On Wed, Jul 23, 2014 at 12:44 PM, Jason Merrill <ja...@redhat.com> wrote:
> On 07/22/2014 02:34 PM, Richard Biener wrote:
>>
>> As discussed during the Cauldron keeping some builtin doesn't help because
>>
>> you are not forced to access the newly created object via the pointer
>> returned
>> by the placement new. That is,
>>
>> template <T>
>> struct Storage {
>> char x[sizeof(T)];
>> Storage() { new (x) T; }
>> T& get() { return reinterpret_cast <T&> (x); }
>> };
>>
>> is valid
>
>
> Yes.
>
>
>> (and used in this way in Boost - with a type different from 'char'
>> to force bigger alignment).
>
>
> But I don't think that should be valid, unless the type contains a char
> array at offset 0, as {std,boost}::aligned_storage; the C++ standard needs
> improvement in this area.
Why especially at offset 0? I'm constructing in the place of 'x', not
'this'. Do you say that
template <class T>
struct Storage {
T& get(i) { return new (x + sizeof (T) * i) T; }
Storage (int n_) n (n_) {}
int n;
char x[sizeof (T)];
};
and doing
Storage *s = new (malloc (sizeof (int) * 4)) Storage (4);
s->get (2);
isn't valid?
> Looks like the small buffer optimization in boost::spirit::hold_any would
> need to be tweaked, as it uses a void* to store anything the same size or
> smaller, but that's the only dodgy case I see.
I've seen other odd cases in GCC bugreports ultimately coming from
Boost & friends (mpl or whatnot). Very likely older Boost versions
of course.
Btw, any reason why the standard treats 'char' and 'unsigned char'
special but not 'signed char'?
That said, as a matter of QOI I think only special-casing character
types would be a bad thing (see your hold_any example).
Richard.
> Jason
>
