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

The macro offsetof expands to an integral constant expression 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).


offsetof throws no exceptions; the expression noexcept(offsetof(type, member)) always evaluates to true.


offsetof is required to work as specified above, even if unary operator& is overloaded for any of the types involved. This cannot be implemented in standard C++ and requires compiler support.


#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

See also

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

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