offsetof
Defined in header <cstddef>
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#define offsetof(type, member) /*implementation-defined*/ |
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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 (until C++17)use of the offsetof
macro is conditionally-supported (since C++17).
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).
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.
Notes
offsetof
cannot be implemented in standard C++ and requires compiler support: GCC, LLVM
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'; }
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 (typedef) | |
(C++11) |
checks if a type is a standard-layout type (class template) |