Why doesn't polymorphism work without pointers/references?

c++ pointers polymorphism virtual
user997112 picture user997112 · Mar 3, 2013 · Viewed 23.5k times · Source

I did find some questions already on SO with similar title- but when I read the answers they were focussing on different parts of the question which were really specific (e.g. STL/containers)..

Could someone please show me why you must use pointers/references for implementing polymorphism? I can understand pointers may help- but surely references only differentiate between pass-by-value and pass-by-reference??

Surely so long as you allocate memory on the heap- so that you can have dynamic binding then this would have been enough- obviously not.

Answer

Luchian Grigore picture Luchian Grigore · Mar 3, 2013

"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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