派生 class returns 已分配基础 class 实例

Question

下面的C++是合法的吗？ derived class 正在分配 base class 并将其转换为 derived。显然，如果派生 (C) 有成员，任何人通过返回的指针访问成员都会失败。但即使没有派生成员——这是合法的吗？ UB ?

struct B { int a; };
struct C : B { 
   static C* get() {
       return static_cast<C*>(new B);
   }
   enum { X, Y,};
};
int main(){
   C* c = C::get();
   return c->X;
}

Answer 1

是UB。标准摘录（强调我的）：

8.2.9 Static cast - bullet 11

A prvalue of type “pointer to cv1 B”, where B is a class type, can be converted to a prvalue of type “pointer to cv2 D”, where D is a class derived from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. If B is a virtual base class of D or a base class of a virtual base class of D, or if no valid standard conversion from “pointer to D” to “pointer to B” exists ([conv.ptr]), the program is ill-formed. The null pointer value is converted to the null pointer value of the destination type. If the prvalue of type “pointer to cv1 B” points to a B that is actually a subobject of an object of type D, the resulting pointer points to the enclosing object of type D. Otherwise, the behavior is undefined.

派生 class returns 已分配基础 class 实例

Derived class returns allocated base class instance

c++

inheritance

language-lawyer

static-cast

8.2.9 Static cast - bullet 11