Is it slower to access std::map elements with at() than operator[]?

Question

When I want get certain element from a std::map, as far as I know at() will do a bounds check. Does that mean it's slower than []?

No, it is not slower. The only difference is, it does not create new element but throws an exception if an element does not exits. — 3CEZVQ
– 3CEZVQ, Commented May 27, 2020 at 3:35
@ S.M. Thanks,no wonder I google so many times,and none of those reference say that ".at" is more slow — scirocc
– scirocc, Commented May 27, 2020 at 3:48
For a lot of containers with [] support it is slower because of the bounds check, but map has very different behaviour. — user4581301
– user4581301, Commented May 27, 2020 at 4:07

rugbyralph288 · Accepted Answer · 2024-01-06 02:09:26Z

Both std::map::at and std::map::operator[] check for the presence of a key. The former throws an exception when not found, the latter creates a default value.

Their complexity is mandatory to be log(n) (look-up). Performance by itself is not mandatory to be identical, but should be similar in practice.

So choose the one to use from their behavior (after the look-up). Similar alternatives might be map::find or in some cases map::insert/insert_or_assign/emplace.

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