c++ - dynamic_cast<> 是否仅限于沿继承层次结构的直接转换？

Question

代码

struct A { };           // virtual details there, but left out
struct B { };           // virtual details there, but left out
struct C : A, B { };    // virtual details there, but left out

C c;
B& b = c;
A& a = dynamic_cast<A&>( b );  // will this cast succeed at run-time?

请注意，为了使代码简单，我省略了虚拟细节。

如果 dynamic_cast<> 仅限于沿继承层次结构的直接转换，那么我希望上面的代码在运行时失败(因为 B& 与 A& 无关)。

但是，如果它更通用/更灵活，代码应该可以工作，因为所引用对象的真实类型是 C 类型(并且 C 可以被称为 B& 或 A&)。

C++ 规范对这种情况有何看法？

Answer 1

dynamic_cast<T>(v)可能会下降到最派生的对象，其中 v是一个子对象，然后向上(可能是不同的分支)找到一个明确的基类 T是指针或引用。来自标准:

5.2.7/8 If C is the class type to which T points or refers, the run-time check logically executes as follows:

• If, in the most derived object pointed (referred) to by v, v points (refers) to a public base class subobject of a C object, and if only one object of type C is derived from the subobject pointed (referred) to by v the result points (refers) to that C object.
• Otherwise, if v points (refers) to a public base class subobject of the most derived object, and the type of the most derived object has a base class, of type C, that is unambiguous and public, the result points (refers) to the C subobject of the most derived object.
• Otherwise, the run-time check fails.