path temp_directory_path(error_code& ec);
Returns: An unspecified directory path suitable for temporary files. An error shall be reported if exists(p) is false or is_directory(p) is false, where p is the path to be returned. The signature with argument ec returns path() if an error occurs.
[ Example: For POSIX-based operating systems, an implementation might return the path supplied by the first environment variable found in the list TMPDIR, TMP, TEMP, TEMPDIR, or if none of these are found, "/tmp".
For Windows-based operating systems, an implementation might return the path reported by the Windows GetTempPath API function. — end example ]