offsetof

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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.

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.

Exceptions

noexcept specification:  

noexcept

  (since C++11)

Possible implementation

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

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

See also

size_t	unsigned integer type returned by the sizeof operator (typedef)
is_standard_layout (C++11)	checks if a type is standard-layout type (class template)