p0908r0
Offsetof for Pointers to Members

The offsetof macro, inherited from C and applicable to standard-layout classes (and, conditionally, other classes) in C++, calculates the layout offset of a member within a class. offsetof is useful for calculating an object pointer given a pointer to one of its members:

struct link {
  ...
};

struct container {
  link l;
};

container* container_from_link(link* x) {
  // x is known to be the .l part of some container
  uintptr_t x_address = reinterpret_cast<uintptr_t>(x);
  size_t l_offset = offsetof(container, l);
  return reinterpret_cast<container*>(x_address - l_offset);
}

This pattern is used in several implementations of intrusive containers, such as Linux kernel linked lists (struct list_head).

Unfortunately, although offsetof works for some unusual member-designators, it does not work for pointers to members. This won’t compile:

template <typename Container, typename Link, Link (Container::* member)>
Container* generic_container_from_link(Link* x) {
  uintptr_t x_address = reinterpret_cast<uintptr_t>(x);
  size_t link_offset = offsetof(Container, member); // error!
  return reinterpret_cast<Container*>(x_address - link_offset);
}

Programmers currently compute pointer-to-member offsets using nullptr casts (i.e., the incorrect folk implementation of offsetof, which invokes undefined behavior), or by jumping through other hoops:

template <typename Container, typename Link, Link (Container::* member)>
Container* generic_container_from_link(Link* x) {
  ...
  alignas(Container) char container_space[sizeof(Container)] = {};
  Container* fake_container = reinterpret_cast<Container*>(container_space);
  size_t link_offset = reinterpret_cast<uintptr_t>(&(fake_container->*member))
      - reinterpret_cast<uintptr_t>(fake_container);
  ...
}

offsetof with pointer-to-member member-designators should simply work. Modern compilers implement offsetof using an extension (__builtin_offsetof in GCC and LLVM), so implementation need not require library changes. To avoid ambiguity, we propose this syntax:

size_t link_offset = offsetof(Container, .*member);

1. Questions

Must a pointer-to-member expression in an offsetof member-designator be a constant expression (such as a template argument)? The C standard requires that “the expression &(t.member-designator) evaluates to an address constant,” which might make this code illegal:

struct container {
  char array[200];
};

int index = /* dynamic value */;
size_t offset = offsetof(container, array[index]);  // questionable

But since several current compilers accept dynamic array indexes, the proposed wording allows any pointer to member.

2. Proposed Wording

In Sizes, alignments, and offsets [support.types.layout], modify the first sentence of ❡1 as follows:

The macro offsetof(type, member-designator) has the same semantics as the corresponding macro in the C standard library header <stddef.h>, but accepts a restricted set of type arguments and a superset of member-designator arguments in this International Standard.

Add this paragraph after ❡1:

An offsetof member-designator may contain pointer-to-member expressions as well as member-designators acceptable in C. A member-designator may begin with a prefix . or .* operator (e.g., offsetof(type, .member_name) or offsetof(type, .*pointer_to_member)). If the prefix operator is omitted, . is assumed.

3. Example online discussions of the issue

• [LLVMdev] Evaluation of offsetof() macro

• Working around offsetof limitations in C++