将指针从基本类型转换为子类型 [英] Casting pointer from base type to child type

问题描述

我正在为我的项目构建一个简单的游戏设计。我有以下类：

I'm building a simple game design for a project of mine. I have the following classes:

class Character
{
public: 
   virtual void Display();
   virtual void SetParameters( char* param, ... );
};

class NonPlayableCharacter : public Character
{

public:
   virtual void Display();
   virtual void SetParameters( char* paaram, ... );
   int GetNPCState();
}

然后我有一堆从Character或NonPlayableCharacter派生的类。我这样定义：

And then I have a bunch of classes that derive either from Character or NonPlayableCharacter. I define it like so:

std::vector<Character*> _allChar;

我的问题是，在任何给定的时间，我想对一个元素执行一些操作矢量。所以从向量中获取一个元素我不能直接调用方法 GetNPCState（），因为向量中的元素是Character *类型。这样做：

My problem is that at any given time I would want to perform some operation on one of the element of the vector. So getting an element out of the vector I can't directly call the method GetNPCState() because the element in the vector are of Character* type. So doing this:

_allChar[0]->GetNPCState();

无法使用。所以我试着用着名的dynamic_cast：

doesn't work. So I tried doing it with the famous dynamic_cast:

NonPlayableCharacter* test = dynamic_cast<NonPlayableCharacter*>(_allChar[0]);
test->GetNPCState();

最后一次尝试的问题是 GetNPCState（）崩溃，因为对象是null，我知道一个事实（通过调试）_allChar [0]不为null。

The problem with this last attempt is that GetNPCState() crashes because the object is null, and I know for a fact (via debugging) that _allChar[0] isn't null.

推荐答案

在C ++中有多种类型的转换（4），其中2个是感兴趣的：

There are several types of casts in C++ (4), of which 2 are of interests here:

• static_cast dynamic_cast 在运行时检查您是否猜到了



• static_cast assumes that you know what you are doing
• dynamic_cast checks, at runtime, that you "guessed" right

>注意：简化，因为 dynamic_cast 也允许涉及虚拟基础的交叉转型和转换。

Note: simplified, as dynamic_cast also allows cross-casts and casts involving virtual bases.

根据目标的性质，有 dynamic_cast 的3个版本：

There are 3 versions of dynamic_cast, really, depending on the nature of the target:

<如果目标是引用（即 dynamic_cast ），则如果检查失败 dynamic_cast throws a std :: bad_cast exception

• 如果目标是指针（即 dynamic_cast <如果检查失败，则 dynamic_cast 返回一个空指针


• 最后，作为特殊情况，如果目标是 void * ，则 dynamic_cast ，而是返回完整对象的地址

• if the target is a reference (ie dynamic_cast<T&>(u)), then if the check fails dynamic_cast throws a std::bad_cast exception
• if the target is a pointer (ie dynamic_cast<T*>(p)), then if the checks fails dynamic_cast returns a null pointer
• finally, as a special case, if the target is void*, then dynamic_cast instead returns the address of the full object

在这种情况下，您可以：

In this case, you may:

• 从 dynamic_cast< NonPlayableCharacter *>（_ allChar [0]） - > getNPCState（）切换到
  dynamic_cast< NonPlayableCharacter& >（* _ allChar [0]）。getNPCState（），并让异常传播


• 测试cast的结果（ NonPlayableCharacter * test 此处）非空性

• switch from dynamic_cast<NonPlayableCharacter*>(_allChar[0])->getNPCState() to dynamic_cast<NonPlayableCharacter&>(*_allChar[0]).getNPCState(), and let the exception propagates
• test the result of the cast (NonPlayableCharacter* test here) for non-nullity

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