[Bug c++/29615] New: Class can't be friends of itself?

[Bug c++/29615] New: Class can't be friends of itself?

[danny dot boelens at artwork-systems dot com]

The following simple C++ file:

<test.cpp>
class A
{
  friend class A;
};

int main() { return 0; }
</test.cpp>

results in the following error when I try to compile it with g++:

$ ~/gcc-build-4.2-20061007/gcc/g++ -o test test.cpp
test.cpp:3: error: class 'A' is implicitly friends with itself

I was using a snapshot (GCC 4.2 2006/10/07), but I got the same behaviour with
several 4.1.x and 4.0.x builds on both Linux and Windows 2000 (Cygwin).

I agree that it's pointless to make a class a friend of itself (which is why I
used severity 'minor'), but I haven't found anything in my C++ spec (ISO/IEC
14882) that say it is not allowed. Please point my to the relevant section in
case I missed anything.

By the way: all other compilers I've tried accept this file without a problem.
These include Visual Studio .NET 2003, CodeWarrior 8.3, CodeWarrior 9.6 and the
online test drive for Comeau. Comeau does exactly what I'd expect:

Your Comeau C/C++ test results are as follows: 

Comeau C/C++ 4.3.8 (Aug 19 2006 13:36:48) for  ONLINE_EVALUATION_Alpha1
Copyright 1988-2006 Comeau Computing.  All rights reserved.
MODE:strict errors C++

"ComeauTest.c", line 3: warning: pointless friend declaration
       friend class A;
       ^

Not a big issue, but I thought it might be a good idea to log this anyway,
since a search didn't result in relevant hits :-)


-- 
           Summary: Class can't be friends of itself?
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: danny dot boelens at artwork-systems dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615

[Bug c++/29615] Class can't be friends of itself?

[bangerth at dealii dot org]

------- Comment #1 from bangerth at dealii dot org  2006-10-27 22:57 -------
Confirmed.


-- 

bangerth at dealii dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bangerth at dealii dot org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-10-27 22:57:33
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615

[Bug c++/29615] Class can't be friends of itself?

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-10-28 18:44 -------
As far as I can tell this has been a pedwarning since the begining of the C++
front-end.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615

[Bug c++/29615] Class can't be friends of itself?

[pcarlini at suse dot de]

-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |pcarlini at suse dot de
                   |dot org                     |
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615

[Bug c++/29615] Class can't be friends of itself?

[pcarlini at suse dot de]

-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|pcarlini at suse dot de     |unassigned at gcc dot gnu
                   |                            |dot org
             Status|ASSIGNED                    |NEW


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615

[Bug c++/29615] Class can't be friends of itself?

[jens-devel at gmx dot de]

------- Comment #3 from jens-devel at gmx dot de  2007-10-15 12:48 -------
Subject:  Class can't be friends of itself?

Hi,

I think gcc should be able to compile something like this.
There is no other way to make a member-variable accessible only from all
objects which are of the same type. Am I wrong?

<test.cpp>
class A
{
  friend class A;

  private:
        int var_accessible_from_all_A_objects;
};

int main() { return 0; }
</test.cpp>

Greetings
Jens


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615

[Bug c++/29615] Class can't be friends of itself?

[potswa at mac dot com]

------- Comment #4 from potswa at mac dot com  2010-03-26 21:08 -------
The error doesn't occur if the friend is a template instance, so it really
doesn't hurt anyone. But it's weird.

template< int z >
class F {
    friend class F<0>; // error only if <0> removed
};

template class F<0>;


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615

[Bug c++/29615] Class can't be friends of itself?

[redi at gcc dot gnu dot org]

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------- Comment #5 from redi at gcc dot gnu dot org  2010-03-27 20:14 -------
With current versions this is only a warning not an error, changing keywords
from rejects-valid to diagnostic

(In reply to comment #3)
> There is no other way to make a member-variable accessible only from all
> objects which are of the same type. Am I wrong?

Yes.

(In reply to comment #4)
> The error doesn't occur if the friend is a template instance, so it really
> doesn't hurt anyone. But it's weird.
> 
> template< int z >
> class F {
>     friend class F<0>; // error only if <0> removed
> };

Well in that case it isn't friends with itself, so not really weird.

This shows that being a template has nothing to do with it:

template< int z >
class F {
    friend class F<z>;
};

warning: class ‘F<z>’ is implicitly friends with itself


-- 

redi at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|rejects-valid               |diagnostic


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615

[Bug c++/29615] Class can't be friends of itself?

[redi at gcc dot gnu dot org]

------- Comment #6 from redi at gcc dot gnu dot org  2010-03-27 20:19 -------
I think the warning is reasonable, but it would be nice if it could be
disabled.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29615