Re: [boost] [MPL] Potential use for multiple sequence types

From: Emily Winch (emily_at_[hidden])
Date: 2002-08-01 15:07:02

Douglas Gregor wrote:
> On Thursday 01 August 2002 03:23 pm, Emily Winch wrote:
>>Douglas Gregor wrote:
> 
> [snip me talking about big heterogenous containers]
> 
>>Hmmm. I think the application for these containers in Mike's paper might
>>well require enough elements to make the memory used by the vast
>>recursion depth of a cons-style list a serious consideration. But there
>>still isn't much point debating it without concrete data about real life
>>compiler problems, timing, and memory usage.
> 
> 
> Maybe Mike has some data on this for us. They might have tried cons-style 
> lists.

I don't think they did, but I could be wrong.

>>If Boost.Tuple was PP'ified, nobody would need to do that.
> 
> <snickers as he considers the EDG preprocessor expanding 10,000 template 
> parameters>

yeah, yeah  :)
That's a problem with the MPL also: if anyone really wants to make a MPL 
list (or vector) of 10,000 elements, EDG-based compilers are not going 
to be a lot of help.

> I'll try to explain my interpretation. You said:
>   "In any case, it's not the case that a requirement for a certain
>     implementation of a heterogenous value container implies a requirement
>     or a type container with the same implementation."
> 
> To paraphrase: the need for a specific implementation of a heterogeneous value 
> container does not imply the need for a type container with the same 
> implementation. So, say we need the type vector that Burton et al. used for a 
> heterogeneous value container; why don't we need to access all of the types 
> in the container?

We do need to do that. And we can, without requiring any kind of type 
list to do so. I think we may be hooked up by my use of type list/value 
list: by type list I meant a list that _doesn't_ carry values. I suppose 
that's misleading, since the value list is really intrinsically a type 
list also.

So: Mike B. doesn't need a vector-based typelist (without values) in 
order to access types in his vector-based value list. Thus, just because 
someone needs a vector-based value list, doesn't imply the MPL should 
contain a vector-based non-value typelist.

I think I need a better set of terminology :)

> Good answer! 

Why thankyou!

Emily