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From: David Abrahams (abrahams_at_[hidden])
Date: 2001-06-06 21:11:38

----- Original Message -----
From: "Aleksey Gurtovoy" <alexy_at_[hidden]>
> David Abrahams wrote:
> > I have moved the boost build system to the "build-development" branch.
> > I have also integrated Daryle's latest dlw_oprs submission into the
> > operators library.
> I should have catched this early, but.. due to default 'void' template
> parameters 'output_iterator_helper' fails to compile on MSVC:
> template <class T,
> class V = void,
> class D = void,
> class P = void,
> class R = void>
> struct output_iterator_helper
> : incrementable<T
> , boost::iterator<std::output_iterator_tag, V, D, P, R
> > > {};
> MSVC problem with 'void' as default type template parameter is an old one;
> one possible workaround is to use 'void*' instead of 'void' if the latter
> just a placeholder for an unsignificant parameter.

I think a better choice might be boost::type<void>. We could then use a
little metaprogram to detect it and replace the type with void where it
really counts.

> It's not exactly the case
> here, as the standard _requires_ 'difference_type' and 'value_type' of
> output iterators to have 'void' type (24.3.1 [lib.iterator.traits]);
> I haven't yet encountered a case when having output iterator value_type ==
> void* instead of plain void would make any difference. Actually, on the
> second thought, if those _are_ the requirements, probably
> 'output_iterator_helper' definition should look like this anyway:
> template <class T>
> struct output_iterator_helper
> : incrementable<T
> , boost::iterator<std::output_iterator_tag, void, void, void, void
> > > {};

Why should reference and pointer be void by default? Do we mean void& and
void* (or something)?

> I am not sure that "void value_type" requirement is a reasonable one,
> though. After reading
> per, it seems to me that it may be a defect as well. Thoughts?

I tend to agree. One could imagine generic output iterators, to which /any/
type could be written, but surely that's not a reasonable requirement to
As a matter of fact, I think the output iterator requirements table is a
complete mess. I guess I'm submitting an issue right away.


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