c++ - 动态转换返回 null

Question

我遇到了 dynamic_cast 返回 null 的问题。我正在尝试从 Card* 向下转换为 GCard*，其中 GCard 是 Card 的派生类(它是多态的)。

下面是失败的代码示例。如果有人能告诉我失败的原因，我将不胜感激。

class Card
{
public:
   Card(Rank, Suit);
   virtual ~Card();
   void SetSelected(bool selected);

   Suit mSuit;
   Rank mRank;
   bool mSelected;
};

class GCard : public Card
{
public:
   GCard(Rank, Suit);
   virtual ~GCard();

   void LoadTexture();
   void SetDepth(int);

   sf::Sprite mSprite;
   int mDepth;
private:
  sf::Texture mTexture;
};

int main(void)
{
   Card c(Two, clubs);
   Card* pC = &c;

   GCard* pG = dynamic_cast<GCard *>(pC);
   if (!pG)
   {
      cout << "Dynamic cast returned null"; // This line is being executed
   }
   return 0;
}

Answer 1

dynamic_cast 正在返回 nullptr 因为您的对象很明显不是 GCard。动态转换并不是将对象转换成它们不是的事物的神秘魔法机制。它所做的只是允许您使用一个对象作为子类类型，如果它已经是该类型。

以下将按您的预期运行:

int main(void)
{
   GCard c(Two, clubs); //Note the key change, the object **IS-A** GCard
   Card* pC = &c;

   GCard* pG = dynamic_cast<GCard *>(pC);
   if (!pG)
   {
      cout << "Dynamic cast returned null"; // This line would no longer be executed
   }
   return 0;
}