如何实现编译时检查，一个downcast在一个CRTP是有效的？ [英] How to implement a compile-time check that a downcast is valid in a CRTP?

问题描述

我有一个普通的旧CRPT（请不要因为访问限制而分心 - 问题不在于他们）：

I have a plain old CRPT (please don't get distracted by access restrictions - the question is not about them):

 template<class Derived>
 class Base {
     void MethodToOverride()
     {
        // generic stuff here
     }
     void ProblematicMethod()
     {
         static_cast<Derived*>(this)->MethodToOverride();
     } 
 };

如下：

 class ConcreteDerived : public Base<ConcreteDerived> {
     void MethodToOverride()
     {
        //custom stuff here, then maybe
        Base::MethodToOverride();
     }
 };

现在 static_cast 我需要一个向下转换（不是向上转换），所以我必须使用显式转换。在所有合理的情况下，转换将是有效的，因为当前对象确实是派生类。

Now that static_cast bothers me. I need a downcast (not an upcast), so I have to use an explicit cast. In all reasonable cases the cast will be valid since the current object is indeed of the derived class.

但是如果我改变层次结构而转换现在变得无效了怎么办？

But what if I somehow change the hierarchy and the cast now becomes invalid?

我可以在某种程度上执行编译时检查，在这种情况下显式的downcast是有效的吗？

May I somehow enforce a compile-time check that an explicit downcast is valid in this case?

推荐答案

在编译期，你只能检查静态类型，这是 static_cast 已经做了。

At compile-time you can only check the static types, and that's what static_cast already does.

给定一个 Base * ，它只是，并且只能在运行时知道其动态类型，也就是说，它是否实际指向 ConcreteDerived 或其他东西。所以如果你想检查这个，它必须在运行时完成（例如使用 dynamic_cast ）

Given a Base*, it is only, and can only be, known at run-time what its dynamic type is, that is, whether it actually points to a ConcreteDerived or something else. So if you want to check this, it has to be done at runtime (for example by using dynamic_cast)

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