Re: [cpp-netlib-devel] MSVC Problem with 'extern' placeholders

[Christian H. <chh...@gm...>]

Hi Dean, I think the MS compiler is right. Defining a variable extern
means it has external linkage. We shouldn't try to be "too smart" in
fooling the compiler. This code can break with every new compiler
version.

I don't see it as a problem when using parenthesis. boost::gil does
something similar

channel_t& c = get_color( pixel, red_t() );

But if you don't like that then there are other ways, like using an
enum ( I know it's old school ) or use total template specialization.
I'm sure there are other ways.

Christian

On 6/11/07, Dean Michael Berris <mik...@gm...> wrote:
> Hi Christian,
>
> On 6/11/07, Christian Henning <chh...@gm...> wrote:
> >
> > And there were no problems. Does that makes sense to you?
> >
>
> Two issues here:
>
> 1. The unit test is a specification on how the interface is meant to
> be. Although your solution would work, the readability of additional
> parentheses aren't very appealing to me. The unit test is an example
> in itself of how the interface *should* look like and I'd like
> to_upper_ (and other future transformations to be implemented) to not
> have the extra parentheses.
>
>   msg << transform(to_upper_, source_) ;
>
> Reads better IMO than
>
>   msg << transform(to_upper_(), source_()) ;
>
> The same way
>
>   boost::bind(&function, _1) ;
>
> Reads better than
>
>   boost::bind(&function, _1()) ;
>
> 2. The premium is put on readability and extensibility. The transform
> template function is meant to extend to more than just a pair of
> parameters. For flexibility's sake, see the example below:
>
>   msg << transform(to_upper_, source_ & destination_)
>     << transform(headers_, to_upper_ & trim_)
>   ;
>
> With template metaprogramming, operator& can be overloaded to produce
> a composite selector and composite transformations. This gives
> 'transform' a flexible enough interface which allows:
>
>   msg << transform(to_upper_, source_) << transform(source_, to_upper_) ;
>
> Of course, with the help of template metaprogramming. In this regard,
> having extra parentheses doesn't help with readability.
>
> NOTE: This is the direction of the transformation interface (and other
> interfaces to the message type in general), to leverage on
> compile-time meta-programming to produce readable and flexible
> interfaces.
>
> The above are the two rationale's for why I opted for 'extern
> to_upper_placeholder to_upper_' instead of making it a typedef in the
> first place. It just so happened that my first iteration
> implementation hit a snag (which I sure hope would be temporary).
>
> So the real question now would be (assuming it works with GCC) how we
> can allow the above parentheses-less while keeping MSVC and other
> compilers happy?
>
> --
> Dean Michael C. Berris
> http://cplusplus-soup.blogspot.com/
> mikhailberis AT gmail DOT com
> +63 928 7291459
>
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