This is perfectly well-defined:
void something(int *x) {
reinterpret_cast<Point*>(x)->y = 42;
}
The Point object (p) and its x member are pointer-interconvertible, from [basic.compound]:
Two objects a and b are pointer-interconvertible if:
- […]
- one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, any base class subobject of that object ([class.mem]), or:
- […]
If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a
reinterpret_cast.
That reinterpret_cast<Point*>(x) is valid and does end up with a pointer that points to p. Hence, modifying it directly is fine. As you can see, the standard-layout part and the first non-static data member part are significant.
Although it’s not like the compilers in question optimize out the extra load if you pass a pointer to p.y in and return p.x instead.