c++ - Do we need to use std::launder when doing pointer arithmetic within a standard-layout object (e.g., with offsetof)?

Question

This question is a follow-up to: Is adding to a "char *" pointer UB, when it doesn't actually point to a char array?

In CWG 1314, CWG affirmed that it is legal to perform pointer arithmetic within a standard-layout object using an unsigned char pointer. This would appear to imply that some code similar to that in the linked question should work as intended:

struct Foo {
    float x, y, z;
};

Foo f;
unsigned char *p = reinterpret_cast<unsigned char*>(&f) + offsetof(Foo, z); // (*)
*reinterpret_cast<float*>(p) = 42.0f;

(I have replaced char with unsigned char for greater clarity.)

However, it seems that the new changes in C++17 imply that this code is now UB unless std::launder is used after both reinterpret_casts. The result of a reinterpret_cast between two pointer types is equivalent to two static_casts: the first to cv void*, the second to the destination pointer type. But [expr.static.cast]/13 implies that this produces a pointer to the original object, not to an object of the destination type, since an object of type Foo is not pointer-interconvertible with an unsigned char object at its first byte, nor is an unsigned char object at the first byte of f.z pointer-interconvertible with f.z itself.

I find it hard to believe that the committee intended a change that would break this very common idiom, making all pre-C++17 usages of offsetof undefined.

Answer 1

You question was:

Do we need to use std::launder when doing pointer arithmetic within a standard-layout object (e.g., with offsetof)?

No.

std::launder won't change anything in this case and therefore has nothing to do with the presented example (imo edit launder out of the question or ask another question).

std::launder is usually just needed in a subset of cases (eg. due to a const member) where you change (or create) an underlying object in some runtime manner (eg. via placement new). Mnemonic: the object is 'dirty' and needs to be std::launder'ed.

Using only a standard layout type cannot result in a situation where you would ever need to use std::launder.