On Fri, Apr 17, 2020 at 12:33 AM JeanHeyd Meneide via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Wed, Apr 15, 2020 at 12:48 PM Mario Charest <stayprivate_at_[hidden]>
> wrote:
>
>>
>> - Do we really need yet another non orthogonal thingy.
>> - Readability
>> - data[10][10]
>> - data.at(10).at(10)
>> - at(at(data,10),10) // I felt a bunch of my neurones explode on that
>> one. And I did not put in the namespaces. That would could mean including
>> <range> ? I'm guessing <range> might be as costly as <string>.
>>
>
> This is on track (but not in yet) to be data[10, 10] for C++23;
> removal of comma in operator[] was received favorable and I believe the
> Committee is in favor of deprecating, removing, and fixing all in one
> cycle, thank god. We can do the same for at() and make it variadic:
> at(data, 10, 10). A little more template boilerplate to do but it's
> do-able, I guess.
>

This is off-topic from std::span::at, but FYI— My understanding (and very
strong preference) is that multi-argument `operator[]` should NOT be used
as a hacky tool to save one character ([i][j] versus [i,j]). That's not its
purpose. The purpose is better language-level support for *multidimensional*
objects, e.g. myQuadtree[x,y], not terse support for *nested
one-dimensional objects*. If I have a std::vector<std::string> vs, I
access the nth character of the mth string with vs[m][n], just like in
C++98. If I have an array of arrays `as`, I access the jth element of the
ith array with as[i][j], just like in C++98.

The library writer would implement a multidimensional operator[] — or a
multidimensional at() — only for user-defined class types that are actually
multidimensional. For example, instead of representing a matrix as a
"one-dimensional array of one-dimensional rows," the author could now
choose to represent it opaquely as a 2D data structure.

    ArrayOfRows rows; rows[i][j] = rows.at(i).at(j);
    Matrix mat; mat[i,j] = mat.at(i,j);

Conclusion: If I have a single-dimensional std::vector<T> v, I do not want
std::at(v, i, j) to compile. Not even if T is std::string!

–Arthur