[swift-dev] [idle] COW wrapper in 30 lines

Dave Abrahams dabrahams at apple.com
Thu Mar 10 18:09:03 CST 2016

on Thu Mar 10 2016, Joe Groff <jgroff-AT-apple.com> wrote:

>> On Mar 10, 2016, at 3:54 PM, Dave Abrahams via swift-dev <swift-dev at swift.org> wrote:
>> 2. The thing in your box has to be a value type for this to work.  It's
>>   pretty uncommon that a thing that's already a value type benefits
>>   from being CoW-ified in this way.  
> It doesn't need to be particularly largeā€”if your value type contains
> two or more refcounted fields, it becomes cheaper to retain/release a
> single indirect box than to copy around the entire value inline.

You do have to trade that off against the cost of the allocation, but
you make a good point.  I'd guess that value instances are probably
copied more often than they're created in other ways.


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