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The macro offsetof(type, member-designator) accepts a restricted set of type arguments in this International Standard. If type is not a standard-layout class, the results are undefined. 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 field that is 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.

When specifying the type argument for the offsetof() macro, pass only a standard-layout class. The full description of a standard-layout class can be found in paragraph 7 of the [class] clause of the C++ Standard, or the type can be checked with the std::is_standard_layout<> type trait. When specifying the member designator argument for the offsetof() macro, do not pass a bit-field, static data member, or function member. Passing an invalid type or member to the offsetof() macro is undefined behavior.

Noncompliant Code Example

In this noncompliant code example, a type that is not a standard-layout class is passed to the offsetof() macro, resulting in undefined behavior.

#include <cstddef>
 
struct D {
  virtual void f() {}
  int i;
};
 
void f() {
  size_t off = offsetof(D, i);
  // ...
}

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In this noncompliant code example, the offset to i is calculated so that a value can be stored at that offset within buffer. However, because i is a static data member of the class, this example results in undefined behavior. According to the C++ Standard, [class.static.data], paragraph 1 [ISO/IEC 14882-2014], static data members are not part of the subobjects of a class.

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