Re: conflicting declaration in template static member

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 26 Mar 2011 07:01:43 -0700 (PDT)

Message-ID:

<c81996c8-b01f-4737-8588-523a2593cd59@x3g2000yqj.googlegroups.com>

On Mar 24, 2:03 pm, Michael Doubez <michael.dou...@free.fr> wrote:

On 24 mar, 11:42, puppi <fabricio.pu...@gmail.com> wrote:

On Mar 24, 6:18 am, Michael Doubez <michael.dou...@free.fr> wrote:

I have the following minimal code:

template< class T >
struct Foo {
    static int const N = T::size;
    static int values[N];};

template< class T > int const Foo<T>::N;
template< class T > int Foo<T>::values[Foo<T>::N] = {};

struct Bar {
    enum { size = 42 } ;
};

int main() {
    Foo<Bar> f;
    return f.N;
}

Which yield under gcc (4.1.3 and 4.3.3):
error: conflicting declaration 'int Foo<T>::values [Foo::N]
error: 'Foo<T>::values' has a previous declaration as 'int
Foo<T>::values [Foo<T>::N]'
error: declaration of 'int Foo<T>::values [Foo<T>::N]' outside of
class is not definition

The compiler seems confused by the size because it accepts the code
   - if I replace the definition of N by:
enum { N = T::size };
  - if I don't use N but T::size

This code pass under Comeau online (not in codepad).

Is this a compiler bug or is it expected behavior ?

Well, under gcc 4.4.5 it compiles fine...

Ok. I suppose this is a bug and it has since been fixed.

Have you tried to remove the declaration/initialization pair:
template< class T > int const Foo<T>::N;
template< class T > int Foo<T>::values[Foo<T>::N] = {};
Since they really aren't necessary, trying to remove them would be my
first try.

They are necessary.
Otherwise you get UB and if used, you are likely to get link errors.

This may have been changed in more recent versions of the draft,
but at least in the current standard, the problem is that
template< class T > int const Foo<T>::N;
is NOT a definition, but only a declaration. When defining data
in this manner, the declaration is only a definition if it
contains an initializer. You should probably try:
template< class T > int const Foo<T>::N = T::size;
(and drop the = T::size in the class definition).

--
James Kanze