on Fri Feb 01 2013, Paul Smith <pl.smith.mail-AT-gmail.com> wrote:
On Fri, Feb 1, 2013 at 7:15 PM, Dave Abrahams <dave@boostpro.com> wrote:
on Fri Feb 01 2013, Paul Smith <pl.smith.mail-AT-gmail.com> wrote:
Okay, I'm truely confused about what you're saying then.
Either the standard library requires moved-from user-types to keep meeting all their requirements, or it only requires them to remain destructible and assignable.
Not knowing if there's specific text about this, I can't say for sure. If, as I think is likely, what it requires of them is not explicitly called out, then it is implied by other requirements. In other words, the library has the freedom to perform any operations on such objects as it may perform on any other object in the same context. For example, in a call to sort(first, last), it is free to compare two objects in the range [first,last) even if one or both of them has been moved-from, so the objects passed to such a call must, technically, preserve less-than-comparability of moved-from objects.
In practice, though, IIUC implementations will only actually use assignment and destruction on moved-from objects.
This was my understanding.
The standard says the former.
If you say so.
Table 20, [moveconstructible]:
rv (the moved-from object) must still meet the requirements of the library component that is using it. The operations listed in those requirements must work as specified whether rv has been moved from or not.
How else should I read it? When I say that "this is what the library requires" I mean exactly that - what it technically requires, not what it will or will not use in practice.
I think you read it correctly. However, be aware that the text in question is in a non-normative note, which means that it doesn't change the meaning of the standard. Those facts are implied by the other text; in particular, the lack of any special and explicit permission for moved-from objects to stop meeting their requirements.
No, I only disavow these parts of what you attributed to me:
"it's not something the standard actually guarantees in general, though, and the requirements are still much stricter, perhaps superfluously so.
I'm not sure what you're disavowing then. You don't say that the standard guarantees that destructibility and assignability are enough, but you do say that this is all it will need in practice. That's called superfluous in my book.
That's a judgement that you're willing to make, but I'm not. -- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost