Sure: 7.1.5.1/4: Except that any class member declared mutable (7.1.1) can be modified, any attempt to modify a const object during its lifetime (3.8) results

Sure: 7.1.5.1/4: Except that any class member declared mutable (7.1.1) can be modified, any attempt to modify a const object during its lifetime (3.8) results in undefined behavior. const_cast is not undefined behavior, but modifying a const object is.