Travis Vitek wrote:
I've already implemented fallback support for many traits. Here is a
list of those that I can think of off of the top of my head...
__rw_is_class
__rw_is_union
__rw_is_empty
__rw_is_polymorphic
__rw_is_abstract
__rw_is_convertible
__rw_is_base_of
__rw_has_trivial_ctor
__rw_has_trivial_copy
__rw_has_trivial_assign
__rw_has_trivial_dtor
__rw_has_nothrow_ctor
__rw_has_nothrow_copy
__rw_has_nothrow_assign
All of the fallbacks are supposed to provide a correct result where
possible, otherwise they should return a pessimistic result. I.e. The
fallback __rw_has_trivial_ctor<T>::value will be true for all (possibly
cv-qualified) scalar types, but will evaluate to false for class types
(struct, union or class).
Now I have to figure out what to do for a few more traits. Specifically
__rw_alignment_of<> and __rw_aligned_storage<>. As it stands now, these
traits require definition of _RWSTD_TT_ALIGN_OF() and
_RWSTD_TT_ALIGNED_POD() to be defined. If the macros are not provided,
we will see a compile failure.
But only for the unusual specializations where the alignment
isn't the same as one of the fundamental types, right?
As I see it, my options are...
1. put code that will prevent instantiation in the the type
declaration (i.e. static_assert)
2. provide a suitable default that is not correct, but can be
used to detect the failure
3. disable traits entirely by defining _RWSTD_NO_EXT_CXX_0X if
the macros have not been defined
4. disable the affected traits (and their tests) if the macros
have not been defined
As I see it, option 4 is probably best. Especially given the recent
discussion of removing all three alignment traits in favor of the new
alignof and alignas keywords and non-trivial unions.
We still need to write up the issue...
As for providing the traits only conditionally, I'm not sure
that's the best approach. If we feel it likely that some of
them will end up getting yanked from the working paper we
should probably avoid providing them at all, otherwise we
might be stuck maintaining them for a long time. Providing
them only for some platforms would also make them difficult
to use portably (we'd have to expose some kind of config
macro for users to check whether the trait is defined or
not).
So my preference would be either (2) or (5), with option 5
being to disable the aligned traits unconditionally until
we know for certain that they aren't going away.
Out of curiosity, what do other implementations do?
Martin
Input anyone?
Travis
