Re: Regarding assignment to struct

On Mon 5/5/2025 1:27 PM, Keith Thompson wrote:

Andrey Tarasevich <noone@noone.net> writes:
[...]

   #include <stdio.h>
>
   struct S { int a[10]; };
>
   int main()
   {
     struct S a, b = { 0 };
     int *pa, *pb, *pc;
>
     pa = &a.a[5],
     pb = &b.a[5],
     pc = &(a = b).a[5],
     printf("%p %p %p\n", (void *) pa, (void *) pb, (void *) pc);
   }
>
This version has no UB.

 I believe it does.  pc points to an element of an object with
temporary lifetime.  The value of pc is then used after the object
it points to has reached the end of its lifetime.  At that point,
pc has an indeterminate value.

Nope. Nowhere in this code the value of `pc` is used beyond the lifetime of the object with temporary lifetime.
Pay attention to the fact that the last 4 lines in above code is a single expression joined by a comma operator, which is the whole point of the corrections that differentiate it from the original version.

N3096 6.2.4p2: "If a pointer value is used in an evaluation after
the object the pointer points to (or just past) reaches the end of
its lifetime, the behavior is undefined. The representation of a
pointer object becomes indeterminate when the object the pointer
points to (or just past) reaches the end of its lifetime."

Not applicable in this case.
--
Best regards,
Andrey