Is my previous statement only valid if placement new used a reinterpret_cast
on it's implementation? Or if h.storage was reinterpret_cast-ed to a void*
when it was passed in? Well, if reinterpret_cast remembered the type, it
would know it wasn't what we were trying to cast it to.
Also, instead of a placement_cast, why not write a placed_new<> template (as
someone said, offset_cast is only used in this situation):
template <typename Storage, typename Object>
Object* const placed_new (Storage* const storage) {
new (storage) Object;
return ...;
}
// .. support construction with (1, 2, 3, ...) parameters as well
"Eric Woodruff" <[EMAIL PROTECTED]> wrote in message
aso09f$akc$[EMAIL PROTECTED]">news:aso09f$akc$[EMAIL PROTECTED]...
>
> "Anthony Williams" <[EMAIL PROTECTED]> wrote in
> message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > Eric Woodruff writes:
> > > "Anthony Williams" <[EMAIL PROTECTED]> wrote in
> > > message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > > > Thus, given that h.storage is properly aligned, (which is the
purpose
> of
> > > the
> > > > other union member), after "new(h.storage) Foo", h.storage contains
a
> Foo
> > > > object. Thus accessing it through a pointer-to-Foo is legal, as Foo
> is the
> > > > dynamic type of the object.
> > > >
> > >
> > > This is precisely my reasoning why reinterpret_cast<> is _not_
> > > implementation defined.
> > >
> > > It must be the case that it is equal to the situation of having
> > >
> > > Foo* -> char */void* -> Foo* if Foo is the dynamic type of the
object.
> >
> > Nowhere do I see a requirement that supports your deductions. Indeed
there
> are
> > very few requirements on reinterpret_cast<>, other than the fact that
what
> it
> > does must be defined by the implementation. For pointers to objects,
these
> > amount to:
> >
> > * T1* can be converted to T2* and back again unchanged (unless T2 has
> stricter
> > alignment requirements).
>
> Precisely, T1 is Foo in this case as you have already pointed out. This is
> simply the identity conversion T1 -> T2 -> T1, which is pretty much the
only
> thing reinterpret_cast can do portably.
>
[snip]
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