Defined in header
#define offsetof(type, member) /*implementation-defined*/
The macro offsetof expands to an integer constant expression of type size_t, the value of which is the offset, in bytes, from the beginning of an object of specified type to its specified member, including padding if any.
the first element is at offset 0 the double is at offset 8
 See also
| unsigned integer type returned by the sizeof operator |
C++ documentation for offsetof