What’s New in Libc++ 12.0.0?
New Features
Random device support has been made optional. It’s enabled by default and can be disabled by building libc++ with -DLIBCXX_ENABLE_RANDOM_DEVICE=OFF. Disabling random device support can be useful when building the library for platforms that don’t have a source of randomness, such as some embedded platforms. When this is not supported, most of <random> will still be available, but std::random_device will not.
Localization support has been made optional. It’s enabled by default and can be disabled by building libc++ with -DLIBCXX_ENABLE_LOCALIZATION=OFF. Disabling localization can be useful when porting to platforms that don’t support the C locale API (e.g. embedded). When localization is not supported, several parts of the library will be disabled: <iostream>, <regex>, <locale> will be completely unusable, and other parts may be only partly available.
If libc++ is compiled with a C++20 capable compiler it will be compiled in C++20 mode. Else libc++ will be compiled in C++17 mode.
Several unqualified lookups in libc++ have been changed to qualified lookups. This makes libc++ more ADL-proof.
The libc++ implementation status pages have been overhauled. Like other parts documentation they now use restructured text instead of html. Starting with libc++12 the status pages are part of libc++’s documentation.
More C++20 features have been implemented. libc++ C++20 Status has the full overview of libc++’s C++20 implementation status.
Work has started to implement new C++2b features. libc++ C++2b Status has the full overview of libc++’s C++2b implementation status.
API Changes
By default, libc++ will _not_ include the definition for new and delete, since those are provided in libc++abi. Vendors wishing to provide new and delete in libc++ can build the library with -DLIBCXX_ENABLE_NEW_DELETE_DEFINITIONS=ON to get back the old behavior. This was done to avoid providing new and delete in both libc++ and libc++abi, which is technically an ODR violation. Also note that we couldn’t decide to put the operators in libc++ only, because they are needed from libc++abi (which would create a circular dependency).
During the C++20 standardization process some new low-level bit functions have been renamed. Libc++ has renamed these functions to match the C++20 Standard. - ispow2 has been renamed to has_single_bit - ceil2 has been renamed to bit_ceil - floor2 has been renamed to bit_floor - log2p1 has been renamed to bit_width
In C++20 mode, std::filesystem::path::u8string() and generic_u8string() now return std::u8string according to P0428, while they return std::string in C++17. This can cause source incompatibility, which is discussed and acknowledged in P1423, but that paper doesn’t suggest any remediation for this incompatibility.
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