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

[Dean M. B. <mik...@gm...>]

Hi Christian,

On 6/11/07, Christian Henning <chh...@gm...> wrote:
> 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.
>

Although it does make sense that extern will cause for
compilation/linking to require that the object be allocated somewhere,
if you check the code again for the transform template method, it
doesn't use the extern variables.

  template <class Algorithm, class Selector>
  inline impl::transform_impl<Algorithm, Selector>
  transform(Algorithm, Selector) {
    return impl::transform_impl<Algorithm, Selector>();
  };

That means, when transform(to_header_, source_) is called, nothing
should require linkage to to_header_ and source_ since transform(...)
doesn't use it (but the type information) anywhere.

> I don't see it as a problem when using parenthesis. boost::gil does
> something similar
>
> channel_t& c = get_color( pixel, red_t() );
>

True, but this doesn't scale well when we start implementing stuff
like operator&, operator|, and other operator overloads.

> 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.
>

Total template specialization doesn't make sense yet, especially if
there's no _easy_ way of invoking transform(...) with placeholders.
Enums are out of the question. ;-)

I guess what I'm really looking for is how boost::bind's _1, _2, _N works... :-D

-- 
Dean Michael C. Berris
http://cplusplus-soup.blogspot.com/
mikhailberis AT gmail DOT com
+63 928 7291459