offsetof
offsetof
Defined in header <cstddef>
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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) |
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