Re: [boost] [flyweight] key value with objects not constructible from a key

Subject: Re: [boost] [flyweight] key value with objects not constructible from a key
From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2008-11-12 17:11:10

Joaquin M Lopez Munoz wrote:
> Scott McMurray <me22.ca+boost <at> gmail.com> writes:
> 
>> On Wed, Nov 12, 2008 at 16:09, Joaquin M Lopez Munoz <joaquin <at> tid.es>
>> wrote:
>>> If an equivalent flyweight already exists then there is no
>>> problem, fw will be assigned the same texture object, this is
>>> how Boost.Flyweight currently works. Now, imagine that
>>> no equivalent texture exists, and yet you can't construct
>>> a texture object from "wood.texture" alone. How would you
>>> have then the above statement behave?
>>>
>> Perhaps there is no constructor for a texture that takes the
>> key, but instead there's some load_texture function that can
>> construct a texture from the key?
> 
> Then you can do something like this:
> 
>   struct texture
>   {
>     ...
>   };
> 
>   texture load_from_texture(const std::string)
>   {
>     ...
>   }
> 
>   struct loaded_texture:texture
>   {
>     loaded_texture(const std::string& str):
>       texture(load_from_texture(str))
>     {}
>   };
> 
>   typedef flyweight<key_value<std::string,loaded_texture> >
>   texture_flyweight;
> 
> Note that loaded_texture can be treated as an implementation
> artifact: texture_flyweight is convertible to texture, which
> is the type you're interested about.
> 

But loaded_texture's can't share any state or be initialized with state 
other than the key.

class TextureFactory
{
public:
    TextureFactory( Log& log ) : m_log( log ) {}

    GLuint LoadImage( const std::string& filename )
    {
       if( ExtensionMatches(filename,".png") )
       {
          m_log << "Loading png: " << filename;
          return m_pngLoader.Load( filename );
       }
       else
       {
          throw UnsupportedImageFormat( filename );
       }
    }

    // rest of factory implementation goes here

private:
   Log& m_log;
   PngLoader m_pngLoader; // heavy object to construct
};

-- 
Michael Marcin