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.. _implementation-defined-behavior:
===============================
Implementation-defined behavior
===============================
Contains the implementation details of the implementation-defined behavior in
libc++. Implementation-defined is mandated to be documented by the Standard.
.. note:
This page is far from complete.
Implementation-defined behavior
===============================
Updating the Time Zone Database
-------------------------------
The 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 <http://eel.is/c++draft/ostream.formatted.print#3>`_ A terminal capable of displaying Unicode
--------------------------------------------------------------------------------------------------------------------------
The 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<CharT, Traits>`` 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);
<http://eel.is/c++draft/print.fun#7>`_. This function is used for other
``std::print`` overloads that don't take an ``ostream&`` argument.
`[sf.cmath] <https://wg21.link/sf.cmath>`_ Mathematical Special Functions: Large indices
----------------------------------------------------------------------------------------
Most functions within the Mathematical Special Functions section contain integral indices.
The 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``
Listed in the index of implementation-defined behavior
======================================================
The order of the entries matches the entries in the
`draft of the Standard <http://eel.is/c++draft/impldefindex>`_.
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