.. _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 <http://eel.is/c++draft/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<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 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

[filebuf.virtuals] <https://eel.is/c++draft/filebuf.virtual>_ Effect of calling basic_filebuf::setbuf with nonzero arguments

Libc++ uses the provided buffer as the underlying buffer for input and output, and does not discard that buffer even when the stream is closed.

[stringbuf.cons] <http://eel.is/c++draft/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 <http://eel.is/c++draft/impldefindex>_.