I'm not sure why anybody would at the same time be implementing SoA AND resizing 20 arrays for a single delete, those things seem to be on either ends of the "I care about performance" spectrum.
The point is that a simple SoA implementation requires this - each field in the monster struct is an item in 20 different arrays. So, removing one monster means removing that item from those 20 arrays.
Now, as others have suggested, you can have a more complex implementation, where instead of removing the monster's fields from those arrays, you just mark them as "dead" or whatever and then skip them when consuming the relevant arrays, with some relatively small extra bookkeeping overhead. Of course, this comes with its own drawbacks, especially if the number of monsters is very dynamic and you are memory constrained.
The point is not to say that SoA is never good for performance, it obviously and certainly is, probably even in most cases. It's just not always best for performance, this was all.
> So, removing one monster means removing that item from those 20 arrays.
Removing from an array is not the same as resizing, which is what I commented on. Resizing is a very deliberate, bad, choice.
If you need to support deletes, you can do this without resizing an array. Either by tracking object lifetimes and inserting tombstones, or by swapping to fill in deleted objects. Both of them retain good performance characteristics. Both of them are easy.
This is not "simple vs complex" this is "I misunderstand vs understand SoA"
Now, as others have suggested, you can have a more complex implementation, where instead of removing the monster's fields from those arrays, you just mark them as "dead" or whatever and then skip them when consuming the relevant arrays, with some relatively small extra bookkeeping overhead. Of course, this comes with its own drawbacks, especially if the number of monsters is very dynamic and you are memory constrained.
The point is not to say that SoA is never good for performance, it obviously and certainly is, probably even in most cases. It's just not always best for performance, this was all.