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. -- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost