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

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