1.. title:: clang-tidy - portability-std-allocator-const 2 3portability-std-allocator-const 4=============================== 5 6Report use of ``std::vector<const T>`` (and similar containers of const 7elements). These are not allowed in standard C++, and should usually be 8``std::vector<T>`` instead." 9 10Per C++ ``[allocator.requirements.general]``: "T is any cv-unqualified object 11type", ``std::allocator<const T>`` is undefined. Many standard containers use 12``std::allocator`` by default and therefore their ``const T`` instantiations are 13undefined. 14 15libc++ defines ``std::allocator<const T>`` as an extension which will be removed 16in the future. 17 18libstdc++ and MSVC do not support ``std::allocator<const T>``: 19 20.. code:: c++ 21 22 // libstdc++ has a better diagnostic since https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48101 23 std::deque<const int> deque; // error: static assertion failed: std::deque must have a non-const, non-volatile value_type 24 std::set<const int> set; // error: static assertion failed: std::set must have a non-const, non-volatile value_type 25 std::vector<int* const> vector; // error: static assertion failed: std::vector must have a non-const, non-volatile value_type 26 27 // MSVC 28 // error C2338: static_assert failed: 'The C++ Standard forbids containers of const elements because allocator<const T> is ill-formed.' 29 30Code bases only compiled with libc++ may accrue such undefined usage. This 31check finds such code and prevents backsliding while clean-up is ongoing. 32