alt Hacker NewsIf you're an old timer, you expect [] to index into an array, and you definitely do not expect it to do a lookup in a map/dict. Older languages just didn't include a dict-like structure in the language (technically, even C++ didn't).
Do you not see that for them, overloading [] for dictionary lookups is "something completely different from what you intuitively expect"?
More strikingly, C++ doesn't distinguish what Rust would call IndexMut and just Index, the use of the [idx] operator in a context where we'll mutate things and one where we don't want that.
Rust's HashMap implements Index because answer = map[name] is a perfectly reasonable thing to do, and if there is no key matching name then we panic, makes sense but it does not provide IndexMut so that you could write map[name] = answer because the edge cases are non-obvious so better to make you write what you meant.
C++ hash tables implement operator[] but the result is mutable, in order that map[name] = 123.0 can work which in Rust would be IndexMut and isn't provided. Because this is true, the index operation always succeeds, if you ask for map["not-present"] it creates the hash table entry for "not-present" and tries to store a default value ready to update it if you later assign to the reference.