In article <39e07d61.15083146@news.nikoma.de>,
Martin Aupperle <MikeAlpha@NoSpam_csi.com> wrote:
>I have a POD like
>
> struct S { int x; int y; };
>
>I can define a const Objekt like
>
> const S s;

You can't.

>and not explicitely initialize it.

You still can't. :)

>Formaly it is initialized by the
>Default-Standard-Constructor which is automatically generated, but is
>empty. The net effect is that we have a completely uninitialized
>constant object!

Not possible.

>The language does not allow uninitialized constants of fundamental or
>array types. Why does it allow that for structure types?

It doesn't.  The best you can get is:

   extern const S s;

- Greg
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Hello,
I have a POD like

 struct S { int x; int y; };

I can define a const Objekt like

 const S s;

and not explicitely initialize it. Formaly it is initialized by the
Default-Standard-Constructor which is automatically generated, but is
empty. The net effect is that we have a completely uninitialized
constant object!

The language does not allow uninitialized constants of fundamental or
array types. Why does it allow that for structure types? I try to
understand the reasoning behind. D&E does not say anything here.


Another question is why we can have partially initialized constants of
PODs but not of arrays:

 const S s2 = { 1 }; // OK - s2.y will get 0

 const int ai[5] = { 1 }; // NOT OK - ai[1]..ai[4] will NOT GET
0


Thanks - Martin.






------------------------------------------------
Martin Aupperle
MikeAlpha@NoSpam_csi.com
(remove NoSpam_)
------------------------------------------------

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Martin Aupperle wrote:
>
> Hello,
> I have a POD like
>
>         struct S { int x; int y; };
>
> I can define a const Objekt like
>
>         const S s;
>
> and not explicitely initialize it. Formaly it is initialized by the
> Default-Standard-Constructor which is automatically generated, but is
> empty. The net effect is that we have a completely uninitialized
> constant object!

No, we have a constant object that has been innitialized by the default
constructor for its type. If you don't like what the compiler-generated
default constructor does you can write your own.

--
Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)
Contributing Editor, C/C++ Users Journal (http://www.cuj.com)

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In article <39e07d61.15083146@news.nikoma.de>,
  MikeAlpha@NoSpam_csi.com (Martin Aupperle) wrote:
> Hello,
> I have a POD like
>
>  struct S { int x; int y; };
>
> I can define a const Objekt like
>
>  const S s;
>
> and not explicitely initialize it. Formaly it is initialized by the
> Default-Standard-Constructor which is automatically generated, but is
> empty. The net effect is that we have a completely uninitialized
> constant object!
>
> The language does not allow uninitialized constants of fundamental or
> array types. Why does it allow that for structure types? I try to
> understand the reasoning behind. D&E does not say anything here.

No, the Standard does not allow uninitialized structure types
of this kind.  You didn't say what scope the declaration of "s"
is in.  It makes a difference.

If "s" is in namespace scope, the members are zero-initialized
(8.5p6).  If "s" has automatic storage duration, the declaration
is ill-formed (8.5p9 says that a const object must have either a
user-declared default constructor or have an initializer).

You can still get uninitialized members, of course, by writing a
constructor that doesn't initialize some or all of the class's
members, but that's not the situation you described.

> Another question is why we can have partially initialized constants of
> PODs but not of arrays:
>
>  const S s2 = { 1 }; // OK - s2.y will get 0
>
>  const int ai[5] = { 1 }; // NOT OK - ai[1]..ai[4] will NOT GET
> 0

What leads you to think this is the case?  8.5.1 treats array
and class aggregates the same, as far as I can tell.

--
William M. Miller, wmm@fastdial.net
Vignette Corporation (www.vignette.com)


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