[boost] Re: Boost Digest, Vol 982, Issue 1

From: Dave Handley (dave.handley_at_[hidden])
Date: 2005-01-24 10:22:06

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?
> 

I think that "multitons" are very useful - but I'm not sure that a
Singleton class should necessarily be bloated out to support them. 
The way I have normally implemented this type of structure is to have
a class that is itself a singleton, but is responsible for generating
and returning one of a fixed (or variable) number of classes.  If you
implement the interface through a static function that delegates
through the singleton GetInst function, you get a very simple
interface which doesn't detract from a nice simple singleton interface
- eg:

class Multiton : public Singleton<Multiton>
{
public:
    class Child;
    static Child *GetInst( int number ) { return GetInst()->GetChild(
number ); }
private:
    Child *GetChild( int number ) { // TODO: check for and return the
correct instance of Child }
};

Is this pretty much what you were thinking?

Dave