Q: ask yourself, how big is a Joint object if you have Line 21 in there?
A: infinite
you need to replace that line with a reference or a pointer to Joint
edit: hint - critical lines are Line 9 and Line 21 - you have defined an object which contains itself - you cannot do this without references or pointers - if you try, as you have, the compiler cannot calculate the value of sizeof( Joint ) because the answer would be infinite