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Re: [std-proposals] New vector range constructor to improve performance with forward ranges

From: Jason McKesson <jmckesson_at_[hidden]>
Date: Fri, 11 Nov 2022 18:51:03 -0500
On Fri, Nov 11, 2022 at 5:12 PM Lénárd Szolnoki via Std-Proposals
<std-proposals_at_[hidden]> wrote:
> Hi,
> I suppose this applies to unsized ranges. But range does not have to be random access to have std::ranges::size, which is O(1) and meant to be cheap.
> For sized ranges it absolutely makes sense to do the preallocation, and it doesn't require traversing the range twice.
> I would say that for unsized ranges it makes more sense to not do this, and do the usual multiple reallocations.

I disagree. For most ranges that don't have a size, traversal is
inexpensive next to the cost of memory allocations (and the requisite
copying/moving of data). Even complex views that need to invoke some
functor (like `filter_view` and the like) will generally be cheaper
than doing potentially dozens of memory allocations. Indeed, it's
pretty rare to find a forward range for whom iteration is so slow that
multiple memory allocation is preferred.

Received on 2022-11-11 23:52:54