[Bug c++/51426] New: explicit instantiation declaration without definition

[Bug c++/51426] New: explicit instantiation declaration without definition

[mkp76 at wp dot pl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51426

             Bug #: 51426
           Summary: explicit instantiation declaration without definition
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mkp76@wp.pl


The below code can be compiled (with -std=C++0x option).

--------------------------
template <typename T>
struct A { };

extern template
struct A<int>;

int main()
{
    A<int> a;
}
--------------------------

According to C++11 standard (14.7 paragraph 5) it shouldn't
"an explicit instantiation definition shall appear at most once in a program"

I was trying to test 'extern templates' based on Bjarne Stroustrup's page:
http://www2.research.att.com/~bs/C++0xFAQ.html#extern-templates

Regards,
Marcin

[Bug c++/51426] explicit instantiation declaration without definition

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51426

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-05 18:54:11 UTC ---
I don't see what that paragraph has to do with it - the explicit instantiation
definition appears zero times, zero is less than one.

The relevant text is 14.7.2 p11
"An entity that is the subject of an explicit instantiation declaration and
that is also used in a way that would otherwise cause an implicit instantiation
(14.7.1) in the translation unit shall be the subject of an explicit
instantiation definition somewhere in the program; otherwise the program is
ill-formed, no diagnostic required."

The program is ill-formed, no diagnostic required.

If you change it so there's something that actually needs a definition you'll
get a linker error:

template <typename T>
struct A { A() { } };

extern template
struct A<int>;

int main()
{
    A<int> a;
}

I don't see a bug here

[Bug c++/51426] explicit instantiation declaration without definition

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51426

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

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

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-12-05 23:46:34 UTC ---
Closing.

[Bug c++/51426] explicit instantiation declaration without definition

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51426

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-06 00:32:48 UTC ---
(In reply to comment #0)
> The below code can be compiled (with -std=C++0x option).

By the way, the program compiles without -std=c++0x, GCC has supported extern
templates since at least GCC 2.95

[Bug c++/51426] explicit instantiation declaration without definition

[mkp76 at wp dot pl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51426

--- Comment #4 from Marcin Pytel <mkp76 at wp dot pl> 2011-12-06 17:49:06 UTC ---
I did some more testing. I was using eclipse and it adds some compilation
flags. The cause is optimization. When you add empty ctor there's the linker
error. But when you compile with -O2 there's no error. When I added something
to ctor (cout) I got linker error even with O2.

Marcin