offsetof

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 subobject, including padding bits if any.

Given an object o of type type and static storage duration, o. member shall be an lvalue constant expression that refers to a subobject of o . Otherwise, the behavior is undefined. Particularly, if member is a static data member , a bit-field , or a member function , the behavior is undefined.

If type is not a PODType (until C++11) standard-layout type (since C++11) , the result of offsetof is undefined (until C++17) use of the offsetof macro is conditionally-supported (since C++17) .

The expression offsetof ( type, member ) is never type-dependent and it is value-dependent if and only if type is dependent.

Exceptions

offsetof throws no exceptions.

Notes

offsetof cannot be implemented in standard C++ and requires compiler support: GCC , LLVM .

member is not restricted to a direct member. It can denote a subobject of a given member, such as an element of an array member. This is specified by C DR 496 .

It is specified in C23 that defining a new type containing an unparenthesized comma in offsetof is undefined behavior, and such usage is generally not supported by implementations in C++ modes: offsetof ( struct Foo { int a, b ; } , a ) is rejected by all known implementations.

Example

#include <cstddef>
#include <iostream>
 
struct S
{
    char   m0;
    double m1;
    short  m2;
    char   m3;
//  private: int z; // warning: 'S' is a non-standard-layout type
};
 
int main()
{
    std::cout
        << "offset of char   m0 = " << offsetof(S, m0) << '\n'
        << "offset of double m1 = " << offsetof(S, m1) << '\n'
        << "offset of short  m2 = " << offsetof(S, m2) << '\n'
        << "offset of char   m3 = " << offsetof(S, m3) << '\n';
}

offset of char   m0 = 0
offset of double m1 = 8
offset of short  m2 = 16
offset of char   m3 = 18

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also