Sunday, February 04, 2007

Ownership idioms, The Move Constructor idiom and auto_ptr

There are properties of objects that are relevant beyond mere identity of the object. One of such properties is Ownership! Who owns the object? Can you transfer the ownership? Questions like these become quite important when you are implementing Resource Acquisition Is Initialization (RAII) idiom. Tom Cargill describes [in PLoPD2] a set of ownership patterns: "Creator as a Sole Owner", "Sequence of Owners", and "Shared Ownership". Lets take a look at the idioms for each one of them.

Creator as a sole owner can be simply implemented using const auto_ptr idiom by Hurb Sutter. Sequence of owners come into play when ownerships moves from one guy to the other. (a.k.a. Move Semantics). Standard auto_ptrs use move semantics. Implementation of move semantics appears to be a juicy issue as far as std::auto_ptrs are concerned. See Meyer's errata like notes on his book More Effective C++ and Sutter's GotW #25 articles. M. D. Wilson's article on Move Constructors is a very good start before you jump into Sutter's above mentioned article that has difficulty level 8/10! And Finally, if you are still hungry for some more spicy generic programming stuff, please see Alexandrescu's article on Move Constructor in Dr. Dobb's May 2003 issue.

Cargill's Shared Ownership patterns take us to the world of reference counting and reference linking. Some idioms in this space allow us to avoid making unnecessary deep copies. Copy-on-Write (COW) idiom is an important one. Another idiom to avoid the overhead is called "Temporary Base Class Idiom" given by Bernd Mohr. It somehow reminds me of the move constructor idiom. Finally, Return Value Optimization (RVO) and Named Return Value Optimization (NRVO) are important compiler optimization techniques that avoid unnecessary calls to the copy-constructor and the copy-assignment operator.


leo said...

Thank you very much, that was exactly what I was looking for.

Anonymous said...

Same here! Thanks for that useful article!