Question

What is the purpose of __builtin_offsetof operator (or _FOFF operator in Symbian) in C++?

In addition what does it return? Pointer? Number of bytes?

Answer 1

It’s a builtin provided by the GCC compiler to implement the offsetof macro that is specified by the C and C++ Standard:

GCC – offsetof

It returns the offset in bytes that a member of a POD struct/union is at.

Sample:

struct abc1 { int a, b, c; }; union abc2 { int a, b, c; }; struct abc3 { abc3() { } int a, b, c; }; // non-POD union abc4 { abc4() { } int a, b, c; };  // non-POD  assert(offsetof(abc1, a) == 0); // always, because there's no padding before a. assert(offsetof(abc1, b) == 4); // here, on my system assert(offsetof(abc2, a) == offsetof(abc2, b)); // (members overlap) assert(offsetof(abc3, c) == 8); // undefined behavior. GCC outputs warnings assert(offsetof(abc4, a) == 0); // undefined behavior. GCC outputs warnings 

@Jonathan provides a nice example of where you can use it. I remember having seen it used to implement intrusive lists (lists whose data items include next and prev pointers itself), but i can’t remember where it was helpful in implementing it, sadly.