1.. _implementation-defined-behavior: 2 3=============================== 4Implementation-defined behavior 5=============================== 6 7Contains the implementation details of the implementation-defined behavior in 8libc++. Implementation-defined is mandated to be documented by the Standard. 9 10.. note: 11 This page is far from complete. 12 13 14Implementation-defined behavior 15=============================== 16 17Updating the Time Zone Database 18------------------------------- 19 20The Standard allows implementations to automatically update the 21*remote time zone database*. Libc++ opts not to do that. Instead calling 22 23 - ``std::chrono::remote_version()`` will update the version information of the 24 *remote time zone database*, 25 - ``std::chrono::reload_tzdb()``, if needed, will update the entire 26 *remote time zone database*. 27 28This offers a way for users to update the *remote time zone database* and 29give them full control over the process. 30 31 32`[ostream.formatted.print]/3 <http://eel.is/c++draft/ostream.formatted.print#3>`_ A terminal capable of displaying Unicode 33-------------------------------------------------------------------------------------------------------------------------- 34 35The Standard specifies that the manner in which a stream is determined to refer 36to a terminal capable of displaying Unicode is implementation-defined. This is 37used for ``std::print`` and similar functions taking an ``ostream&`` argument. 38 39Libc++ determines that a stream is Unicode-capable terminal by: 40 41* First it determines whether the stream's ``rdbuf()`` has an underlying 42 ``FILE*``. This is ``true`` in the following cases: 43 44 * The stream is ``std::cout``, ``std::cerr``, or ``std::clog``. 45 46 * A ``std::basic_filebuf<CharT, Traits>`` derived from ``std::filebuf``. 47 48* The way to determine whether this ``FILE*`` refers to a terminal capable of 49 displaying Unicode is the same as specified for `void vprint_unicode(FILE* 50 stream, string_view fmt, format_args args); 51 <http://eel.is/c++draft/print.fun#7>`_. This function is used for other 52 ``std::print`` overloads that don't take an ``ostream&`` argument. 53 54`[sf.cmath] <https://wg21.link/sf.cmath>`_ Mathematical Special Functions: Large indices 55---------------------------------------------------------------------------------------- 56 57Most functions within the Mathematical Special Functions section contain integral indices. 58The Standard specifies the result for larger indices as implementation-defined. 59Libc++ pursuits reasonable results by choosing the same formulas as for indices below that threshold. 60E.g. 61 62- ``std::hermite(unsigned n, T x)`` for ``n >= 128`` 63 64 65`[stringbuf.cons] <http://eel.is/c++draft/stringbuf.cons>`_ Whether sequence pointers are initialized to null pointers 66---------------------------------------------------------------------------------------------------------------------- 67 68Libc++ does not initialize the pointers to null pointers. It resizes the buffer 69to its capacity and uses that size. This means the SSO buffer of 70``std::string`` is used as initial output buffer. 71 72 73Listed in the index of implementation-defined behavior 74====================================================== 75 76The order of the entries matches the entries in the 77`draft of the Standard <http://eel.is/c++draft/impldefindex>`_. 78