检查交叉转换是否可能工作？ [英] Checking whether a cross-cast could possibly work?

问题描述

我知道使用 dynamic_cast 在类层次结构中执行交叉是合法的。例如，如果我有类如下所示的类：

I know that it's legal to use dynamic_cast to do a "cross-cast" across a class hierarchy. For example, if I have classes that look like this:

  A   B
   \ /
    C

如果我有一个 A * 在 C 类型的对象处，我可以使用

If I have an A* pointer that's pointing at an object of type C, then I can use

A* aPtr = /* ... something that produces a C* ... */
B* bPtr = dynamic_cast<B*>(aPtr);

以获取指向 B C 的对象。

我提到这个的原因是，编写上面的代码，可能编译器还没有看到 C 的定义，即使看到 A B 。这意味着编译器可能没有检测到 A 和 B 之间的任何类型的连接，但它仍然无论如何编译代码，因为有可能存在 C 类，并且在某些情况下 dynamic_cast 。

The reason I mention this is that at the time that I write the above code, it's possible that the compiler has not yet seen the definition of C even though it's seen A and B. This means that it's possible that the compiler does not detect any sort of connection between A and B, but it still has to compile the code anyway because it's possible for a class like C to exist and for the dynamic_cast to succeed under some circumstance.

问题是，这意味着我可以不小心地交叉到一个错误类型的对象。假设我有类似这样的类：

The problem is that this means that I can accidentally cross-cast to an object of the wrong type. Suppose that I have classes that look like this:

A   B    D
 \ /   
  C

这里， D 是一些随机无关的类。如果我尝试写这样的：

Here, D is some random unrelated class. If I try writing something like this:

A* aPtr = /* ... get a C* pointer ... */
D* dPtr = dynamic_cast<D*>(aPtr);

然后，这个 dynamic_cast ，因为没有可能的方法连接 A 和 D 。如果我使用 D 意外，因为我打算使用 B ，编译器会给我没有任何迹象表明，有一个无意义的转换。

Then this dynamic_cast will always fail at runtime, since there's no possible way to connect A and D. If I'm using D accidentally because I meant to use B, the compiler will give me no indication whatsoever that I have a meaningless cast.

我的问题是：有一些方法，我可以让编译器警告我，运行时总是会失败？ / strong>我会很高兴与语言级解决方案或一些编译器设置任何主要的编译器，可以检测到这一点。如果有一个外部工具，这也很好;我只想知道是否可以捕获这类错误。

My question is: is there some way that I can get the compiler to warn me that the cast will always fail at runtime? I'd be happy with a language-level solution or some compiler setting for any major compiler that could detect this. If there's an external tool, that's fine as well; I just want to know if it's possible to catch this class of errors.

推荐答案

这是不可能在编译时检测到。引入关系的类 C 可以在尚未编写的动态可加载库中找到，而编译器无法证明。

It's not possible to detect this at compile-time. The class C that introduces the relationship could be found in a dynamically loadable library that hasn't even been written yet, and the compiler can't prove otherwise.

可能有一些例外。如果 A 只有私有构造函数（或私有析构函数），那么编译器可以确定没有新的子类不会被命名为朋友 A 。

There may be a few exceptions though. If A has only private constructors (or a private destructor) then the compiler can be certain that there will be no new subclasses that aren't named as friends by A.

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