>Eric Lemings wrote:
>
>> Travis Vitek wrote:
>>
>>
>> I'm looking at the standard, and it appears that the following is
>> legal...
>>
>> struct incomplete_t;
>>
>> std::aligned_union<0, void, incomplete_t>::type
>> aligned_t;
>>
>> [...]
>>
>> Now I could implement aligned_union<> to ignore incomplete
>> types (they have no alignment requirement), but this might
>> cause problems if someone tried to use the result.
>
>How so? If someone uses only such types in the list (probably a very
>rare use case), a zero result should be expected. If alignable types
>are mixed with such non-alignable types, the result would be as if no
>such non-alignable types were given.
What if none of the provided types have an alignment requirement or
size? What should the alignment be? What should the size be?
>
>So I'd say just ignore non-alignable types.
So you see no problem with the following code failing at runtime due to
misalignment or insufficient space?
struct incomplete_t;
// supposed to be 'suitable for use as uninitialized
// storage for any object whose type listed and will
// be at least Len bytes
std::aligned_union<1, incomplete_t>::type aligned_buf;
struct incomplete_t
{
long double val;
// [...]
};
void save_value (const long double& v)
{
((incomplete_t*)&aligned_buf)->val = v;
}
I'm pretty sure that the standard assumes we will use
std::alignment_of<> and std::aligned_storage<> to implement
std::aligned_union<>. If that is the case, then the requirement for the
type to be complete would be implied.
Travis
