How can you determine the byte offset of a field within a structure?

Answer 1

For C++ you can do this:

#define member_offset(Struct, Field) (&(reinterpret_cast(0)->member))

because the byte offset is usually the structure address + offset, so by setting the address to 0, you get 0 + offset == offset.

AnswerYou should the "offsetof" macro. This is a standard macro that has been a part of the C standard for 20 years now (it's part of C89 and C99), so all C and C++ compilers should support it.

#include
size_t offsetof(type, member);

For example:

struct s { int x, y; };
offsetof(struct s, y);