C++ Undefined Behavior Access to nonexistent member through pointer to member


Example

When accessing a non-static member of an object through a pointer to member, if the object does not actually contain the member denoted by the pointer, the behavior is undefined. (Such a pointer to member can be obtained through static_cast.)

struct Base { int x; };
struct Derived : Base { int y; };
int Derived::*pdy = &Derived::y;
int Base::*pby = static_cast<int Base::*>(pdy);

Base* b1 = new Derived;
b1->*pby = 42; // ok; sets y in Derived object to 42
Base* b2 = new Base;
b2->*pby = 42; // undefined; there is no y member in Base