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 18:13:28

Joaquin M Lopez Munoz wrote:
>
> 
> The following might be one possible approach to your envisioned
> scenario using the technique proposed by Scott. There is one
> significant difference with the way TextureFactory is defined,
> namely that construct is required to return a Texture (your
> version does the construction using placement new).

I only chose the placement new because flyweight already has an 
allocator template parameter. If allocation became part of the factory 
then I think more cool option occur like polymorphic flyweights where a 
key constructs a derived type via the factory but the flyweight and 
calling code only knows about the base type.

> Complete sample follows:
> 
> 
> typedef flyweight<key_value<KeyAndFactory,LoadedTexture> >
> TextureFlyweight;
> 
> int main()
> {
>   Log log1,log2;
>   TextureFactory factory1(log1),factory2(log2);
> 
>   TextureFlyweight fw1("hello",factory1);
>   TextureFlyweight fw2("hello",factory2);
>   TextureFlyweight fw3("bye",factory1);
>   const Texture& t1=fw1;
> 
>   assert(fw1==fw2);
>   assert(fw1!=fw3);
> }
> 
> 
> Is this more similar to what you have in mind?
> 

That is certainly a lot closer. I didn't know that flyweights could have 
multiple construction parameters. I didn't see any examples of that in 
the docs, it seems like a pretty cool feature you might show off somewhere.

-- 
Michael Marcin