C ++不能通过虚拟基本A从基本A转换为派生类型B. [英] C++ cannot convert from base A to derived type B via virtual base A

问题描述

我有三个类：

  class A {}; 
 
 class B：virtual public A {}; 
 class C：virtual public A {}; 
 
 class D：public B，public C {}; 
 

尝试从A *到B *的静态转换我得到以下错误：

 无法通过虚拟基础A从基础A转换为派生类型B 
  




解决方案

为了理解你需要在对象模型中潜入的转换系统。

<一个简单的层次模型的经典表示是遏制：如果 B 源自 A ，则 B 对象将事实上包含一个 A 子对象及其自己的属性。

使用这个模型，downcasting是一个简单的指针操作，通过在编译时已知的偏移量，它取决于 B 的内存布局。

这是 static_cast ：静态转换被称为静态，因为对于转换所需的计算是在编译时完成的，无论是指针运算还是转换*）。

但是，当 virtual 继承时，事情往往变得更加困难。主要的问题是使用 virtual 继承所有子类共享子对象的同一个实例。为了这样做， B 将有一个指向 A 的指针，而不是 A 正确，并且 A 基类对象将在 B 之外进行实例化。 / p>

因此，编译时不可能推导出必要的指针运算：它取决于对象的运行时类型。

每当有一个运行时类型依赖，你需要RTTI（运行时类型信息），使用RTTI的转换是 dynamic_cast 的工作。

总结：

• 编译时下载： static_cast


• 运行时下载： dynamic_cast

其他两个也是编译时强制转换，但它们是如此具体，很容易记住它们是什么...和他们是臭的，所以最好不要使用它们在所有反正。

（*）正如@curiousguy在注释中所指出的，这只适用于downcasting。一个 static_cast 允许向上转换，无论虚拟或简单的继承，虽然那么转换也是不必要的。

I have three classes:

class A {};

class B : virtual public A {};
class C : virtual public A {};

class D: public B, public C {};

Attempting a static cast from A* to B* I get the below error:

cannot convert from base A to derived type B via virtual base A

解决方案

In order to understand the cast system you need to dive in the object model.

The classic representation of a simple hierarchy model is containment: that if B derives from A then the B object will in fact contain a A subobject alongside its own attributes.

With this model, downcasting is a simple pointer manipulation, by an offset known at compilation time which depends from the memory layout of B.

This is what static_cast do: a static cast is dubbed static because the computation of what is necessary for the cast is done at compile-time, be it pointer arithmetic or conversions (*).

However, when virtual inheritance kicks in things tend to become a bit more difficult. The main issue is that with virtual inheritance all subclasses share a same instance of the subobject. In order to do that, B will have a pointer to a A, instead of a A proper, and the A base class object will be instantiated outside of B.

Therefore, it's impossible at compilation time to be able to deduce the necessary pointer arithmetic: it depends on the runtime type of the object.

Whenever there is a runtime type dependency, you need RTTI (RunTime Type Information), and making use of RTTI for casts is the job of dynamic_cast.

In summary:

• compile-time downcast: static_cast
• run-time downcast: dynamic_cast

The other two are also compile-time casts, but they are so specific that it's easy to remember what they are for... and they are smelly, so better not use them at all anyway.

(*) As noted by @curiousguy in the comments, this only holds for downcasting. A static_cast allows upcasting regardless of virtual or simple inheritance, though then the cast is also unnecessary.

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