Re: [Boost-users] Problem with simple type extraction

Subject: Re: [Boost-users] Problem with simple type extraction
From: Christian Henning (chhenning_at_[hidden])
Date: 2010-08-16 16:33:01

Hi Nathan,

On Mon, Aug 16, 2010 at 3:15 PM, Nathan Crookston
<nathan.crookston_at_[hidden]> wrote:
> On Mon, Aug 16, 2010 at 12:32 PM, Christian Henning <chhenning_at_[hidden]> wrote:
>> It works for me now too. Any idea why that is?
>
> Well, apparently the reference type is declared as const somewhere in
> the GIL code (not the referenced pixel, of course).  I tried to find
> exactly where the const was added in the code, but I had some trouble
> following all the many metafunction instantiations, etc -- there are
> quite a few for the declaration given.

I did not think of const since I thought there is
gray1_image_t::view_t::const_reference. Turns out there isn't a
const_reference. And maybe I got fooled by the MSVC debugger which
told me that there is no const in actual type of
gray1_image_t::view_t::reference.

>
> I did modify the metafunction slightly to give correct results
> regardless of const (and volatile) qualifiers.  Perhaps it would be of
> use to you:
>
> //In addition to previous includes
> #include <boost/type_traits/remove_cv.hpp>
>
> namespace boost {
> namespace gil {
>
> template< typename T >
> struct bit_field_type_impl
> {
>  typedef T type;
> };
>
> template< typename BitField, typename ChannelBitSizes,
>          typename Layout, bool IsMutable >
> struct bit_field_type_impl< bit_aligned_pixel_reference< BitField
>                                                       , ChannelBitSizes
>                                                       , Layout
>                                                       , IsMutable > >
> {
>  typedef BitField type;
> };
>
> template< typename T >
> struct bit_field_type
>  : bit_field_type_impl<typename boost::remove_cv<T>::type > {};
>
>
> } // namespace gil
> } // namespace boost

Thanks alot for your code suggestions I'll incorporate them into my code.

Regards,
Christian