Boost C++ Libraries: Ticket #8606: Impossible to convert 0-size tuple to (empty) sequence any more

summary changed

vz-boost@… — Thu, 23 May 2013 00:51:05 GMT

summary Warnings when converting 0-size tuple to sequence with MSVC → Impossible to convert 0-size tuple to (empty) sequence any more

After investigating this further, it turns out that doing this with variadic macros is not obvious at all because there doesn't seem to be any way to distinguish between empty tuple and a 1 element tuple as BOOST_PP_VARIADIC_SIZE returns 1 for both of them.

Also, the reason for the difference in behaviour between g++ and MSVC was due to the fact that variadic support is not enabled for Boost Debian package for some reason (the compiler does support variadic macros, of course), so the situation is completely different there, sorry for the confusion.

Anyhow, if there is really no way to handle empty tuples with BOOST_PP_TUPLE_TO_SEQ using variadic macros, could the same functionality be at least provided with non-variadic macros under a different name? It's really useful to handle tuples of all sizes consistently.

Edward Diener — Sun, 17 May 2015 02:36:19 GMT

There is no such thing as a 0 size tuple. The notation of '()' is a tuple whose size is 1 and whose single element is empty. Therefore attempting to convert a zero-sized tuple to any other Boost PP data type, by specifying that the size is 0, is undefined.

status changed; resolution set

Edward Diener — Sun, 17 May 2015 04:50:12 GMT

status new → closed
resolution → fixed

vz-boost@… — Sun, 17 May 2015 12:07:35 GMT

Replying to eldiener:

There is no such thing as a 0 size tuple.

Terminological discussions aside, this worked in old Boost versions and was genuinely useful so this is objectively a regression. I know Boost doesn't guarantee backwards compatibility but there is no indication that this breakage was intentional.

Edward Diener — Sun, 17 May 2015 14:04:13 GMT

It is not useful to have a tuple of 0 size and not be able to distinguish between '()' as a tuple of size 0 or a tuple of size 1 with the single element as empty. Since emptiness is perfectly allowable in the preprocessor, and since '(x,)' is clearly a tuple whose size is 2 with the second element being empty, it would always be wrong to make a special case for '()' and declare that this is tuple of size 0. Furthermore treating '()' as a apecial case would make it impossible to have a tuple of size 1 whose single element is empty.

The VC++ preprocessor is a C++ non-standard conforming preprocessor and it is only with heroic effort that Paul Mensonides was able to get VC++ to handle Boost PP processing as much as it does ( there are numerous workarounds in Boost PP code just to handle VC++ ). The gcc preprocessor is vastly closer to the C++ standard.