Hi Nathan, On Mon, Aug 16, 2010 at 12:09 PM, Nathan Crookston wrote:

Hi Christian,

On Mon, Aug 16, 2010 at 8:49 AM, Christian Henning wrote:

...

Hi there, I cannot understand why the code below gives me a static assertion. As far as I know I use a very common technique to to extract a type from templated class using specialization over this class.

I was surprised that this didn't work as well.  After some investigation, I found that adding a 'const' to the specialization worked for me:

It works for me now too. Any idea why that is? Thank you very much! I have spend way too much time for these couple of lines of code. ;-) Christian

Hi Nathan, On Mon, Aug 16, 2010 at 3:15 PM, Nathan Crookston wrote:

On Mon, Aug 16, 2010 at 12:32 PM, Christian Henning 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

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 {};

} // namespace gil } // namespace boost

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