# Extra Clang Tools 22.1.2 documentation

## clang-tidy - portability-std-allocator-const

# portability-std-allocator-const

Report use of `std::vector<const T>` (and similar containers of const elements). These are not allowed in standard C++, and should usually be `std::vector<T>` instead.”

Per C++ `[allocator.requirements.general]`: “T is any cv-unqualified object type”, `std::allocator<const T>` is undefined. Many standard containers use `std::allocator` by default and therefore their `const T` instantiations are undefined.

libc++ defines `std::allocator<const T>` as an extension which will be removed in the future.

libstdc++ and MSVC do not support `std::allocator<const T>`:

    // libstdc++ has a better diagnostic since https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48101
    std::deque<const int> deque; // error: static assertion failed: std::deque must have a non-const, non-volatile value_type
    std::set<const int> set; // error: static assertion failed: std::set must have a non-const, non-volatile value_type
    std::vector<int* const> vector; // error: static assertion failed: std::vector must have a non-const, non-volatile value_type

    // MSVC
    // error C2338: static_assert failed: 'The C++ Standard forbids containers of const elements because allocator<const T> is ill-formed.'

Code bases only compiled with libc++ may accrue such undefined usage. This check finds such code and prevents backsliding while clean-up is ongoing.