1*4bdff4beSrobert===================== 2*4bdff4beSrobertHeader Removal Policy 3*4bdff4beSrobert===================== 4*4bdff4beSrobert 5*4bdff4beSrobertPolicy 6*4bdff4beSrobert------ 7*4bdff4beSrobert 8*4bdff4beSrobertLibc++ is in the process of splitting larger headers into smaller modular 9*4bdff4beSrobertheaders. This makes it possible to remove these large headers from other 10*4bdff4beSrobertheaders. For example, instead of including ``<algorithm>`` entirely it is 11*4bdff4beSrobertpossible to only include the headers for the algorithms used. When the 12*4bdff4beSrobertStandard indirectly adds additional header includes, using the smaller headers 13*4bdff4beSrobertaids reducing the growth of top-level headers. For example ``<atomic>`` uses 14*4bdff4beSrobert``std::chrono::nanoseconds`` and included ``<chrono>``. In C++20 ``<chrono>`` 15*4bdff4beSrobertrequires ``<format>`` which adds several other headers (like ``<string>``, 16*4bdff4beSrobert``<optional>``, ``<tuple>``) which are not needed in ``<atomic>``. 17*4bdff4beSrobert 18*4bdff4beSrobertThe benefit of using minimal headers is that the size of libc++'s top-level 19*4bdff4beSrobertheaders becomes smaller. This improves the compilation time when users include 20*4bdff4beSroberta top-level header. It also avoids header inclusion cycles and makes it easier 21*4bdff4beSrobertto port headers to platforms with reduced functionality. 22*4bdff4beSrobert 23*4bdff4beSrobertA disadvantage is that users unknowingly depend on these transitive includes. 24*4bdff4beSrobertThus removing an include might break their build after upgrading a newer 25*4bdff4beSrobertversion of libc++. For example, ``<algorithm>`` is often forgotten but using 26*4bdff4beSrobertalgorithms will still work through those transitive includes. This problem is 27*4bdff4beSrobertsolved by modules, however in practice most people do not use modules (yet). 28*4bdff4beSrobert 29*4bdff4beSrobertTo ease the removal of transitive includes in libc++, libc++ will remove 30*4bdff4beSrobertunnecessary transitive includes in newly supported C++ versions. This means 31*4bdff4beSrobertthat users will have to fix their missing includes in order to upgrade to a 32*4bdff4beSrobertnewer version of the Standard. Libc++ also reserves the right to remove 33*4bdff4beSroberttransitive includes at any other time, however new language versions will be 34*4bdff4beSrobertused as a convenient way to perform bulk removals of transitive includes. 35*4bdff4beSrobert 36*4bdff4beSrobertFor libc++ developers, this means that any transitive include removal must be 37*4bdff4beSrobertguarded by something of the form: 38*4bdff4beSrobert 39*4bdff4beSrobert.. code-block:: cpp 40*4bdff4beSrobert 41*4bdff4beSrobert #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20 42*4bdff4beSrobert # include <algorithm> 43*4bdff4beSrobert # include <iterator> 44*4bdff4beSrobert # include <utility> 45*4bdff4beSrobert #endif 46*4bdff4beSrobert 47*4bdff4beSrobertWhen users define ``_LIBCPP_REMOVE_TRANSITIVE_INCLUDES``, libc++ will not 48*4bdff4beSrobertinclude transitive headers, regardless of the language version. This can be 49*4bdff4beSrobertuseful for users to aid the transition to a newer language version, or by users 50*4bdff4beSrobertwho simply want to make sure they include what they use in their code. 51*4bdff4beSrobert 52*4bdff4beSrobert 53*4bdff4beSrobertRationale 54*4bdff4beSrobert--------- 55*4bdff4beSrobert 56*4bdff4beSrobertRemoving headers is not only an issue for software developers, but also for 57*4bdff4beSrobertvendors. When a vendor updates libc++ several of their upstream packages might 58*4bdff4beSrobertfail to compile, forcing them to fix these packages or file a bug with their 59*4bdff4beSrobertupstream packages. Usually upgrading software to a new language standard is 60*4bdff4beSrobertdone explicitly by software developers. This means they most likely will 61*4bdff4beSrobertdiscover and fix the missing includes, lessening the burden for the vendors. 62