[Bug c++/57977] New: zero-length const array in union prohibits default copy xtor

[Bug c++/57977] New: zero-length const array in union prohibits default copy xtor

[daniel.santos at pobox dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57977

            Bug ID: 57977
           Summary: zero-length const array in union prohibits default
                    copy xtor
           Product: gcc
           Version: 4.7.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: daniel.santos at pobox dot com

#include <iostream>

union a {
    struct {
        const char string[0];
    } b;
};


int main(int argc, char *argv[]) {
    std::cout << "size = " << sizeof(union a) << std::endl;
    return 0;
}

Produces:

bug.cpp:6:4: error: member ‘a::<anonymous struct> a::b’ with copy assignment
operator not allowed in union
bug.cpp:6:4: note: unrestricted unions only available with -std=c++11 or
-std=gnu++11


So the zero-length array is, of course, a gcc extension.  I believe it is a bug
because it is zero-sized and no code needs to be generated to copy union
a::b::string.

My justification for using this is to aid in const correctness. I actually use
this in C in a kernel module where I create and populate it once and then
access it several times, so I just cast it as non-const when I populate it.  I
prefer to use the exact same header files for kernel & userland where possible.
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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/57977] zero-length const array in union prohibits default copy xtor
Date: Thu, 25 Jul 2013 05:37:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW977

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It is not the zero length which is causing the copy constructor to be created
but rather the const part which causes the copy constructor to happen.

[Bug c++/57977] zero-length const array in union prohibits default copy xtor

[daniel.santos at pobox dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57977

--- Comment #2 from Daniel Santos <daniel.santos at pobox dot com> ---
Don't you mean the part which prohibits its creation?

[Bug c++/57977] zero-length const array in union prohibits default copy xtor

[daniel.santos at pobox dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57977

--- Comment #3 from Daniel Santos <daniel.santos at pobox dot com> ---
Hmm, I guess it's actually the copy assignment operator.  Either way, it makes
sense if the const union member was "real", in this case, the copy assignment
for this member would be a no-op (were we to copy it as that type), so the
constness of this member should be ignored (as I understand it).

[Bug c++/57977] zero-length const array in union prohibits default copy ctor

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57977

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-12-03
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
           Keywords|                            |rejects-valid

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed