Re: [Boost-users] [mpl] a few questions about set and map

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2006-07-03 01:40:56

David Abrahams wrote:
> Yuval Ronen <ronen_yuval_at_[hidden]> writes:
> 
>> David Abrahams wrote:
>>> The default version of front always takes at least two
>>> instantiations (begin<...> and deref<...>).  If you can do front in
>>> one instantiation and you want the optimization, you'd better
>>> specialize it.
>> If this single instantiation you save is important (which sounds to me 
>> like too small a thing to mess with, but if you claim otherwise, I'll 
>> take your word for it), 
> 
> It's at least a 50% savings, and quite possibly more than 50%.

It's at most 50%, not at least. Remember that I compare specializing 
'front' to using the default 'front' + specializing 'begin'. Any 
optimization you put in 'front' you can also put in 'begin', so you save 
exactly one instantiation saved, which is at most 50%.

But again, if you say that it's important, then I'll accept it.

>> then wouldn't you want to specialize 'back' as well as 'front', if
>> possible? You might be able save an instantiation there as well...
> 
> You might want to do that, yes... "if possible" being key words here.
> 
>> Moreover, if this single instantiation saving is important, should it be 
>> specialized for *all* sequences, including those already present in mpl?
> 
> I don't have an opinion on it, and I don't see where you're going with
> that question.

Where I was going (or was trying to go - we'll see if I'll actually get 
there) is to show that there is a contradiction between two things. The 
first is the claim that a single instantiation is worth saving. The 
second is Forward Sequence requiring 'front' and not 'back'. If we're 
that much into saving, then it should require back for the same reason 
as it requires front. And also all sequences should take advantage of 
this, and actually specialize front and back to save this instantiation.

The other option is to declare that an extra single instantiation s not 
that bad, not require front/back, and leave all possible optimizations 
to begin/end. The default front/back will work fine.

I hope I could explain myself better this time.