How can I understand the below code

ArduCopter
1

##I have some puzzle with the code below：

#define AP_VAROFFSET(type, element) (((ptrdiff_t)(&((const type *)1)->element))-1)

define here

question：
is that want to using the class type and variable name to find out the address of the var in the class？
but a class type would not has the variable define only if the class is defined to a object.

so,I has a test on linux, and the result is : 0xfffffffffffffffd