菱形继承:为什么向下转换不起作用? [英] Diamond shape inheritance: why downcasting is not working?

问题描述

提前致谢.我正在DevC ++环境中从事C ++项目.
我观察到的是，在菱形继承的情况下，向上转换为基类然后向下转换为派生类是不可能的吗?
你能告诉我为什么会这样吗?

以下是我正在运行的代码.

Hi,

Thanks in advance. I am working on C++ project in DevC++ environment.
What I have observed is that in case of diamond-shape inheritance, upcasting to base class and then downcasting to derived class back is not possible?
Could you please tell me why is this so?

Following is the code which I am running.

class DiamondBase
{
      public:
             int DiamondBaseData1;
};

// virtual base class will share base-class-subobject with DiamondDerived2 class
class DiamondDerived1: virtual public DiamondBase
{
      public:
             int DiamondBaseData2;

             void DiamondDerived1Fun(void){};

};

// virtual base class will share base-class-subobject with DiamondDerived1 class
class DiamondDerived2: virtual public DiamondBase
{
      public:
             int DiamondBaseData3;

             void DiamondDerived2Fun(void){};
};

class VirtualBaseDerived: public DiamondDerived1, public DiamondDerived2
{
      public:
             int DiamondBaseData4;

};

void Main()
{
   DiamondDerived2 obj1;
   DiamondBase* pBase = &obj1;  // upcasting of DiamondDerived2

  ((DiamondDerived1*)pBase)->DiamondDerived1Fun();  // error: cannot convert from base to derived via "virtual-base"

  ((DiamondDerived2*)pBase)->DiamondDerived2Fun(); // error: cannot convert from base to derived via "virtual-base"

}

推荐答案

签出
http://stackoverflow.com/questions/3747066/c-cannot-convert-from-base-a-to-derived-type-b-via-virtual-base-a [dynamic_cast.

Check out
http://stackoverflow.com/questions/3747066/c-cannot-convert-from-base-a-to-derived-type-b-via-virtual-base-a[^]
Note that to use dynamic_cast, you must have ''polymorphic classes'', i. e. your classes should have at least one virtual method. Try defining, say, a virtual destructor for each of your classes. Then use dynamic_cast.

要正确地转换为具有虚拟基类的类，某些
有关对象实际类型的运行时信息
可能需要强制转换.

如果我们使用 dynamic_cast ，就可以实现这一点.另一方面，您正在static_cast中尝试的常规转换没有这样的功能.

To perform correct cast to a class with virtual base classes some
runtime information about the actual type of the object being
casted may be required.

And this can be acheived if we use dynamic_cast. On the other hand, the normal casting you are trying in static_cast which do not have a facility like this.

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