On 1/31/13 12:16 AM, Dave Abrahams wrote:
on Mon Jan 28 2013, Joel de Guzman <djowel-AT-gmail.com> wrote:
On 1/28/13 12:18 AM, Dave Abrahams wrote:
on Sat Jan 26 2013, Joel de Guzman <djowel-AT-gmail.com> wrote:
I think we are OK!
Come to think of it, the situation is a lot like a "singular" iterator:
"Iterators can also have singular values that are not associated with any sequence. [snip] Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value, the assignment of a non-singular value to an iterator that holds a singular value, and, for iterators that satisfy the DefaultConstructible requirements, using a value-initialized iterator as the source of a copy or move operation."
Indeed, for a singular valued iterator, i, you can assign to i and destruct i, place i in a container, etc. You just cannot dereference i, access its underlying value through ->, compare i with another iterator, etc.
Yes, it's like a singular iterator, but it's a lot more like a null pointer. A singular iterator is like (and may in fact be) an uninitialized pointer, over which the program does not and cannot exert any control. A null pointer, on the other hand, has been initialized to a known state, but one in which some operations (e.g. *p, ++p, --p, p[0], ...) are not allowed. Or you could think of it as being like the a zero-valued primitive numeric type, which you can't use as a divisor.
The same is true with a nulled recursive_wrapper.
Being honest about this is just a matter of documentation. I do not see any problem at all with having a "singular" recursive_wrapper value.
Does it break backward-compatibility, by changing the guarantees users have about the possible values of certain variant types? If so, the documentation should make that clear as well.
Thank you Dave for your insights. That is sensible and reasonable. In the end, it's all about the variant's invariant alright :-P. I can see now how it may break backward compatibility. Yet, that would be an extremely rare situation and IMO, the practicality of a highly efficient move far outweighs this backward-compatibility issue. Regards, -- Joel de Guzman http://www.boostpro.com http://boost-spirit.com