when unit gets killed does it automatically removed from memory ?

read title(Not Hero Unit)

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if it's removed,
and I have unit global variable pointed to that killed unit. what happen if I used that variable without set it to other unit.

As far as I know, the unit still exists in the memory. After all, it can still be resurrected by a Paladin. However, I do know some information is no longer stored about the unit when it dies, like buffs it had. But it's still there, why would there otherwise be a boolean condition "unit is dead", if you couldn't reference a dead unit anyway.

However, there is probably a very limited ammount of things you can do with a dead unit. You definitely can't give it any orders. The rest,... well, it depends, I suppose. If you are unsure about anything, you can always make a test map to try it out.

Even after the corpse of killed unit is gone, is it still in memory ?

once the corpse is gone, afaik any reference to the unit is destroyed