#define offsetof(type, member) /* implementation-defined */
The macro offsetof expands to an integral constant expression of type std::size_t, the value of which is the offset, in bytes, from the beginning of an object of specified type to its specified subobject, including padding bits if any.
Given an object o of type type and static storage duration, o.member shall be an lvalue constant expression that refers to a subobject of o. Otherwise, the behavior is undefined. Particularly, if member is a static data member, a bit-field, or a member function, the behavior is undefined.
If type is not a PODType(until C++11)standard-layout type(since C++11), the result of offsetof is undefined(until C++17)use of the offsetof macro is conditionally-supported(since C++17).
The expression offsetof(type, member) is never type-dependent and it is value-dependent if and only if type is dependent.
offsetof throws no exceptions.
The expression noexcept(offsetof(type, member)) always evaluates to true.
(since C++11) [edit] NotesThe offset of the first member of a standard-layout type is always zero (empty-base optimization is mandatory).
(since C++11)offsetof cannot be implemented in standard C++ and requires compiler support: GCC, LLVM.
member is not restricted to a direct member. It can denote a subobject of a given member, such as an element of an array member. This is specified by C DR 496.
It is specified in C23 that defining a new type containing an unparenthesized comma in offsetof is undefined behavior, and such usage is generally not supported by implementations in C++ modes: offsetof(struct Foo { int a, b; }, a) is rejected by all known implementations.
#include <cstddef>
#include <iostream>
struct S
{
char m0;
double m1;
short m2;
char m3;
// private: int z; // warning: 'S' is a non-standard-layout type
};
int main()
{
std::cout
<< "offset of char m0 = " << offsetof(S, m0) << '\n'
<< "offset of double m1 = " << offsetof(S, m1) << '\n'
<< "offset of short m2 = " << offsetof(S, m2) << '\n'
<< "offset of char m3 = " << offsetof(S, m3) << '\n';
}
Possible output:
offset of char m0 = 0 offset of double m1 = 8 offset of short m2 = 16 offset of char m3 = 18[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior CWG 273 C++98offsetof may not work if unary operator& is overloaded required to work correctly even
operator& is overloaded LWG 306 C++98 the behavior was not specified when type is not a PODType the result is undefined in this case LWG 449 C++98 other requirements of offsetof were
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