-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 05/03/2010 12:46 PM, Stewart, Robert wrote:
Not exactly useless -- I *do* know enough to avoid things that are completely useless. :-) The purpose was to initialize them to the value of another integer, such as a parameter, before going into a loop that modifies them based on their current values. With copy-on-write, the initial allocation can be skipped, which (depending on the number of times it needs to loop) can be a significant savings.
You say you copy an instance and then mutate it. Guess what! The copy involves manipulating the reference count and the mutating operations must all check whether the instance is unique. The first mutating operation actually makes a unique copy and the rest just check superfluously.
The first operation *may* make a unique copy, if the operation results in a change to the number. There's no guarantee that it will. For example, in earlier versions of the library (when the divide function wasn't split up the way it is now), I needed to do a calculation to see whether the divisor and dividend needed to be normalized. If they did (about half the time), they had to be mutated; if they didn't, they kept their original values. With copy-on-write, it was simple to make mutable copies of them, and only actually mutate them (and pay the price for it) when it was needed.
Thus, in the cited example, since there will be a mutated copy, COW pessimizes the code. In single threaded code, the overhead is small, assuming good branch prediction in the processor, but it isn't zero. (When used in MT code, of course, the overhead is worse.)
When used in MT code, the library doesn't use copy-on-write, so that's not a problem.
My point is your example in no way justifies COW. Provide another real use case from your library that isn't served by move semantics, and you'll have convinced folks that COW is appropriate.
I already have: it's quite a bit faster. :-) Though as I said, I may have identify something that's slowing down the move stuff. I'll post more information when I have it. I'll be happy to remove copy-on-write if I can make the library as fast with move semantics. - -- Chad Nelson Oak Circle Software, Inc. * * * -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkvfCn0ACgkQp9x9jeZ9/wR6jgCfa8qyIUZhUU7jO4hrDgT9U8Ms UkAAoIsqbBUrcWOGnKQ5uyK6mZW0RRZx =UOqu -----END PGP SIGNATURE-----