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_at_[hidden]
Date: 2008-11-13 05:34:05

Michael Marcin escribió:
> 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.
>   

I don't see how the use of allocators hint at the possibility to have 
polymorphic flyweights.
flyweight<T> treats T as values --they are stored in a factory typically 
implemented
with an associative container, and STL containers are not polymorphic. 
In order to have
polymorphism you'd have to use something like flyweight<T*>.

>> Is this more similar to what you have in mind
>
> That is certainly a lot closer.

I'm glad about it. This kind of techniques to bring external value 
factories into play
were discussed in conversations with Alberto Barbati:

http://lists.boost.org/Archives/boost/2008/09/142227.php

where it was also pointed out that a future extension of key-value 
flyweights
might accept additional "factory functors" to make this type of constructs
explicit and thus avoid such admittedly contrived tricks. I don't plan 
to have
this in the initial version of the library so as to be able to gain more 
real-life
feedback, I hope in the meantime the techniques just described serve your
purposes.

Regarding the terminology issue with what "factory" really means within B.F,
I think it'll help to clarify things to distinguish these two entities:

* A *flyweight* factory is the entity in charge of producing flyweight 
objects
on demand. This is what's called a factory in B.F.
* A *value* factory is the entity that produces values from keys and other
conextual information, like your TextureFactory. This concept is not
currently explicit in B.f, though it can be more or less acommodated as
shown previously and might receive official treatment in future releases
of the lib via "factory functors".

>  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.
>   

It is documented in the reference and mentioned at a comment in one of the
snippets of the tutorial:

  // flyweight<T> can be constructed in the same way as T objects can,
  // even with multiple argument constructors

I  might try to give it more prominence in the tutorial, though.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo