offsetof - cppreference.com (original) (raw)

| | | | | ------------------------------------------------------------- | | | | #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.

[edit] Exceptions

offsetof throws no exceptions.

[edit] Notes

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.

[edit] Example

#include #include   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.

[edit] See also