The behavior is well-defined :
C++ standard, section § 5.2.11/7 [const cast]
[ Note: Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier may produce undefined behavior. —end note ]
GetA() does not write any member of Bob, so this program does not involve undefined behavior.
