C ++：对于CRTP，在派生类中定义的类在基类中不可访问 [英] C++: With CRTP, class defined in the derived class is not accessible in the base class

问题描述

这里是（简化的）基类：

Here is the (simplified) base class:

template <class T>
class SharedObject
{
protected:
    QExplicitlySharedDataPointer <typename T::Data> d;
};

这里是派生：

class ThisWontCompile : public SharedObject <ThisWontCompile>
{
private:
    friend class SharedObject;
    struct Data : public QSharedData
    {
        int id;
    };
};

是否有任何解决方法可从访问 ThisWontCompile :: Data SharedObject ？

Is there any workaround to access ThisWontCompile::Data from SharedObject? What exactly can and what exactly cannot be done with the derived object from the base object?

推荐答案

这实际上是不相关的无障碍和友谊，它与CRTP的使用相关。考虑下面的示例也会出现问题：

This actually isn't related to the accessibility and friendship, it's related to the use of CRTP. Consider the following example that also exhibits the problem:

template <class T>
struct Base
{
    typedef typename T::Data Data;
};

struct ThisWontCompile : public Base<ThisWontCompile>
{
    struct Data { };
};

问题是 ThisWontCompile 它用作 Base 的模板参数的时间，因此它只能用作 Base 中的不完整类型。

The problem is that ThisWontCompile is incomplete at the time it is used as a template argument to Base, so it can only be used as an incomplete type in Base.

有关您特定问题的一些解决方案，请参阅这个问题，特别是Martin的建议使用traits类，它基本上看起来像这样：

For a handful of solutions to your specific problem, consult the answers to this other question, especially Martin's recommendation to use a traits class, which would basically look like this:

// Base
template <typename T>
struct BaseTraits;

template <typename T>
struct Base
{
    typedef typename BaseTraits<T>::Data Data;
};

// Derived
struct Derived;

template <>
struct BaseTraits<Derived>
{
    struct Data { };
};

struct Derived : public Base<Derived>
{
};

typename BaseTraits< Derived> :: Data 可以在 Derived 和 Base 中使用。如果 Derived 本身是一个模板，您可以对traits类使用部分特化。

typename BaseTraits<Derived>::Data can be used in both Derived and in Base. If Derived is itself a template, you can use a partial specialization for the traits class.

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