Strenghten assumption about dynamic type changes (placement new)

[Richard Biener]

On Wed, Jul 23, 2014 at 12:44 PM, Jason Merrill <jason@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
>