.. _implementation-defined-behavior: =============================== Implementation-defined behavior =============================== This document contains the implementation details of the implementation-defined behavior in libc++. The C++ standard mandates that implementation-defined behavior is documented. .. note: This page is far from complete. Implementation-defined behavior =============================== Updating the Time Zone Database ------------------------------- The C++ standard allows implementations to automatically update the *remote time zone database*. Libc++ opts not to do that. Instead calling - ``std::chrono::remote_version()`` will update the version information of the *remote time zone database*, - ``std::chrono::reload_tzdb()``, if needed, will update the entire *remote time zone database*. This offers a way for users to update the *remote time zone database* and give them full control over the process. `[ostream.formatted.print]/3 `_ A terminal capable of displaying Unicode -------------------------------------------------------------------------------------------------------------------------- The C++ standard specifies that the manner in which a stream is determined to refer to a terminal capable of displaying Unicode is implementation-defined. This is used for ``std::print`` and similar functions taking an ``ostream&`` argument. Libc++ determines that a stream is Unicode-capable terminal by: * First it determines whether the stream's ``rdbuf()`` has an underlying ``FILE*``. This is ``true`` in the following cases: * The stream is ``std::cout``, ``std::cerr``, or ``std::clog``. * A ``std::basic_filebuf`` derived from ``std::filebuf``. * The way to determine whether this ``FILE*`` refers to a terminal capable of displaying Unicode is the same as specified for `void vprint_unicode(FILE* stream, string_view fmt, format_args args); `_. This function is used for other ``std::print`` overloads that don't take an ``ostream&`` argument. `[sf.cmath] `_ Mathematical Special Functions: Large indices ---------------------------------------------------------------------------------------- Most functions within the Mathematical Special Functions section contain integral indices. The C++ standard specifies the result for larger indices as implementation-defined. Libc++ pursuits reasonable results by choosing the same formulas as for indices below that threshold. E.g., - ``std::hermite(unsigned n, T x)`` for ``n >= 128`` `[stringbuf.cons] `_ Whether sequence pointers are initialized to null pointers ---------------------------------------------------------------------------------------------------------------------- Libc++ does not initialize the pointers to null pointers. It resizes the buffer to its capacity and uses that size. This means the SSO buffer of ``std::string`` is used as initial output buffer. Listed in the index of implementation-defined behavior ====================================================== The order of the entries matches the entries in the `draft of the Standard `_.