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: Joaquin M Lopez Munoz (joaquin_at_[hidden])
Date: 2008-11-12 16:59:30

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.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo