It can be quite tricky to get right once you get into ownership issues. If you delete an item, undo'ing that should reinstate it. So that undo step becomes the owner of the data of the item, otherwise it couldn't reinstate it. This can already become a bit complicated if you're dealing with memory management of this item. Do resources that the item uses get deleted immediately, or do you want to keep them around in your undo state too? If the resource requires a lot of processing then that may be the only option.

This then grows more complex if items have parent-child relationships and you also need to start storing all the information required to completely reinstate all the children, possibly also in the same order they were originally created in.

You know, tricky.