From: Joel de Guzman (joel_at_[hidden])
Date: 2006-08-23 11:20:36
Christian Henning wrote:
>> Creating a type vector depending on the template parameter, S, is
>> certainly OK. I'll have to ignore the "bad pseudo code" though ;)
>> You have to do it another way. Tell me precisely what you wish
>> to do and I may be able to provide an example. Actually, I am
>> compiling a small set of simple use-case examples. You wouldn't
>> mind if I use our first example, would you?
>>
>
> No, of course not. How about a real life example? I trying to create a
> generic histogram operator for GIL ( Generic Image Library - see
> http://opensource.adobe.com/gil/index.html ).
>
> To create a histogram I need to count all colors of an image and put
> them into a map. Since there are many different color definitions,
> like rgb-8bit, cymk32, gray-8bit, etc., I was intend to create the key
> type of the histogram map as a tuple. For example:
>
> struct rgb8
> {
> static const int num_channels = 3;
>
> unsigned char red;
> unsigned char green;
> unsigned char blue;
>
> };
>
> struct cymk32
> {
> static const int num_channels = 4;
>
> unsigned int cyan;
> unsigned int yellow;
> unsigned int magenta;
> unsigned int key; // black
> };
>
> As you can see a color types can be defined very differently.
> Sometimes a color can even be a mixture of integer and floating point
> values, see HSV or HSL color space.
>
> Now imagine a function that takes any kind of image that's made of of
> a certain color space. By the way this is not GIL-comlaint code. I'm
> simplifying for the sake of understanding.
>
> template <class IMAGE> void create_histogram( const IMAGE& image )
> {
> typedef IMAGE::color_t color_t;
>
> //somehow create type vector and subsequently a tuple containing
> //the color channel types
>
> typedef std::map< color_tuple, unsigned int > histogram_t;
>
> histogram_t histogram;
>
> for( IMAGE::iterator it = image.begin()
> ; it != image.end()
> ; ++it )
> {
> ++histogram[*it];
> }
> }
>
> Does that makes sense now?
Not sure. Why not simply make IMAGE::color_t the key? Then give
std::map a template key compare function templated by color_t;
the template can be specialized for each color definition.
Regards,
-- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net