Why doesn’t polymorphism work without pointers/references?

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“Surely so long as you allocate memory on the heap” – where the memory is allocated has nothing to do with it. It’s all about the semantics. Take, for instance:

Derived d;
Base* b = &d;

d is on the stack (automatic memory), but polymorphism will still work on b.

If you don’t have a base class pointer or reference to a derived class, polymorphism doesn’t work because you no longer have a derived class. Take

Base c = Derived();

The c object isn’t a Derived, but a Base, because of slicing. So, technically, polymorphism still works, it’s just that you no longer have a Derived object to talk about.

Now take

Base* c = new Derived();

c just points to some place in memory, and you don’t really care whether that’s actually a Base or a Derived, but the call to a virtual method will be resolved dynamically.

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