[Bug c++/26205] New: pointer to member template parameter can't be null

[Bug c++/26205] New: pointer to member template parameter can't be null

[jeffp at doomsday dot org]

struct Simple
{
    int a;
};

template <typename T, int T::*U> struct Test
{
    void Foo(T *obj, int v)
    {
        if (U != 0)
            obj->*U = v;
    }
};

int main(int argc, const char **argv)
{
    Test<Simple, &Simple::a> t1;    // ok
    Test<Simple, 0> t2;             // error, but should not be (works in MSVC
7/8)

    Simple s;
    t1.Foo(&s, 5);
}


Note: it also won't let you specify a default parameter of 0 for the template
parameter:

template <typename T, int T::*U = 0> struct Test   // won't do this either

MSVC 7/8 handle the first case, but also can't handle the default parameter
(they crash with an internal compiler error).  MSVC 7/8 also won't handle
specialization of the Foo function for the 0 case, though the resultant code
generated is the same since the branch is optimized out.  I only mention this
to give you some other nuances to check for once the base bug is fixed.


-- 
           Summary: pointer to member template parameter can't be null
           Product: gcc
           Version: 3.4.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jeffp at doomsday dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26205

[Bug c++/26205] pointer to member template parameter can't be null

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-02-10 01:19 -------
Comeau gives the following error:


"ComeauTest.c", line 19: error: argument of type "int" is incompatible with
          template parameter of type "int Simple::*"
      Test<Simple, NULL> t2;             // error, but should not be (works in
                   ^


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26205

[Bug c++/26205] pointer to member template parameter can't be null

[bangerth at dealii dot org]

------- Comment #2 from bangerth at dealii dot org  2006-02-15 19:37 -------
icc gives essentially the same error as that one mentioned by Andrew for
Comeau (not surprisingly). As a workaround, you can always use an explicit
cast:
    Test<Simple, (int Simple::*)0> t2;


I believe the code is ok anyway, see 4.11/1.

W.


-- 

bangerth at dealii dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-02-15 19:37:18
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26205

[Bug c++/26205] pointer to member template parameter can't be null

[jeffp at doomsday dot org]

------- Comment #3 from jeffp at doomsday dot org  2006-02-15 19:55 -------
The casting work-around doesn't compile under GCC.  It requires that the
template parameter be a constant-expression (which is true), which results in
the following error:

error: a casts to a type other than an integral or enumeration type cannot
appear in a constant-expression

I have yet to be able to figure out a work-around.  MSVC allows the cast, but
works with or without it.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26205

[Bug c++/26205] pointer to member template parameter can't be null

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26205

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|gcc-bugs at gcc dot gnu.org        |
         Resolution|---                         |DUPLICATE

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Nothing to fix here, besides maybe the casting bit in c++98 mode which is
PR10541.

*** This bug has been marked as a duplicate of bug 10541 ***