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order_includes.md

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Order of includes

Use standard order for readability and to avoid hidden dependencies:

  • C library,
  • C++ library,
  • other libraries' .h,
  • your project's .h.

Note Within each section the includes should be ordered alphabetically.

All of a project's header files should be listed as descendants of the project's source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory). For example, mpm/include/material/bingham.h should be included as

#include "material/bingham.h"

In dir/foo.cc or dir/foo_test.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:

  • dir2/foo2.h (preferred location — see details below).
  • C system files.
  • C++ system files.
  • Other libraries' .h files.
  • Your project's .h files.

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.

dir/foo.cc and dir2/foo2.h are often in the same directory (e.g. base/basictypes_test.cc and base/basictypes.h), but can be in different directories too.

Caution You should include all the headers that define the symbols you rely upon, except in the unusual case of forward declaration. If you rely on symbols from bar.h, don't count on the fact that you included foo.h which (currently) includes bar.h: include bar.h yourself, unless foo.h explicitly demonstrates its intent to provide you the symbols of bar.h. However, any includes present in the related header do not need to be included again in the related cc (i.e., foo.cc can rely on foo.h's includes).

For example, the includes in material/base/material_base.cc might look like this:

#include "base/material_base.h"  // Preferred location.

#include <sys/types.h>
#include <unistd.h>

#include <algorithm>
#include <vector>

#include "base/material_container.h"

Exception

Sometimes, system-specific code needs conditional includes. Such code can put conditional includes after other includes. Of course, keep your system-specific code small and localized.