offsetof
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 |
(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
#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
unsigned integer type returned by the sizeof operator (typedef) |
|
(C++11)
|
checks if a type is a standard-layout type (class template) |
C documentation for offsetof |
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