Re: Iterators in Java and C++

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Lloyd Bonafide writes:


Before you can make that argument, you need to define what "better" means.
And, what "better" might mean to you, in this context, may not necessarily
mean the same thing to someone else, who uses a different definition of
"better".

For example, I happen to believe that C++'s iterators are "better", and I
happen to believe that they're better for the same reason it's argued that
Java's java.lang.Iterator is "better". The argument -- that you carry around
one object in Java, versus two objects in C++ (the beginning and the ending
iterator) -- and that because of this Java is better, is full of crap.

Only if your pristine fingers are too tired to type a few extra keywords,
and only your brain lacks the capacity to keep track of two things at the
same time, only then I would perhaps concede that Java Iterator
implementation is better. In terms of speed, performance, and efficiency,
C++ spanks the monkey out of Java's iterators for most use cases.

What the Java fan club fails to comprehend is that each iteration, in Java,
involves two virtual function calls: hasNext() and next(), while equivalent
C++ code is likely to involve more than a pair of CPU instructions:
increment and comparison, since the C++ generated code is likely to inline
and keep both iterators in registers (most C++ STL iterators usually get
optimized into nothing more than glorified pointers). In the worst case, a
C++ iterator would be a basic, no-frills object, which would incur only a
slightly bigger overhead than a direct pointer.

Meanwhile, since all Java methods are virtual, unless explicitly specified
otherwise; and I'm rather skeptical that even modern Java VMs will be able
to optimize away the required virtual function calls, one for each method:
hasNext(), and next(), on each iteration.

What you actually have, in terms of Java iterators, is something that in C++
would be equivalent to:

class Iterator {

public:
    virtual Iterator *next()=0;
    virtual bool hasNext()=0;
};

I'm ignoring an obvious issue here with memory allocation. When you factor
in the fact that, with most Java iterators, their next() method has to
allocate another instance of an Iterator object, and that Java's famous VM
has to keep track of references to each instance of every Iterator object
that gets instantiated, and garbage-collect it, from where I'm standing,
anyone who claims that Java implements iteration better than C++, from a
practical standpoint, are out of their freaking skulls.

So, really, for the same reasons that it's argued that Java's iterator
implementation is "better", I am going to argue that when you actually
examine what has to happen, you have no choice but to reach a completely
opposite conclusion, if you want to be intellectually honest about it. I
just can't accept that modern Java VMs, with all their JIT wizardly, can
optimize all this bloat away; especially since Java iterators are defined
mostly as interface objects, and all containers subclass the iterator
interface, and implement whatever next() and hasNext() means to them. That's
what they must do, there's just no way around it, and I see slim-to-none
chance that at runtime you can avoid two virtual function calls for each
loop iteration, and having to deal with instantiating a brand new iterator
object on each loop iteration, and having to deal with garbage-collecting
the entire mess, when you're done.

Yeah, that's MUCH "better" than just keeping two words in a pair of
registers, then incrementing the first one and comparing it against the
second one, on each loop iteration.

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