operator[] can’t be marked noexcept as it has a narrow contract and by convention (“Lakos rule”) we do not mark functions with narrow contracts as noexcept…
Notice how operator[] is not marked as noexcept in array, vector, string or string_view!

From: Std-Proposals <std-proposals-bounces_at_[hidden]> On Behalf Of Vishal Oza via Std-Proposals
Sent: Thursday, November 28, 2019 6:46 PM
To: sotrdg sotrdg via Std-Proposals <std-proposals_at_[hidden]>
Cc: Vishal Oza <vickoza_at_[hidden]>
Subject: Re: [std-proposals] function at() for span<> that throws exception on out-of-range, like vector and array ::at()

I would first measure if there are performance issues adding at to the span's operator [] other then the performance issues with the at in general. If there are no performance issues with the operator[] then a justification to adding at to std::span is to adding noexcept the operator[] and keeping the exception version with the at operator. This might be another benefit to low latency C++ programmers to use std::span's operator[] as exceptions can be slow. The main issue I see is should the operator[] do any bound checking I am not sure that should be the case with C++ programmers who are looking for the highest performance.

On Thu, Nov 28, 2019 at 5:37 AM Roman Babinicz via Std-Proposals <std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>> wrote:

Could we add operators at() to span<> container?

  value_type & at(size_type n);
  const value_type & at(size_type n) const;

they would have same behaviour as in vector, array - that is throws an
exception if n is outside of size of the container.

1) can we have that?

2) if yes, then should I write a proposal paper or how else to help in this?
Can it make it to C++20?

Imo it is a notable defect that span can not be a drop in replacement
for code taking a (reference to) array/vector that is using at().

3)
By the way, in case of span that has specific size (not dynamic_extent)
we could perhaps have a static assert and the function might be constexpr.




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