offsetof

Example from MSVC's implementation: #define offsetof(s,m) \ (size_t)&reinterpret_cast<const volatile char&>((((s *)0)->m)) // ^^^^^^^^^^^ As can be seen, it dereferences a null pointer, which normally invokes undefined behaviour. Is this an exception to the rule or what is going on? Where the language standard says "undefined behavior", any given compiler can define the behavior. Implementation code in the standard library typically relies on that. So there are two questions: (1) Is the code UB with respect to the C++ standard? That's a really hard question, because it's a well known almost

In ANSI C, offsetof is defined as below. #define offsetof(st, m) \ ((size_t) ( (char *)&((st *)(0))->m - (char *)0 )) Why won't this throw a segmentation fault since we are dereferencing a NULL pointer? Or is this some sort of compiler hack where it sees that only address of the offset is taken out, so it statically calculates the address without actually dereferencing it? Also is this code portable? JaredPar At no point in the above code is anything dereferenced. A dereference occurs when the * or -> is used on an address value to find referenced value. The only use of * above is in a type