Associative array

[Witold]

On Friday, 13 October 2023 at 11:32:29 UTC, Imperatorn wrote:
> Hello there.
>
> I'm wondering if anyone would be opposed to us/me making 
> associative array having resize/reserve public?
>
> Currently there exists private members in druntime etc for this 
> but they are not exposed.
>
> Basically I would like to be able to reserve n buckets before 
> populating.
>
> For reference, see std::unordered_map, which is kvp, which has 
> rehashing etc:
>
> ```cpp
>   template<typename _Key, typename _Tp,
> 	   typename _Hash = hash<_Key>,
> 	   typename _Pred = equal_to<_Key>,
> 	   typename _Alloc = allocator<std::pair<const _Key, _Tp>>>
>     class unordered_map
>     {
>       typedef __umap_hashtable<_Key, _Tp, _Hash, _Pred, _Alloc> 
>  _Hashtable;
>       _Hashtable _M_h;
> ```
>
> https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/unordered_map.h
>
> https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/hashtable.h


In my opinion C++ / STL containers in `std` are pretty meh, and 
one probably should not be guided by them. unordered_map design 
and requirements in particular are bad.

I like `reserve`, but I would say: show real world example, and 
benchmark (not microbenchmark, but actual app), where `reserve` 
helped (i.e. it is also easy to use reserve and actually make 
code / memory / cpu usage worse). Then maybe.

For example, `reserve` on tree based associative array, does not 
really make sense.

If you need `reserve`, maybe you need to not relay on any 
specific AA implementation provided by D runtime by default, but 
rather use library or own solution, that you control over time. 
Otherwise, change in a compiler or runtime, could obliterate any 
performance improvements you had before. (i.e. what if it was 
critical to performance, and at next dmd version, `reserve` 
started just being ignored, then you are in a big trouble).