Re: aliasing

[Martin Uecker <uecker@tugraz.at>]

Am Montag, dem 18.03.2024 um 11:55 +0100 schrieb Martin Uecker:
> Am Montag, dem 18.03.2024 um 09:26 +0100 schrieb Richard Biener:
> > On Mon, Mar 18, 2024 at 8:03 AM Martin Uecker <uecker@tugraz.at> wrote:
> > 
> 
> > 
> > Let me give you an complication example made valid in C++:
> > 
> > struct B { float x; float y; };
> > struct X { int n; char buf[8]; } x, y;
> > 
> > void foo(struct B *b)
> > {
> >   memcpy (x.buf, b, sizeof (struct B)); // in C++:  new (x.buf) B (*b);
> 
> Let's make it an explicit store for the moment
> (should not make a difference though):
> 
>     *(struct B*)x.buf = *b;
> 
> >   y = x; // (*)
> > }
> > 
> > What's the effective type of 'x' in the 'y = x' copy? 
> 
> Good point. The existing wording would take the declared
> type of x as the effective type, but this may not be
> what you are interested in. Let's assume that x has no declared
> type but that it had effective type struct X before the
> store to x.buf (because of an even earlier store to 
> x with type struct X).
> 
> There is a general question how stores to subobjects
> affect effective types and I do not think this is clear
> even before this proposed change.

Actually, I think this is not allowed because:

"An object shall have its stored value accessed only by an
lvalue expression that has one of the following types:

— a type compatible with the effective type of the object,
...
— an aggregate or union type that includes one of the
aforementioned types among its members (including,
recursively, a member of a subaggregate or contained union), or

— a character type."

... and we would need to move "a character type" above
in the list to make it defined.

Martin