Libc++ ABI stability¶
Libc++ aims to preserve a stable ABI to avoid subtle bugs when code built under the old ABI is linked with code built under the new ABI. At the same time, libc++ wants to make ABI-breaking improvements and bugfixes in scenarios where the user doesn’t mind ABI breaks.
To support both cases, libc++ allows specifying an ABI version at
build time. The version is defined with CMake option
Currently supported values are
1 (the stable default)
2 (the unstable “next” version). At some point “ABI version 2” will be
frozen and new ABI-breaking changes will start being applied to version
but this has not happened yet.
To always use the most cutting-edge, most unstable ABI (which is currently
but at some point will become
3), set the CMake option
Internally, each ABI-changing feature is placed under its own C++ macro,
_LIBCPP_ABI_XXX. These macros’ definitions are controlled by the C++ macro
_LIBCPP_ABI_VERSION, which is controlled by the
at build time. Libc++ does not intend users to interact with these C++ macros
The exception to this is MSVC environments. Libc++ does not currently have users that require a stable ABI in MSVC environments, so MSVC-only changes may be applied unconditionally.