offsetof

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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.

Contents 1 Exceptions 2 Notes 3 Example 4 Defect reports 5 See also [edit] Exceptions

offsetof throws no exceptions.

The expression noexcept(offsetof(type, member)) always evaluates to true.

(since C++11) [edit] Notes

The 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 in offsetof is undefined behavior, and such usage is only partially supported by some implementations in C++ modes: offsetof(struct Foo { int a; }, a) is supported by ICC and some old versions of GCC, while offsetof(struct Foo { int a, b; }, a) is rejected by all known implementations because of the comma in the definition of Foo.

[edit] Example Run this code #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

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offsetof

offsetof

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