来自纯抽象基础的typedef继承 [英] typedef inheritance from a pure abstract base

问题描述

编辑：找到重复

我已经将一些问题代码缩小到最简单的工作情况，以说明如下：我的纯粹的抽象基类中的typedef不被派生类继承。在下面的代码中，我想继承 system_t typedef到 ConcreteTemplateMethod 中：

I've whittled down some problem code to the simplest working case to illustrate the following: my typedef in a pure abstract base class is not being inherited by the derived class. In the code below I'd like to inherit the system_t typedef into the ConcreteTemplateMethod:

#include <iostream>

// pure abstract template-method
template <typename T>   // T == Analyzer<U>
class TemplateMethod {
  public:
    typedef T system_t;

    virtual void fn (const system_t& t) const = 0;
};


template <typename T>
class Analyzer {
  public:
    void TemplatedAlgorithm (const TemplateMethod< Analyzer <T> >& a) const {
      printf ("Analyzer::TemplatedAlgorithm\n");
      a.fn(*this);  // run the template-method
    }

    void fn () const {
      printf ("Analyzer::fn\n");
    }
};


// concrete template-method
template <typename T>
class ConcreteTemplateMethod : public TemplateMethod < Analyzer<T> > {
  public:
    typedef Analyzer<T> system_t;

    virtual void fn (const system_t& t) const {
      printf ("ConcreteTemplateMethod::fn\n");
      t.fn(); // perform Analyzer's fn
    }
};

int main () {

  Analyzer <double> a;
  ConcreteTemplateMethod<double> dtm;
  a.TemplatedAlgorithm(dtm);

  return 0;
}

此代码根据预期进行编译和运行。在 ConcreteTemplateMethod 中，需要以下内容，删除时会导致编译器错误：

This code compiles and runs as expected. In the ConcreteTemplateMethod the following is required, and when removed causes compiler errors:

typedef Analyzer<T> system_t;

请注意， system_t 但是，在基类中，code> typedef '。为什么在继承时我必须包含另一个typedef？

Note that the system_t type is already typedef'ed in the base class, however. Why must I include another typedef when inheriting?

我意识到我可以在派生中定义 system_t 的类型名称 ConcreteTemplateMethod 通过使用 typename TemplateMethod<分析器< T> > :: system_t& ，但这有点冗长，我想避免每次我不得不将 typedef 继承并需要使用相同的 system_t 。有没有办法，我可以在基础中定义 TemplateMethod ？

I realize that I can qualify the typename of system_t in the derived ConcreteTemplateMethod by using typename TemplateMethod< Analyzer<T> >::system_t&, but that's a bit verbose, and I'd like to avoid having to re-typedef to the base everytime I inherit and need to use that same system_t. Is there a way around this that I can define in the base TemplateMethod?

推荐答案

你应该做

typedef typename TemplateMethod<X>::system_t system_t;

以继承typedef。 typedef不会自动继承（如果编译器是兼容的）。

to "inherit" typedef. typedef is not automatically inherited (if compiler is compliant).

如果您浏览堆栈溢出，则会在某处复制此问题。

if you look through stack overflow, there will be duplicate of this question somewhere.

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