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From: Jonathan Wakely (cow_at_[hidden])
Date: 2005-03-23 13:05:12

On Wed, Mar 23, 2005 at 06:15:14PM +0100, Thorsten Ottosen wrote:

> "Jonathan Wakely" <cow_at_[hidden]> wrote in message
> | On Wed, Mar 23, 2005 at 04:06:09PM +0000, Joaquin M Lopez Munoz wrote:
> |
> | > "I can't think of any way that this extension could
> | > break a conforming program, considering that users
> | > are not permitted to forward-declare standard
> | > library components"
> | >
> | > Is this really so? Why cannot I forward declare a
> | > stdlib component?
> |
> | Opening namespace std for anything except specialisations of standard
> | library templates (with user-defined types as parameters) is illegal.
> yes, but why?

Because the Standard says so! :-)

I think that's the rule that the DR refers to when it says "users are
not permitted ...", whether it's possible/advisable in practice is a
different matter.

Boost already opens namespace std for various reasons (e.g declaring C
library names in std::) so forward decls would be no worse than the
current situation - technically illegal, but harmless and even necessary
in some cases.

I can't see any real compiler ever rejecting the code, only a really
strict validation test suite of some kind.


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