typeid 作用于正在构造或析构的对象，而且该对象与当前构造或析构函数不属于同一个类或基类

示例：

struct V {
    virtual void foo();
};

struct A: virtual V {};
struct B: virtual V { B(V*, A*); };

struct C: A, B {
    C(): B((A*)this, this) {}
};

B::B(V* v, A* a) {
    typeid(*v);        // Well-defined, V is the base of B
    typeid(*a);        // Undefined behavior, A is not a base of B
}