Re: [boost] Re: enable_if review

From: Jaakko Jarvi (jajarvi_at_[hidden])
Date: 2003-10-10 17:24:03

Hi Gaby, 

First, the thing we were talking about in the previous email is
probably correct behavior according to the standard, so it is probably
not a g++ bug :) The Intel compiler has the same behavior.

We're currently figuring out again the behavior of different
compilers, and cannot repeat the same cases that motivated
lazy_enable_if. Compilers have been updated since we last tested.
We'll report later...

  Jaakko & Jeremiah

In our last exciting episode Gabriel Dos Reis wrote:
> Lines: 30

> Jaakko Jarvi <jajarvi_at_[hidden]> writes:

> | In our last exciting episode David Abrahams wrote:
> | > > 1. lazy_enable_if
> | > >
> | > > Shouldn't be this named apply_enable_if. It very similar to the appropriate
> | > > MPL concept and seems logical to be named the same.
> | 
> | > I had the same thought, FWIW.  I just couldn't make it sound right in
> | > the ear, though.  
> | 
> | > On second thought, I wonder if we need it at all.  Isn't 
> | 
> | >    mpl::apply_if<condition, nullary_metafunction, disabled>::type
> | 
> | > equivalent, where disabled is a struct with no nested ::type?
> | 
> | This wouldn't work. At the outermost level (apply_if), ::type always
> | exists, so this would lead to an error, not disabling the function, on
> | compilers (e.g. g++) where SFINAE is not applied when the invalid type
> | occurs nested in another instantiation.

> Jaakko --

>   I'm trying to catch up with this thread.  Please could you elaborate
> on the problem with g++ here? 

> Thanks,

> -- Gaby
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Cheers,
Jaakko Järvi