Question

If a class instance uses 20 bytes, and its subclass uses 24 bytes since it as more members, how is it possible to store an instance of the subclass in a variable of the parent class?

Like:

Subclass s;
ParentClass p ;
p = s;

Answer 1

You don’t “store” instances “in” pointers. Pointers merely point at the start of the instance in memory.

The pointer object itself contains a memory address, and the space that this takes up is always the same no matter what — or how much — data may be found at that address.

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Edit (since the question has changed almost completely)

In the example you’ve added to your question, the code doesn’t do what you think it does. You are not storing a Subclass in a ParentClass; instead, you are slicing off the derived bits of s, and copying only its base bits into p.

You asked elsewhere how you can fit a Derived in an array of Base; you can’t.