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.
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.
Input anyone?
Travis
