向下转换使用 dynamic_cast returns null

Question

我正在尝试将基础 class 对象转换为具有 dynamic_cast 的派生 class 对象，但 dynamic_cast returns 为空。是否可以使用 dynamic_cast 进行向下转换？

struct A {
  virtual ~A() {}
};

struct B : A {};


int main()
{
    A* a = new A();

    B* b = dynamic_cast<B*>(a);
    if(b){
      std::cout << "b has value" << std::endl;
    }else{
      std::cout << "no value" << std::endl;
    }
}

此代码打印出 "no value"。

Answer 1

因为a实际上指向A，而不是B，所以dynamic_cast会失败。

Is it possible to downcast using dynamic_cast?

是的，你可以，例如如果 a 恰好指向 B，

A* a = new B;
B* b = dynamic_cast<B*>(a);

见http://en.cppreference.com/w/cpp/language/dynamic_cast

5) If expression is a pointer or reference to a polymorphic type Base, and new_type is a pointer or reference to the type Derived a run-time check is performed:

a) The most derived object pointed/identified by expression is examined. If, in that object, expression points/refers to a public base of Derived, and if only one subobject of Derived type is derived from the subobject pointed/identified by expression, then the result of the cast points/refers to that Derived subobject. (This is known as a "downcast".)

...

c) Otherwise, the runtime check fails. If the dynamic_cast is used on pointers, the null pointer value of type new_type is returned. If it was used on references, the exception std::bad_cast is thrown.

Answer 2

即每个设计。 dynamic_cast 用于测试指向基 class 对象的指针是否实际指向子 class 对象。如果它是一个 subclass 对象，那么 dynamic_cast 会给你一个有效的指针，如果不是，你就得到一个 nullptr.

由于您创建了 A class 对象，并且 A 不是 B 的子 class，因此 dynamic_cast通常返回空指针。

向下转换使用 dynamic_cast returns null

Downcasting using dynamic_cast returns null

c++

dynamic-cast

downcast