CRTP 类中派生类型成员的使用 [英] Use of member of derived type in CRTP class

问题描述

我有一个奇怪的重复模板模式类和一个像这样的派生类:

I have a curiously recurring template pattern class and a derived class like so:

template<class Derived>
class A {
  typedef typename Derived::C D;
  D x;
};
class B : public A<B> {
public:
  class C { };
};

由于在编译器尝试定义 D 时未完全定义 B，因此无法编译.如何获得类似的结果，即 A 的成员属于 B 中定义的类型?还是我必须强制在 B 之外定义 C?

This fails to compile due to B not being fully defined when the compiler attempts to define D. How can I achieve a similar result, i.e. have members of A that are of a type defined in B? Or do I have to force C to be defined outside of B?

推荐答案

或者我是否必须强制在 B 之外定义 C?

Or do I have to force C to be defined outside of B?

是的，不幸的是你必须这样做.通常您可以在A 之前定义一个模板类，并将其专门用于B，包含C 类型.这允许您在 A 中使用它.

Yes, unfortunately you have to do this. Usually you can define a template class before A and specialize it for B, containing the C type. This allows you to use it in A.

template<typename T>
struct members;

template<class Derived>
class A {
  typedef typename members<Derived>::C D;
  D x;
};

template<>
struct members<class B> {
  class C { };
};
class B : public A<B> {
public:
};

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