Beginnings of std:forward_list

[Paolo Carlini]

Hi,

and first, thanks for working on this...
> 2. I'm not sure I got the allocator technology right.
>    This area is new to me.
>   
Actually, I think this is one of the most important issues, at the 
moment: unless I'm badly wrong (sorry in case), forward_list, like list 
and other containers, uses internally *two* different allocators, to 
allocate nodes and to allocate elements (to be precise, in current 
std::list they are _Node_alloc_type and _Tp_alloc_type). In our current, 
ABI-frozen, std::list, and I think in your prototype, we are storing 
only one of those allocators in the class, and constructing on the fly 
the other in case of need. That is in general suboptimal. Thus, when we 
are designing a new experimental container certainly we want to 
seriously deal with the issue: our specific approach we'll be useful for 
all the C++0x containers. The basic idea to implement it in a space 
efficient way is due to Howard Hinnant: he basically designed 
compressed_pair for that, to store efficiently, exploiting the empty 
base optimization, a *pair* of allocators. Therefore, I would suggest 
you starting from that, looking for that class (I think something is 
also in Boost, but then make sure to not plagiarize the actual code!), 
learn from it, play with it for forward_list (or a "mini" version of it, 
to begin)....

Then, there are of course other design decisions, which I consider less 
critical, at the moment...

Thanks,
Paolo.