1 2==================== 3Libc++ ABI stability 4==================== 5 6Libc++ aims to preserve a stable ABI to avoid subtle bugs when code built under the old ABI 7is linked with code built under the new ABI. At the same time, libc++ wants to make 8ABI-breaking improvements and bugfixes in scenarios where the user doesn't mind ABI breaks. 9 10To support both cases, libc++ allows specifying an ABI version at 11build time. The version is defined with CMake option ``LIBCXX_ABI_VERSION``. 12Currently supported values are ``1`` (the stable default) 13and ``2`` (the unstable "next" version). At some point "ABI version 2" will be 14frozen and new ABI-breaking changes will start being applied to version ``3``; 15but this has not happened yet. 16 17To always use the most cutting-edge, most unstable ABI (which is currently ``2`` 18but at some point will become ``3``), set the CMake option ``LIBCXX_ABI_UNSTABLE``. 19 20Internally, each ABI-changing feature is placed under its own C++ macro, 21``_LIBCPP_ABI_XXX``. These macros' definitions are controlled by the C++ macro 22``_LIBCPP_ABI_VERSION``, which is controlled by the ``LIBCXX_ABI_VERSION`` set 23at build time. Libc++ does not intend users to interact with these C++ macros 24directly. 25