each .c or .cpp file you compile.
I think that the main reason to put as little as possible in the .h is to avoid in some cases an error because of a loop of inclusion. Example: two classes need one another for their implementations, but not for their declarations. Putting both includes in the .cpps will avoid an error.
@Qu'est-cet'yont That's a reason why you can't put certain things in the .h files. This answer is about why you should put even less than that.
@BrendanLong I see, although in my sense including several times a same header doesn't matter if you put the correct macros inside to include the content only once. Therefore, I think that putting even less is to reduce probabilities to get an error in the future with modifications of the code.
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The only time you should include a header within another .h file is if you need to access a type definition in that header; for example:

#ifndef MY_HEADER_H #define MY_HEADER_H #include <stdio.h> void doStuffWith(FILE *f); // need the definition of FILE from stdio.h #endif

If header A depends on header B such as the example above, then header A should include header B directly. Do NOT try to order your includes in the .c file to satisfy dependencies (that is, including header B before header A); that is a big ol' pile of heartburn waiting to happen. I mean it. I've been in that movie several times, and it always ended with Tokyo in flames.

Yes, this can result in files being included multiple times, but if they have proper include guards set up to protect against multiple declaration/definition errors, then a few extra seconds of build time isn't worth worrying about. Trying to manage dependencies manually is a pain in the ass.

Of course, you shouldn't be including files where you don't need to.

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2 Comments

Why not just forward declare File class File; and include stdio in .cpp where doStuffWith define?
@olekstolar: In this particular case that won't work because FILE is a typedef name, not a class name. We need that typedef xxx FILE; definition from stdio.h. In other cases yes, you can forward declare incomplete types like struct foo; or class bar; and not include the header file, but now you've increased your maintenance burden. You have to keep your forward declarations in sync with what's in the header file - it's just easier to include the header file. No, the types in stdio.h are not likely to change, but in user-defined headers they can change a lot.
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Put as many includes in your cpp as possible and only the ones that are needed by the hpp file in the hpp. I believe this will help to speed up compilation, as hpp files will be cross-referenced less.

Also consider using forward declarations in your hpp file to further reduce the include dependency chain.

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Oo. The forward declarations thing is interesting.
Parappa, forward declarations are very useful in circular reference scenarios. But would they be a good practice in other scenarios? (I new in C++, so I am honestly asking)
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If I #include <callback.h>, I don't want to have to #include lots of other header files to get my code to compile. In callback.h you should include everything needed to compile against it. But nothing more.

Consider whether using forward declarations in your header file (such as class GtkButton;) will suffice, allowing you to reduce the number of #include directives in the header (and, in turn, my compilation time and complexity).

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2 Comments

I disagree. Including the whole world in H files increases the dependancy chain and therefore compile times.
My answer didn't recommend including the whole world in the header file, I suggested including just enough so that the user of the API doesn't have to spend time searching for dependencies.
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I propose to simply include an All.h in the project that includes all the headers needed, and every other .h file calls All.h and every .c/.cpp file only includes its own header.

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