[Bug c++/44673] New: static const variable works in if/else, fails at linking in ternary

[Bug c++/44673] New: static const variable works in if/else, fails at linking in ternary

[Hodapp87 at gmail dot com]

Reproduced on 4.6.0 and 4.1.2 as well.

Here is the header file:
class TestClass
{
public:
        void testFn() const;
private:
        static const int VAR1 = 9;
        static const int VAR2 = 5;
};

And here is the source file including it:
#include "testclass.h"
void TestClass::testFn() const {
        int b = 1;
        int c = (b == 0) ? VAR1 : VAR2;
        int d;
        if (b == 0) d = VAR1;
        else d = VAR2;
}
int main(void) { return 0; }

This compiles, but does not link (g++ is called with no options, only the
source). The following error is given:
gcc-test.o: In function `TestClass::testFn() const':
gcc-test.cpp:(.text+0x14): undefined reference to `TestClass::VAR1'
gcc-test.cpp:(.text+0x1b): undefined reference to `TestClass::VAR2'
collect2: ld returned 1 exit status

If I comment out the line "int c = (b == 0) ? VAR1 : VAR2;" in the source, then
the linker errors disappear.

I have been told that VAR1 and VAR2 are declared and initialized, but not
defined, and therefore this code is not valid C++. If it is indeed invalid,
then why does only the ternary statement cause a linking error, but not the
equivalent if/else?


-- 
           Summary: static const variable works in if/else, fails at linking
                    in ternary
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: Hodapp87 at gmail dot com
 GCC build triplet: i486-linux-gnu
  GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2010-06-25 21:31 -------
You need indeed a definition.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2010-06-25 21:32 -------
Note if case does not need a diagnostic according to the C++ standard.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[Hodapp87 at gmail dot com]

------- Comment #3 from Hodapp87 at gmail dot com  2010-06-25 21:45 -------
(In reply to comment #1)
> You need indeed a definition.
> 

Why does it require a definition in the ternary case, but not in the if/else?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2010-06-25 21:47 -------
In the case of if, the value was "inlined" and in the case of ?:, it is not.  I
had a patch which changed the behavior but lost it when I moved companies.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[redi at gcc dot gnu dot org]

------- Comment #5 from redi at gcc dot gnu dot org  2010-06-25 23:05 -------
The result of "(b == 0) ? VAR1 : VAR2" is an lvalue, that's the difference
between the two cases.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[redi at gcc dot gnu dot org]

------- Comment #6 from redi at gcc dot gnu dot org  2010-06-25 23:08 -------
in this variation the result is not an lvalue, so you can get away without a
definition:

 (b == 0) ? (int)VAR1 : VAR2


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[manu at gcc dot gnu dot org]

------- Comment #7 from manu at gcc dot gnu dot org  2010-06-26 00:18 -------
(In reply to comment #4)
> In the case of if, the value was "inlined" and in the case of ?:, it is not.  I
> had a patch which changed the behavior but lost it when I moved companies.

And what did your patch do exactly?


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[pinskia at gcc dot gnu dot org]

------- Comment #8 from pinskia at gcc dot gnu dot org  2010-06-26 00:25 -------
(In reply to comment #7)
> (In reply to comment #4)
> > In the case of if, the value was "inlined" and in the case of ?:, it is not.  I
> > had a patch which changed the behavior but lost it when I moved companies.
> 
> And what did your patch do exactly?

It was able to look into xyz?decl1:decl2 and create xyz?ABC:DEF; but I cannot
remember where I placed it.  It worked for lvalues too because this part was
done after (a?b:c) = d; was converted into a?b = d: c = d; or the address
expression was pushed into the ?:.  


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[manu at gcc dot gnu dot org]

-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
  GCC build triplet|i486-linux-gnu              |
   GCC host triplet|i486-linux-gnu              |
 GCC target triplet|i486-linux-gnu              |
   Last reconfirmed|0000-00-00 00:00:00         |2010-06-26 00:31:32
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[manu at gcc dot gnu dot org]

------- Comment #9 from manu at gcc dot gnu dot org  2010-06-26 00:30 -------
Then, I reopen this as an enhancement request. If you ever find/redo the patch
or someone else decides to fix this in the same way, it would a nice
improvement for usability.


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|minor                       |enhancement
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[redi at gcc dot gnu dot org]

------- Comment #10 from redi at gcc dot gnu dot org  2010-06-26 10:26 -------
I disagree that it would be useful, it would encourage non-portable code, and
I'd rather be told about the missing definition.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[redi at gcc dot gnu dot org]

------- Comment #11 from redi at gcc dot gnu dot org  2010-06-26 10:33 -------
[class.static.data]/3
If a static data member is of const literal type, its declaration in the class
definition can specify a brace-or-equal-initializer ... The member shall
still be defined in a namespace scope if it is used in the program and
the namespace scope definition shall not contain an initializer.

[basic.def.odr] explains what "used" means.

The code is invalid, I don't think it does users a service to accept it.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[manu at gcc dot gnu dot org]

------- Comment #12 from manu at gcc dot gnu dot org  2010-06-26 10:35 -------
(In reply to comment #11)
> [class.static.data]/3
> If a static data member is of const literal type, its declaration in the class
> definition can specify a brace-or-equal-initializer ... The member shall
> still be defined in a namespace scope if it is used in the program and
> the namespace scope definition shall not contain an initializer.
> 
> [basic.def.odr] explains what "used" means.
> 
> The code is invalid, I don't think it does users a service to accept it.

So is g++ accepting invalid code?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[redi at gcc dot gnu dot org]

------- Comment #13 from redi at gcc dot gnu dot org  2010-06-26 10:38 -------
(In reply to comment #11)
> definition

Bah, that entity is the "fi" ligature, it should say definition


(In reply to comment #12)
> So is g++ accepting invalid code?

Yes, but it's a violation of the ODR, no diagnostic is required (it's not
possible to tell until link time that there's no definition.) 

There are DOZENS of duplicates of this bug, they are all invalid.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[manu at gcc dot gnu dot org]

------- Comment #14 from manu at gcc dot gnu dot org  2010-06-26 10:53 -------
> (In reply to comment #12)
> > So is g++ accepting invalid code?
> 
> Yes, but it's a violation of the ODR, no diagnostic is required (it's not
> possible to tell until link time that there's no definition.) 
> 
> There are DOZENS of duplicates of this bug, they are all invalid.

However, it links in the if-else case. I understand that ?: may be used as a
lvalue, but in this case, it is not and the value may be inlined in the same
way as in if-else (I guess that is what Andrew's patch did). If the standard is
clear that this should not work, then so be it. 

Let's close this as INVALID, because I don't know how we could improve the
diagnostics here to help users. I wish we could but I cannot see how it can be
done. I guess that would require an integrated super-smart linker, I wonder if
the LLVM project is going to suggest that next?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[redi at gcc dot gnu dot org]

------- Comment #15 from redi at gcc dot gnu dot org  2010-06-26 13:46 -------
(In reply to comment #14)
> 
> However, it links in the if-else case.

The variable is not "used" in the [basic.def.odr] sense, because the
lvalue-to-rvalue conversion occurs immediately.

> I understand that ?: may be used as a
> lvalue, but in this case, it is not and the value may be inlined in the same
> way as in if-else (I guess that is what Andrew's patch did). If the standard is
> clear that this should not work, then so be it. 

The lvalue-to-rvalue conversion doesn't happen immediately to either the second
or third operand, it happens to the result of the conditional expression.

Andrew's idea might be allowed by the as-if rule, but I'm not sure. It must not
change the behaviour of valid programs. And in any case, the original example
is invalid. Supporting it is not important IMHO.

> Let's close this as INVALID, because I don't know how we could improve the
> diagnostics here to help users. I wish we could but I cannot see how it can be
> done. I guess that would require an integrated super-smart linker, I wonder if
> the LLVM project is going to suggest that next?

closing again


-- 

redi at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |scott at lurndal dot org

--- Comment #17 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-03-02 00:35:32 UTC ---
*** Bug 52455 has been marked as a duplicate of this bug. ***

[Bug c++/44673] static const variable works in if/else, fails at linking in ternary

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44673

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |derzuomaia at gmail dot com

--- Comment #16 from Andrew Pinski <pinskia at gcc dot gnu.org> 2010-09-28 20:33:38 UTC ---
*** Bug 45825 has been marked as a duplicate of this bug. ***