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Subject: Re: [boost] [xint] Third release is ready, requesting preliminary review
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-05-02 16:19:21

On 05/02/2010 08:29 AM, Chad Nelson wrote:
> On 05/02/2010 03:19 AM, Jeffrey Hellrung wrote:
>> Regarding COW specifically: I'm guessing COW will be a tough sell,
> I keep hearing that, but why? It's an internal detail, one that provides
> (or at least seems to) a very noticeable speed boost under some
> circumstances, and is disabled when it can't be safely used. Why would
> anyone, other than developers doing work on the library (i.e. me), care
> one way or another that the library uses it?

Because COW is not thread-safe ;) And I'm very curious how this speed
"boost" (pun intended?) has come about.

As Pavel has pointed out, yes, if you make 10 unmodified copies of a COW
object, it's much faster than 10 unmodified copies of a non-COW object.
  But if I make 10 reference-to-consts of the same object, that's *even
faster*. Can you give a "real" example where you'd actually *want* to
produce an unmodified copy of an object, rather than just creating a
reference-to-const? And doesn't the reference-to-const make your
intentions ("Dude, I'm not modifying this thing!") that much clearer?

The only example I can think of where COW might come in handy is if you
do something like

cow* p_x1 = new cow();
cow* p_x2 = new cow(*p_x1); // no deep copy / shared ownership
// ... [A] Read from *p_x1, maybe? ...
delete p_x1; // *p_x2 now has exclusive ownership

I.e., the lifetimes of the handles are not in a subset relation (the
lifetime of *p_x1 begins before, but also *ends* before, the lifetime of
*p_x2). There's no way to avoid the copy with move semantics without
affecting the code at [A] (*p_x1 is probably only guaranteed to be
assignable at that point). However, I'd venture to say that 99% of the
time, variable lifetimes are in fact subset related, and certainly make
the reasoning of code easier.

So, bottom line: Give me a real example of an algorithm where COW gives
superior performance to move semantics, and I'll buy into the COW

- Jeff

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