Doug Gregor <dgregor@cs.indiana.edu> writes:
On Feb 17, 2006, at 8:48 AM, Thorsten Ottosen wrote:
I have just added serialization to Boost.Ptr container. I briefly discussed an issue with Robert, namely that the current implementation calls clear() before reading the new values.
The alternative is to provide roll-back guarantee by default by creating a local container and swap() in the end. By default this would temporarily use more memory.
IMO we should go for the strong guarantee. Any thoughts?
I think the basic guarantee is enough here. Truly paranoid users can copy+swap themselves, but for most purposes you are better off not having two copies of the container in memory. Most people need better performance more than they need the strong guarantee.
That's one right way to look at it. To look at it another way, if you provide the strong guarantee by copy-and-swap up front, there's no way to recover the efficiency for the (majority of) people who only need the basic guarantee. The *only* reason to use copy-and-swap around the implementation of an operation is to make it less error-prone. This: X& operator=(X rhs) { rhs.swap(*this); return *this; } is almost idiot-proof if you have a working copy ctor. Although for many classes it comes at a substantial efficiency cost when compared with a more laboriously worked-out assignment operator that gives the basic guarantee, I can't fault anyone for going for correctness and low maintenance cost first, and efficiency second. Incidentally, I don't know of any operations other than assignment whose maintainability are helped by copy/swap in this way. -- Dave Abrahams Boost Consulting www.boost-consulting.com