C++ Stories

The main difference between those two is that [] is unchecked (and can generate undefined behaviour if you try to access an element which is not there), while .at() may throw std::out_of_range (so it's a well defined behaviour).

In C++26, the Standard introduces the notion of a hardened implementation . Whether a standard-library implementation is hardened, and how that mode is enabled, is implementation-defined.

In other words, if you switch this "hardened" mode you'll get some well specified error/violation rather than just an undefined behaviour.

For .at() you may get an exception… so why do we need a new alternative? That's fair question. In short at() and [] has different interfaces and performance/error-handling approaches. What's more important you cannot turn exceptions off easily (you can, and std::terminate will be called, but that's not very flexible).

So Hardening does not change operator[] into at() - it detects a programming error and terminates instead of allowing memory-unsafe undefined behaviour.

"A contract violation occurs" - this is the key thing here. The "hardening" feature is expressed in terms of Contracts that also were accepted into C++26.