C++ 不能通过虚基 A 从基 A 转换为派生类型 B

问题描述

我有三个班级:

class A {};

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

class D: public B, public C {};

尝试从 A* 静态转换到 B* 我收到以下错误:

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

推荐答案

为了理解强制转换系统，您需要深入研究对象模型.

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

简单层次模型的经典表示是包含:如果 B 派生自 A 那么 B 对象实际上将包含一个 A 子对象以及它自己的属性.

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

使用此模型，向下转换是通过编译时已知的偏移量进行简单的指针操作，这取决于 B 的内存布局.

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

这就是 static_cast 的作用:静态转换被称为静态转换，因为转换所需的计算是在编译时完成的，无论是指针算术还是转换 (*).

This is what static_cast does: 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 (*).

然而，当 virtual 继承开始时，事情往往变得有点困难.主要问题是使用 virtual 继承所有子类共享子对象的相同实例.为了做到这一点，B 将有一个指向 A 的指针，而不是一个 A 本身，而 Acode> 基类对象将在 B 之外实例化.

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 the same instance of the subobject. In order to do that, B will have a pointer to an A, instead of an A proper, and the A base class object will be instantiated outside of B.

因此，在编译时不可能推导出必要的指针算法:这取决于对象的运行时类型.

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.

每当有运行时类型依赖项时，您都需要 RTTI(运行时类型信息)，而利用 RTTI 进行强制转换是 dynamic_cast 的工作.

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.

总结:

• 编译时向下转换:static_cast
• 运行时向下转换:dynamic_cast

另外两个也是编译时强制转换，但它们非常具体，以至于很容易记住它们的用途……而且它们很臭，所以无论如何最好不要使用它们.

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.

(*) 正如@curiousguy 在评论中所指出的，这仅适用于向下转换.static_cast 允许向上转换，而不管是虚拟继承还是简单继承，尽管这样转换也是不必要的.

(*) 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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