Does the weak_ptr need to be removed from the map "instantly" when it expires?
Could you just clean up the expired weak_ptrs at some later point? For
example, when an iterator is incremented it might remove all the weak_ptrs it
encounters while it is trying to find the next live weak_ptr in the map.
In my current usage scenario and implementation, I rely on values being removed 'instantly'.
The thing is that I never iterate these maps, I only query values associated to keys ; so in this case, a cleanup could be triggered, but it would increase the 'query' time cost - I tend to see this weak_key_map as a lightweight construction if possible...