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by kasey_junk 2555 days ago
Note that your model is also an abstraction. Modern architectures move data around between a variety of memory locations (and store it in multiple locations) which are all addressed via the pointer ‘transparently’. It’s not really a memory address anymore and hasn’t been for some time.

That’s not to say your model is bad, it’s the same one I have in my head. But it’s no more ‘real’ than coordinates or library cards or P.O. Box number.
2 comments
roelschroeven 2555 days ago
Isn't it still the model that is exposed to the programmer, even though internally all kinds of things are going on to make it all go faster? For example, nowadays many/most processors have separate caches for instructions and data, meaning that they strictly speaking don't conform to the Von Neumann architecture. But the programmer doesn't see the different caches, ideally doesn't even see the cache at all.

It's like the execution model, where processors do a lot of out-of-order execution, but do a lot of work so they can present a model to the programmer as if everything happens in-order.

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kasey_junk 2555 days ago
That depends on the architecture, the os, the compiler & the runtime.

Part of the confusion in teaching pointers (I think) is that we act like they are a lower level abstraction than they are (because they used to be). On modern commodity hardware with mainstream languages pointers have very little to do with memory addresses. Accessing memory via them will not happen in constant time, they may or may not perform better when used, even if the language doesn’t copy them on use the OS or processor might etc.

I just find when I try to teach people the memory address model of pointers there are so many exceptions that the model isn’t helpful. Like the OP I struggle with an alternative.

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ci5er 2555 days ago
Agreed. But it depends on the computer. On the old vectorized machines like the Cray Y-MP, it was always just a mental model. On even modern embedded systems w/o virtual memory, it's pretty close to being really real.

(I find that the P.O. Box number thing to be a useful general abstraction for me, but it doesn't seem to convey the concept well to people who are confused. Hence the intent behind the intent of my question: "Does anyone know how to effectively teach pointers?")

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