=============================== 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 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] 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.