Boost C++ Libraries: Ticket #6760: PUSH_BACK/_FRONT on empty array broken

attachment set

yves.pausch@… — Tue, 03 Apr 2012 15:16:56 GMT

attachment → boost_pp_ticket_6760.cpp

sample to reproduce the problem

pmenso57 — Tue, 03 Apr 2012 20:11:13 GMT

There is no such thing as a zero-element array or tuple.

(0, ())

should be

(1, ())

because it is a one-element array whose element is nothing--just as with

(2, (,))
(3, (,,))

and so on.

If there was a zero-element array, it would be something like:

(0, ~) // (0,) in >= C99 or >= C++11

but that rogue value would require special handling in all of the array operations that defer to tuple operations.

owner changed

pmenso57 — Tue, 03 Apr 2012 20:12:25 GMT

owner changed from No-Maintainer to pmenso57

status changed; resolution set

pmenso57 — Tue, 03 Apr 2012 20:13:04 GMT

status new → closed
resolution → invalid

frank.richter@… — Tue, 03 Apr 2012 21:39:47 GMT

The reference docs make no statement about the validity of empty tuples or arrays. Only one place makes an implication: “Sequences are data structures that merge the properties of both lists and tuples with the exception that a seq cannot be empty.” - the implication being that lists and tuples (and thus by extension arrays) _can_ be empty.

So now you stated tuples (and by extension arrays) cannot be empty. Judging from the list documentation, an empty list is not possible, either (as I read it, the tail can be BOOST_PP_NIL, but it's stated the head can only be an element).

So what container _can_ be empty? The documentation doesn't say. And at least until 1.49 and empty array was a working solution. So if you're not planning to provide some support of zero-element tuples/arrays, at the very least consider the whole thing a documentation issue, and that some more explicit words on empty containers would be desireable.