| ===================== |
| Header Removal Policy |
| ===================== |
| |
| Policy |
| ------ |
| |
| Libc++ is in the process of splitting larger headers into smaller modular |
| headers. This makes it possible to remove these large headers from other |
| headers. For example, instead of including ``<algorithm>`` entirely it is |
| possible to only include the headers for the algorithms used. When the |
| Standard indirectly adds additional header includes, using the smaller headers |
| aids reducing the growth of top-level headers. For example ``<atomic>`` uses |
| ``std::chrono::nanoseconds`` and included ``<chrono>``. In C++20 ``<chrono>`` |
| requires ``<format>`` which adds several other headers (like ``<string>``, |
| ``<optional>``, ``<tuple>``) which are not needed in ``<atomic>``. |
| |
| The benefit of using minimal headers is that the size of libc++'s top-level |
| headers becomes smaller. This improves the compilation time when users include |
| a top-level header. It also avoids header inclusion cycles and makes it easier |
| to port headers to platforms with reduced functionality. |
| |
| A disadvantage is that users unknowingly depend on these transitive includes. |
| Thus removing an include might break their build after upgrading a newer |
| version of libc++. For example, ``<algorithm>`` is often forgotten but using |
| algorithms will still work through those transitive includes. This problem is |
| solved by modules, however in practice most people do not use modules (yet). |
| |
| To ease the removal of transitive includes in libc++, libc++ will remove |
| unnecessary transitive includes in newly supported C++ versions. This means |
| that users will have to fix their missing includes in order to upgrade to a |
| newer version of the Standard. Libc++ also reserves the right to remove |
| transitive includes at any other time, however new language versions will be |
| used as a convenient way to perform bulk removals of transitive includes. |
| |
| For libc++ developers, this means that any transitive include removal must be |
| guarded by something of the form: |
| |
| .. code-block:: cpp |
| |
| #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20 |
| # include <algorithm> |
| # include <iterator> |
| # include <utility> |
| #endif |
| |
| When users define ``_LIBCPP_REMOVE_TRANSITIVE_INCLUDES``, libc++ will not |
| include transitive headers, regardless of the language version. This can be |
| useful for users to aid the transition to a newer language version, or by users |
| who simply want to make sure they include what they use in their code. |
| |
| |
| Rationale |
| --------- |
| |
| Removing headers is not only an issue for software developers, but also for |
| vendors. When a vendor updates libc++ several of their upstream packages might |
| fail to compile, forcing them to fix these packages or file a bug with their |
| upstream packages. Usually upgrading software to a new language standard is |
| done explicitly by software developers. This means they most likely will |
| discover and fix the missing includes, lessening the burden for the vendors. |