Re: [Mingw-users] "type attributes are honored only at type definition"

[Danny S. <dan...@cl...>]

Greg Chicares wrote:
> Building a cvs snapshot of wxWidgets produces many warnings
> with MinGW gcc-3.4.2 (their makefile uses '-W'):
>
>    C:/wx-cvs-20050216/wxWidgets/build/msw[0]$ \
>      grep "type attributes are honored only at type definition" \
>      buildlog |wc -l
>    33673
>
> Without that diagnostic and its attendant "In file included from"
> output, the output would be only about 500 lines long. Yet the
> warning is benign in this case. Is there a way to turn off this
> specific warning, while still using '-W'? If not, should there be?

Hello Greg

In FSF sources, the warning is  also emitted with just  -Wall.  Downgrading the
warning to a  -Wextra is a local patch.


>
> Admittedly coming to the party rather late, and reading
>    http://gcc.gnu.org/ml/gcc-patches/2003-12/msg01004.html
> now, I wonder whether one more case should be distinguished:
>
>    // Case 2a:
>    // Forward declaration with DLLEXPORT attribute
>    // followed by definition with DLLEXPORT attribute repeated.
>
>    struct __attribute__((__dllexport__)) Foo2a; // WARNING HERE
>    struct __attribute__((__dllexport__)) Bar2a; // WARNING HERE
>
>    struct __attribute__((__dllexport__)) Bar2a
>    {
>      int useFoo2a (const Foo2a&);
>    };
>
>    struct __attribute__((__dllexport__)) Foo2a
>    {
>      int foo2a_fun(void) const;
>    };
>
>    int Bar2a::useFoo2a(const Foo2a& _x)
>    {
>      return  _x.foo2a_fun();
>    }
>
>     void test2a()
>    {
>      Bar2a aBar2a;
>      Foo2a aFoo2a;
>      aBar2a.useFoo2a (aFoo2a);
>    }
>
> That differs from the original "Case2" in two ways:
>   - the attribute is on both the declaration and the definition
>   - dllexport instead of dllimport (it makes less sense to import
>       a class and then define a member function out of line)
>
> That's the way wxWidgets is written, and I imagine that some
> compiler they support wants it that way. MSDN says it's OK:
>
>
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclang/html/_pl
uslang_using_dllimport_and_dllexport_in_c.2b2b.asp
>> You can declare C++ classes with the dllimport or dllexport attribute.
>
> The case in favor of selectively turning this warning off
> so that "Case2a" would compile without warnings:
>   - it's an established usage, condoned by MSDN
>   - it's a best practice to strive for clean builds with as
>       many warnings enabled as possible
>   - omitting '-W' turns off many other warnings that are valuable
>   - detecting this case, where a forward declaration happens
>       not to conflict with a later definition, is probably hard;
>       instead of having the parser look ahead for the definition,
>       just let a user say "I know what I'm doing"
>
> The case against is that there are already many flags to enable
> and disable particular warnings, and they shouldn't be allowed
> to proliferate endlessly. This seems to be of concern only for
> the windows platform at present, although when this attribute
> is adapted for "visibility", a specific warning-suppression flag
> may become more generally handy. Another reason against this is
> that if you turn off this warning, but don't put the seemingly
> redundant attribute on the class definition, then things won't
> work as you expect (but I guess that's like all sharp tools).


>
> Probably there are other pros and cons that I'm missing.
> Is this the right place to discuss this?
>

Probaly the best place is in GCC's bugzilla.. The place where the warning is
emitted is not specific to dllexport/dllimport, but affects all type atributes
(i0e aligned, packed).  Only windows code seems to be in the habit of adding
attributes to forward declarations of user defined types.

I once tried to avoid these warnings by changing windows specific code for
handling dllimport/dllexport.  I can't remember  why I stopped try.ing  Now,
that code is in the generic backend.  I'll look again.

Perhaps  -fms-extensions should disable this warning for dllexport  attribute on
classes.


Danny
>
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