Re: Library proposal - indexed_array

From: Julien Blanc (julien.blanc_at_[hidden])
Date: 2022-06-24 08:08:25

Le 2022-06-23 22:41, Hadriel Kaplan a écrit :

Hi,

Thanks for this feedback.

> Ours is purely for enum-based indexing, and only for scoped enums that
> start at value 0 and increase sequentially – i.e. contiguous, no
> “holes”. We happen to be able to verify that property of enums at
> compile-time, and can static_assert that it’s true, so ours just
> ends up doing a static_cast under-the-hood. (I can’t quite tell what
> indexed_array does in the end, since it also deals with non-contiguous
> index values.)

The current strategy is the following:
* detect if the set of indexing values is contiguous (ie, no holes). 
Duplicate values are removed (this is needed to support enums with 
aliases such as first or last).
   * If that's the case, use a static_cast and an offset for index 
computation. If the offset happens to be zero, this is fully optimized 
by the compiler.
   * if that's not the case, the current implementation use an mp_foreach 
loop (iterate over all indexes to recompute the index in the underlying 
array).

This detection is performed at compile time, and has been firstly 
designed to support described-enums, which we get as an integer 
list-like. Supporting them without run-time penalty was a design goal.

There is indeed a performance penalty with using non-contiguous enums 
with the default indexer. Since several of my use cases for 
non-contiguous enums involves mostly either two discontinuous ranges, or 
a single range + a few single values, i may improve these specific cases 
in the future (this is an implementation detail that will not change the 
client-side usage of the library).

Regards,

Julien