-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 05/02/2010 04:19 PM, Jeffrey Lee Hellrung, Jr. 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.
I've just posted the results of my testing. As to *why* it's that much faster, I can only provide educated guesses.
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?
If something's not going to be modified, you might as well make it a constant reference. I assume the savings come from somewhere else, since I already use constant references wherever I can get away with it.
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
If I read your example correctly, then that's about what I was thinking of: algorithms that have to start with a copy of one or more of the passed-in parameters, but then need to modify it/them. The division algorithm requires this, just as one example off the top of my head.
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 implementation.
I'll give you one better: the actual timings, which you can reproduce yourself if you doubt their accuracy. :-) Just let me know if you want the code I used to come up with them... the rules I read say that I'm not supposed to post any files to this list, so if you want them, I'll need to upload the changes somewhere. - -- 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/ iEYEARECAAYFAkvd6EkACgkQp9x9jeZ9/wTurgCg5H8jKM5lMR2QQZuM6Vwv9+/K skcAn0GPPpV8fT7mqHYjv9ckiZ4QJUUm =JF6s -----END PGP SIGNATURE-----