21 Language support library [language.support]

21.2 Common definitions [support.types]

21.2.4 Sizes, alignments, and offsets [support.types.layout]

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 in this International Standard.
Use of the offsetof macro with a type other than a standard-layout class is conditionally-supported.187
The expression offsetof(type, member-designator) is never type-dependent and it is value-dependent if and only if type is dependent.
The result of applying the offsetof macro to a static data member or a function member is undefined.
No operation invoked by the offsetof macro shall throw an exception and noexcept(offsetof(type, member-designator)) shall be true.
The type ptrdiff_­t is an implementation-defined signed integer type that can hold the difference of two subscripts in an array object, as described in [expr.add].
The type size_­t is an implementation-defined unsigned integer type that is large enough to contain the size in bytes of any object ([expr.sizeof]).
[Note
:
It is recommended that implementations choose types for ptrdiff_­t and size_­t whose integer conversion ranks are no greater than that of signed long int unless a larger size is necessary to contain all the possible values.
end note
]
The type max_­align_­t is a POD type whose alignment requirement is at least as great as that of every scalar type, and whose alignment requirement is supported in every context ([basic.align]).
See also: ISO C 7.
19
Note that offsetof is required to work as specified even if unary operator& is overloaded for any of the types involved.