[ruby-core:117178] [Ruby master Feature#20300] Hash: set value and get pre-existing value in one call

Issue #20300 has been updated by Eregon (Benoit Daloze). That implementation is incorrect (i.e. less correct than current implemenation), and `Hash#exchange_value` is needed to be thread-safe for Set, see https://bugs.ruby-lang.org/issues/20301. ---------------------------------------- Feature #20300: Hash: set value and get pre-existing value in one call https://bugs.ruby-lang.org/issues/20300#change-107268 * Author: AMomchilov (Alexander Momchilov) * Status: Open ---------------------------------------- When using a Hash, sometimes you want to set a new value, **and** see what was already there. Today, you **have** to do this in two steps: ```ruby h = { k: "old value" } # 1. Do a look-up for `:k`. old_value = h[:k] # 2. Do another look-up for `:k`, even though we just did that! h[:k] = "new value" use(old_value) ``` This requires two separate `Hash` look-ups for `:k`. This is fine for symbols, but is expensive if computing `#hash` or `#eql?` is expensive for the key. It's impossible to work around this today from pure Ruby code. One example use case is `Set#add?`. See https://bugs.ruby-lang.org/issues/20301 for more details. I propose adding `Hash#exchange_value`, which has semantics are similar to this Ruby snippet: ```ruby class Hash # Exact method name TBD. def exchange_value(key, new_value) old_value = self[key] self[key] = new_value old_value end end ``` ... except it'll be implemented in C, with modifications to `tbl_update` that achieves this with a hash-lookup. I'm opening to alternative name suggestions. @nobu came up with `exchange_value`, which I think is great. Here's a PR with a PoC implementation: https://github.com/ruby/ruby/pull/10092 ```ruby h = { k: "old value" } # Does only a single hash look-up old_value = h.exchange_value(:k, "new value") use(old_value) ``` -- https://bugs.ruby-lang.org/