View source for cpp/types/offsetof

===Notes===
{{rrev|since=c++11|
The offset of the first member of a standard-layout type is always zero ([[cpp/language/ebo|empty-base optimization]] is mandatory).
}}

{{tt|offsetof}} cannot be implemented in standard C++ and requires compiler support: [https://github.com/gcc-mirror/gcc/blob/68ec60c4a377b532ec2d265ea542107c36b1d15c/gcc/ginclude/stddef.h#L406 GCC], [https://github.com/llvm-mirror/clang/blob/release_70/lib/Headers/stddef.h#L120 LLVM].

{{tt|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 {{stddoc|lang=c|n2396.htm#dr_496|C DR 496}}.

It is specified in C23 that defining a new type containing an unparenthesized comma in {{tt|offsetof}} is undefined behavior, and such usage is generally not supported by implementations in C++ modes: {{c|offsetof(struct Foo { int a, b; }, a)}} is rejected by all known implementations.