offsetof

From Cppreference

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.

[edit] Notes

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

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

The offset of the first member of a standard-layout type is always zero.

[edit] Exceptions

[edit] Equivalent definition

#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';
}

Output:

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

[edit] See also