[boost] Re: Multiton

From: Pavel Vozenilek (pavel_vozenilek_at_[hidden])
Date: 2005-01-24 06:04:32

"Jason Hise" wrote:

> Pavel and I have been discussing a sibling class to the singleton, namely 
> a multiton.  Such a class would manage a unique object instance for each 
> key provided to GetInst.  It should be able to use many of the same 
> policies that the singleton uses.
>
> Pavel thinks that it could be useful to allow GetInst to take any number 
> of parameters.  Would this actually be useful, and if so, how could such a 
> design be achieved?
>

The idea is to have library which allows to create
"parametrized singletons", like:

City& c1 = City::get_inst("Belgium", "Ghent");
...
City& c2 = City::get_inst("Belgium", "Ghent");
assert(&c1 == &c2);

The name "Multiton" was already used with
similar library in Ruby.

The problem is what syntax to use to define Multiton, like:

template
<
   typename T,
   typename Parameters = TYPELIST2(std::string, std::string),
   typename Lifetime = ...
>
class Multiton
{
   ....
};

and how to 'generate' flat structure from the typelist.
(The structure would be then used as key in lookup map.)

/Pavel