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From Alan Conway <acon...@redhat.com>
Subject Re: C++ usage of CopyOnWriteArray : background; adding federation for the HeadersExchange
Date Wed, 23 Dec 2009 14:14:38 GMT
On 12/23/2009 06:13 AM, Sam Joyce wrote:
> Hi Alan,
> thanks for this. I hadn't realized this was the use pattern; more
> comments in code please folks.

Perhaps you can add some while you still have the new-user perspective, that's 
the best time to write documentation :)

> looking at the add() fn, I see what you mean, it takes a copy, does the
> add and then replaces the main array element.
> I think CopyOnWriteArray is the correct type to use in the
> HeadersExchange. Personally I like it because I get to use things like
> add_unless() etc. (less coding for me) and I like the idea of a wrapper
> class like this that protects the coder from having to manage the
> locking directly; leads to less race conds because of misplaced locks.
>  From what you've mentioned below Alan, I'd guess the best option would
> be to add a new method to CopyOnWriteArray, something like
> replace_or_add(T& t, F f). the match function being the one to identify
> an existing element. I'm open to ideas for a better method name :) I
> think it should work a bit like add_unless(), except doing a
> copy/remove/replace
> thoughts?
> regards,
> Sam.
>

How about a general-purpose modify function like this:
     // t takes a const vector<T>& , m takes a non-const vector<T>&
     template <class Test, class Modifier>
     bool modify_if(Test t, Modifier m) {
         Mutex::ScopedLock l(lock);
         if (array && t(*array)) {
             ArrayPtr copy(new std::vector<T>(*array));
             m(*copy);
             array = copy;
             return true;
         }
         return false;
     }

That will let you do anything you like to the array - you could rewrite all the 
modifiers in terms of this one (although they'd be a little less efficient 
because the current ones use the results of the test part in the modify part.)

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