offsetof
From cppreference.com
| Defined in header <cstddef>
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| #define offsetof(type, member) /*implementation-defined*/ |
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The macro offsetof expands to a constant 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.
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)
[edit] Possible implementation
#define offsetof(type,member) ((std::size_t) &(((type*)0)->member)) |
[edit] Example
Run this code
#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
[edit] See also
| unsigned integer type returned by the sizeof operator (typedef) | |
| (C++11) |
checks if a type is standard-layout type (class template) |
| C documentation for offsetof
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