146035553Spatrick 246035553Spatrick==================== 346035553SpatrickLibc++ ABI stability 446035553Spatrick==================== 546035553Spatrick 6*76d0caaeSpatrickLibc++ aims to preserve a stable ABI to avoid subtle bugs when code built under the old ABI 7*76d0caaeSpatrickis linked with code built under the new ABI. At the same time, libc++ wants to make 8*76d0caaeSpatrickABI-breaking improvements and bugfixes in scenarios where the user doesn't mind ABI breaks. 946035553Spatrick 10*76d0caaeSpatrickTo support both cases, libc++ allows specifying an ABI version at 11*76d0caaeSpatrickbuild time. The version is defined with CMake option ``LIBCXX_ABI_VERSION``. 12*76d0caaeSpatrickCurrently supported values are ``1`` (the stable default) 13*76d0caaeSpatrickand ``2`` (the unstable "next" version). At some point "ABI version 2" will be 14*76d0caaeSpatrickfrozen and new ABI-breaking changes will start being applied to version ``3``; 15*76d0caaeSpatrickbut this has not happened yet. 1646035553Spatrick 17*76d0caaeSpatrickTo always use the most cutting-edge, most unstable ABI (which is currently ``2`` 18*76d0caaeSpatrickbut at some point will become ``3``), set the CMake option ``LIBCXX_ABI_UNSTABLE``. 19*76d0caaeSpatrick 20*76d0caaeSpatrickInternally, each ABI-changing feature is placed under its own C++ macro, 21*76d0caaeSpatrick``_LIBCPP_ABI_XXX``. These macros' definitions are controlled by the C++ macro 22*76d0caaeSpatrick``_LIBCPP_ABI_VERSION``, which is controlled by the ``LIBCXX_ABI_VERSION`` set 23*76d0caaeSpatrickat build time. Libc++ does not intend users to interact with these C++ macros 24*76d0caaeSpatrickdirectly. 25