Use in-place construction for value for unordered_node_map::emplace() if the key+value size is large?

[user202729]

As explained in https://github.com/boostorg/unordered/issues/241 , if the object is movable, the current implementation of emplace() first construct the key-value pair on the stack, then move it to the heap if the key did not exist.

This usually improve the performance, but it's not always the case.

sizeof(key)+sizeof(value) ↓ \ Key already exists →	often	rare
small	move() is better (saves a new/delete pair)	about the same
large	move() is negligibly better (cost of new/delete is negligible compared to actually constructing the pair)	worse! (move() is expensive)

Suggestion: if sizeof(key)+sizeof(value) is larger than a certain threshold (say, 60 bytes — often, at this rate the compiler's optimizer give up and the key/value actually touch the memory instead of staying in registers? Or let the user configure this?) then switch to always allocate on the heap.

What do you think?

[joaquintides]

Hi, beginning with Boost 1.85, Boost.Unordered has an optimization by which emplace(key, value) does not construct any intermediate object when the type of key is key_type (or a const reference to that etc.) Does this address your issue?