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Section: 188.8.131.52 [fs.op.equivalent] Status: C++20 Submitter: Billy Robert O'Neal III Opened: 2017-02-27 Last modified: 2021-02-25
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See discussion on the LWG mailing list with subject "Is equivalent("existing_thing", "not_existing_thing") an error?", abreviated below:Billy O'Neal:
The existing "an error is reported" effects say that an error is reported for !exists(p1) && !exists(p2), but I'm not sure that treating equivalent("existing_thing", "not_existing_thing") as "false, no error" makes any more sense than for equivalent("not_existing_thing", "not_existing_thing").It's also unfortunate that the current spec requires reporting an error for is_other(p1) && is_other(p2) — there's no reason that you can't give a sane answer for paths to NT pipes. (Do POSIX FIFOs give garbage answers here?)
I'm fine with an error if either path does not exist. See also Late 29: I would much preferfile_identity identity(const path&, bool resolve = true);
which would of course produce an error if the path did not exist (or, with the default resolve, was a broken symlink).See Late 30 and 32 (31 has been resolved). FIFOs pose no trouble: you can even fstat(2) on the naked file descriptors produced by pipe(2). (That said, I observe the strange inconsistency that Linux but not macOS gives both ends of a pipe the same st_ino.) POSIX has no reason that I know of to treat any file type specially for equivalent().
I think such a file_identity feature would be useful but we can always add it in addition to equivalent post-C++17.
Looks good to me. Maybe submit this as an issue right away in the hopes it can go in C++17?
Set priority to 0; Tentatively Ready
This wording is relative to N4640.
Make the following edits to 184.108.40.206 [fs.op.equivalent]:
bool equivalent(const path& p1, const path& p2); bool equivalent(const path& p1, const path& p2, error_code& ec) noexcept;
-1- Let s1 and s2 be file_statuss determined as if by status(p1) and status(p2), respectively. -2- Effects: Determines s1 and s2. If (!exists(s1) && !exists(s2)) || (is_other(s1) && is_other(s2)) an error is reported (27.10.7).-3- Returns: true, if s1 == s2 andp1 and p2 resolve to the same file system entity, else false. The signature with argument ec returns false if an error occurs. -4- Two paths are considered to resolve to the same file system entity if two candidate entities reside on the same device at the same location. This is determined as if by the values of the POSIX stat structure, obtained as if by stat() for the two paths, having equal st_dev values and equal st_ino values. -5- Throws: As specified in 27.10.7.