`.retain()` unfortunately isn't what I'm talking about, since it's at least O(n) per call. It might be better if you can arrange to call it just once after a ba

`.retain()` unfortunately isn't what I'm talking about, since it's at least O(n) per call. It might be better if you can arrange to call it just once after a batch of operations, but that isn't always the case. "Delete as you go" usually† adds no space/time complexity to the operations you're already doing (though it does affect the constant factor). It does mean you're giving up on predictable destructor calls, but generally the "heavy" destructor was called by whoever made it expire (e.g. the death of the last remaining shared owner if that's what expiry is based on). † If many expirations happen at once, this can easily drop to merely amortized time. It's also possible for a container to offer, say, O(1) iteration via dedicated pointers but have a delete operation that's more complicated.