[Boost-users] Re: (newbie) design question (is there an MPL solution?)

From: David Abrahams (dave_at_[hidden])
Date: 2004-08-10 16:12:19

Timothy Perrigo <tperrigo_at_[hidden]> writes:

> Dave wrote:
>> Need more info.
>>
>> Does each id map to just one type, or might there be different types
>> using the same ID in different registries of type R?
>>
>> Is the id a runtime value, or is it known at compile-time?
>>
>
> Thanks for the reply...I'll try to give as much background information
> as I can without clouding the issue.  We're working on a constraint
> processing system.  Particular problems will be modeled with variables
> with values within certain domains and subject to certain constraints.
> These constraints, when propagated, will narrow the domains of the
> variables, but some form of searching will be required to bind all the
> variables to values and find a feasible solution.  The idea behind the
> registry I mentioned is for it to serve as a snapshot of the current
> state of the system (at a specific point in a search, for example).
> The "same" variables will exist in different registries, but may have
> different domains since their sets of possible values may have been
> reduced due to decisions made during the search.  

Actually all that background info clouds the issue.

> So to answer your first question, a particular id will always
> correspond to a variable of a given type, though its value at that
> point in time (i.e, in that registry) may differ from the same
> variable in a different registry.  

OK.

> Right now, the id is a runtime value; whenever a new constrained
> variable is created, it is given a new id.

But does it *need* to be a runtime value?  If so, something like
Gennadiy's solution is the only choice.  If not, you can do something
much more efficient with MPL.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com