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Defined in header <cstddef>
#define offsetof(type, member) /*implementation-defined*/

The macro offsetof expands to a constant 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 member, including padding if any.

If type is not a standard layout type, the behavior is undefined.

If member is a static member or a member function, the behavior is undefined.

The offset of the first member of a standard-layout type is always zero (empty-base optimization is mandatory)

[edit] Possible implementation

#define offsetof(type,member) ((std::size_t) &(((type*)0)->member))

[edit] Example

#include <iostream>
#include <cstddef>
struct S {
    char c;
    double d;
};
int main()
{
    std::cout << "the first element is at offset " << offsetof(S, c) << '\n'
              << "the double is at offset " << offsetof(S, d) << '\n';
}

Possible output:

the first element is at offset 0
the double is at offset 8

[edit] See also

unsigned integer type returned by the sizeof operator
(typedef) [edit]
checks if a type is standard-layout type
(class template) [edit]
C documentation for offsetof