17 Language support library [support]

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 document.

Use of the offsetof macro with a type other than a standard-layout class ([class.prop]) is conditionally-supported.169

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]).

Recommended practice: An implementation should choose types for ptrdiff_t and size_t whose integer conversion ranks ([conv.rank]) are no greater than that of signed long int unless a larger size is necessary to contain all the possible values.

The type max_align_t is a trivial standard-layout 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/IEC 9899:2018, 7.19