offsetof

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) launder (C++17) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

Type support

Basic types
Fixed width integer types (C++11)
ptrdiff_t size_t max_align_t (C++11) byte (C++17)	nullptr_t (C++11) offsetof NULL
Numeric limits
numeric_limits
C numeric limits interface
Runtime type information
type_info type_index (C++11)	bad_typeid bad_cast

 

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 subobject, including padding 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 behavior 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.

The expression noexcept(offsetof(type, member)) always evaluates to true.

(since C++11)

Notes

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

(since C++11)

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 in offsetof is undefined behavior, and such usage is only partially supported by some implementations in C++ modes: offsetof(struct Foo { int a; }, a) is supported by ICC and some old versions of GCC, while offsetof(struct Foo { int a, b; }, a) is rejected by all known implementations because of the comma in the definition of Foo.

Example

Run this code

#include <iostream>
#include <cstddef>
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';
}

Possible output:

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.

DR	Applied to	Behavior as published	Correct behavior
CWG 273	C++98	offsetof may not work if unary operator& is overloaded	required to work correctly even if operator& is overloaded

See also

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