Friday, February 11, 2011

low-level differences between HASA^ISA, in terms of function pointers

Some interviewer (MS or CS) asked me about the differences.

1) Subclass instance has the _same_address_ as base object, that's why you can cast the ptr (or reference) up and down the inheritance hierarchy. See post on AOB ie address of basement.

2) Also, all inherited methods are available in the "collection" of function pointers of the derived object. In other words, derived object advertises all those inherited "features" or "services", if you think of a family of interchangeable components. Derived object can _stand_in_ for the base TYPE.

In OO languages, a pure interface is basically a collection of function pointers. Has-a doesn't expose/advertise the internal object's function pointers, so the wrapper can't "stand in" for that type.

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