[Bug regression/26355] New: defining static members of specialized template classes doesn't work

[Bug regression/26355] New: defining static members of specialized template classes doesn't work

[vz-gcc at zeitlins dot org]

I'm not 100% sure if this is a compiler bug or a bug in my code but I think
that what I'm trying to do should be valid according to 14.7.5/4 of the C++
Standard. Please consider the following example:

% cat stsp.cpp
enum V { V1, V2, V3 };

template <V v> struct Data { static int Value; };

int Data<V1>::Value;

extern int GetIt() { return Data<V1>::Value; }


Compiling this with g++ 4.0 or 4.1 doesn't work:

% g++-4.0 -c -Wall stsp.cpp
stsp.cpp:5: error: too few template-parameter-lists
% g++-4.1 -c -Wall stsp.cpp
stsp.cpp:5: error: too few template-parameter-lists

While it works with all the previous versions (down to 2.95!).

g++4 does accept the explicit specialization of the static member if you
prepend "template <>" to "Data<V1>::Value" line which, IMHO, wrong too. But it
doesn't define the symbol in the object file in this case.

If this is really not allowed then it would be nice to make the error message
more clear.


-- 
           Summary: defining static members of specialized template classes
                    doesn't work
           Product: gcc
           Version: 4.0.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: regression
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vz-gcc at zeitlins dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26355

[Bug regression/26355] defining static members of specialized template classes doesn't work

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-02-18 22:26 -------
This is invalid code.  This is a dup of bug 22095.

The diagnostic issue is filed under PR 20118.

*** This bug has been marked as a duplicate of 22095 ***


-- 

pinskia at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26355

[Bug regression/26355] defining static members of specialized template classes doesn't work

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-02-18 22:28 -------
Also see PR 11930 for more help with the issue adding template<> does not emit
the variable.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26355

[Bug regression/26355] defining static members of specialized template classes doesn't work

[vz-gcc at zeitlins dot org]

------- Comment #3 from vz-gcc at zeitlins dot org  2006-02-18 22:47 -------
First, thanks a lot Andrew for your lightning fast reply, this is really
amazing -- and incredibly helpful!

Second, really sorry, rereading the explicit specialization section once again
I see that I was indeed wrong and that "template <>" is really needed.

Third, unfortunately PR 11930 does not help with my test case:
% cat -n stsp.cpp
     1  enum V { V1, V2, V3 };
     2
     3  template <V v> struct Data { static int Value; };
     4
     5  template struct Data<V1>;
     6
     7  template <> int Data<V1>::Value;
     8
     9  int main() { return Data<V1>::Value; }
% g++-4.0 -o stsp -Wall stsp.cpp
/tmp/ccywXRfg.o: In function `main':stsp.cpp:(.text+0x1d): undefined reference
to `Data<(V)0>::Value'
collect2: ld returned 1 exit status


It looks like the compiler interprets the line 7 as declaration and not
definition? If I add an initializer (i.e. change the line to end with "Value =
17" or "Value(17)") then it works just fine. Unfortunately in the real code the
static member is a class and, worse, a class with only default ctor so I can't
use "Value()" (which would be parsed as a function declaration) to force
recognizing it such.

Sorry in advance if I'm missing something again but it doesn't seem normal that
no definition is emitted for the example above, does it?

Thanks again for your help!


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26355

[Bug regression/26355] defining static members of specialized template classes doesn't work

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2006-02-19 16:38 -------
You need to add an initializer to make

template <> int Data<V1>::Value;

a definition.  Otherwise it's just an explicit specialization decl.  Or
if you do not want to specialize for V1, use

template <V v> int Data<v>::Value;


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26355

[Bug regression/26355] defining static members of specialized template classes doesn't work

[vz-gcc at zeitlins dot org]

------- Comment #5 from vz-gcc at zeitlins dot org  2006-02-19 16:50 -------
In reply to comment 4: I do realize that adding an initializer fixes the
problem. But what to do if the static member is an object of a class which only
has a default ctor? E.g.

enum V { V1, V2, V3 };

struct Int { Int() : x(0) { } int x; };
template <V v> struct Data { static Int Value; };

template struct Data<V1>;
template <> Int Data<V1>::Value;

int main() { return Data<V1>::Value.x; }


How to fix this example to link?

Note that the above (with or without the explicit instantiation) is just fine
for Intel and Comeau compilers (and g++ < 4 of course) but not g++ 4.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26355