Offset from member pointer without temporary instance
c++
Solution
How about:
template<class T>
struct {
ptrdiff_t get_offset( int (T::*mem) )
{
union {
int used;
T unused;
} dummy;
return reinterpret_cast<char*>(&(dummy.unused.*mem))
- reinterpret_cast<char*>(&dummy.unused);
}
};
The address of a union member doesn't depend on the union member being constructed. Works already in C++03, but then only for PODs.
Problem
I would like to get the offset of a standard layout member variable when provided with a poiner to that variable. I cannot use `offsetof` since I have a pointer and not a name. The current code I have looks something like this, and I am wondering if there is a standards compliant way to get rid of the `dummy` variable. ``` template<class T> struct { ptrdiff_t get_offset( int (T::*mem) ) { T dummy; return reinterpret_cast<char*>(&(dummy.*mem)) - reinterpret_cast<char*>(&dummy); } }; ``` This function should only be callable with `int` member variable points (this is intentional). I am quite certain that the compiler doesn't actually create the `dummy` variable but it'd still be nice if I could get rid of it. I can't use a null pointer since dereferencing null is not defined (though it probably works on all common compilers). A C++03 solution would be good, or a C++11 solution is also of interest (but not usable by me now). NOTE: I'm already aware this is only standards compliant is T is a standard layout type.