Saturday, July 3, 2010

explict cloning in c++, java, collections...

Say you have a collection<Throwable> holding Errors, Exceptions .. which are subclasses. Let's clone them.

In a C++ container, you need Throwable pointers to Error objects. To clone each object, you would need a virtual clone() method.

In a java collection, if Error doesn't implement clone(), then the Throwable clone() won't help as it can't access subclass's additional fields (molecules on a single onion layer). I gave the UBS interviewer a workaround -- collection's iterating loop to use reflection to get the class of each element (Errors), then call constructor reflectively, finally set each field reflectively.

Serialization is another solution, as described in http://en.wikipedia.org/wiki/Clone_(Java_method)

Can you create methods using reflection? I said no you need bytecode instrumentation or perhaps dynamic proxy.

Unlike assignment, clone() should create a brand new object. (Try Object.java javadoc.) Stack-var would get out of scope. So cloned objects should use new() and pointer and heap.

Can clone() return the new pointer unwrapped (i.e. deferenced)? Put another way, do you return the object by pbref or pbclone? We already made a clone, so pbref is more efficient.

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