Guidelines for #include directives

The following include order is used in GROMACS. An empty line should appear between each group, and headers within each group sorted alphabetically.

1. Each source file should include gmxpre.h first.

2. If a source file has a corresponding header, it should be included next. If the header is in the same directory as the source, then it is included without any path (i.e., relative to the source), otherwise relative to src/ (the latter case should be rare).

3. If the file depends on defines from config.h, that comes next.

4. This is followed by standard C/C++ headers, grouped as follows:

   1. Standard C headers (e.g., <stdio.h>)
   2. C++ versions of the above (e.g., <cstdio>)
   3. Standard C++ headers (e.g., <vector>)

   Preferably, only one of the first two groups is present, but this is not enforced.

5. This is followed by other system headers: platform-specific headers such as <unistd.h>, as well as external libraries such as <gtest/gtest.h>.

6. GROMACS-specific libraries from src/external/, such as "thread_mpi/threads.h".

7. GROMACS-specific headers that are not internal to the including module, included with a path relative to src/.

8. In test files, headers not internal to the module, but specific to testing code, are in a separate block at this point, paths relative to src/.

9. Finally, GROMACS headers that are internal to the including module are included using a relative path (but never with a path starting with ../; such headers go into group 7 instead). For test files, this group contains headers that are internal to tests for that module.

All GROMACS headers are included with quotes ("gromacs/utility/path.h"), other headers with angle brackets (<stdio.h>). Headers under src/external/ are generally included with quotes (whenever the include path is relative to src/, as well as for thread-MPI and TNG), but larger third-party entities are included as if they were provided by the system. The latter group currently includes gtest/gmock.

If there are any conditionally included headers (typically, only when some #defines from config.h are set), these should be included at the end of their respective group. Note that the automatic checker/sorter script does not act on such headers, nor on comments that are between #include statements; it is up to the author of the code to put the headers in proper order in such cases. Trailing comments on the same line as #include statements are preserved and do not affect the checker/sorter.

The guidelines are enforced by an automatic checker script that can also sort/reformat include statements to follow the guidelines. See Source tree checker scripts for details.

Enforcing a consistent order and style has a few advantages:

• It makes it easy at a quick glance to find the dependencies of a file, without scanning through a long list of unorganized #includes.
• Including the header corresponding to the source file first makes most headers included first in some source file, revealing potential problems where headers would not compile unless some other header would be included first. With this order, the person working on the header is most likely to see these problems instead of someone else seeing them later when refactoring unrelated code.
• Consistent usage of paths in #include directives makes it easy to use grep to find all uses of a header, as well as all include dependencies between two modules.
• An automatic script can be used to re-establish clean code after semi-automatic refactoring like renaming an include file with sed, without causing other unnecessary changes.