刚才才发现
查了一下，原因是：
I actually proposed
I wasn’t there for the meeting, but from what I understand the feeling was that contracts (which at that time seemed highly likely for ‘20) were a better way to go for bounds checking, and that we should not be throwing exceptions for programmer errors like out-of-bounds access.
https://www.reddit.com/r/cpp/comments/djqdu2/why_is_stdspan_is_missing_at/
std::span 没有 at 成员函数。查了一下，原因是：
I actually proposed
at() for std::span as part of P1024r0. It was soundly defeated in LEWG, 0|0|4|4|5.I wasn’t there for the meeting, but from what I understand the feeling was that contracts (which at that time seemed highly likely for ‘20) were a better way to go for bounds checking, and that we should not be throwing exceptions for programmer errors like out-of-bounds access.
https://www.reddit.com/r/cpp/comments/djqdu2/why_is_stdspan_is_missing_at/
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