Friday, March 2, 2012

HASA ^ ISA ^ vs PointTo in c++

In java, hasa always means point2 (or pointTo), assuming The Small object is never a primitive.... In c++, hasa means Host object's has a piece of its real estate carved out for the Small object "embedded" therein [1]. This is a profound difference from java. Here are some consequences --

* Even if you put a custom op-new into class Small to prevent heap-instantiation of Small, class Host can still instantiate a Small on heap during its own construction. See [[More effC++]]

* const (on the host object or a non-static method) means real-estate const. So the non-ref field should not change. This governs hasa, and has no effect on points-to.

* If a c++ Host object points-to a Small object, then memory leak is quite possible, so Host dtor had better delete that Small pointer. Therefore c++ points-to suffers from memory leak but no such issue with c++ HASA. Host dtor automatically calls Small dtor, in the DCB sequence (see blog post on DCB)

ISA also involves embedding. Therefore, the custom op-new in Small won't prevent a Small instantiation on heap if Small subclass has its own op-new.

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