Re: [Boost-users] [boost-users] How to put together a tuple type?

From: Christian Henning (chhenning_at_[hidden])
Date: 2006-08-22 23:38:20

>
> 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?

Christian