使用模板模板类参数作为参数 [英] Use template template class argument as parameter

问题描述

现代C ++设计提供以下示例：

Modern C++ Design gives the following example:

template <class T> struct EnsureNotNull
{
    static void Check(T*& ptr)
    {
      if (!ptr) ptr = GetDefaultValue();
    }
};

template
<
   class T,
   template <class> class CheckingPolicy = EnsureNotNull,
   template <class> class ThreadingModel
>
class SmartPtr
  : public CheckingPolicy<T>
  , public ThreadingModel<SmartPtr>
{
...
  T* operator->()
  {
    typename ThreadingModel<SmartPtr>::Lock guard(*this);
    CheckingPolicy<T>::Check(pointee_);
    return pointee_;
  }
private:
  T* pointee_;
};

我想不出怎样的ThreadingModel模板会以某种形式构造，它可以接受的SmartPtr为参数，在我看来，一些疯狂的递归将要发生。这是怎么可能的？

I couldn't figure how ThreadingModel template would be constructed in a fashion that It could accept SmartPtr as parameter, in my mind some crazy recursion is going to happen. How can this be possible?

编辑：

我试过Potatoswatter （sorry lol）comment：

I've tried Potatoswatter (sorry lol) comment:

template <class SmartPtr> struct SingleThreadingModel
{
  class Lock
  {
    public: 
      Lock(SmartPtr&)
      {
      }
  };
};

但它没有工作。

这里是gcc给我的错误：

here is the error that gcc is giving me:

main.cpp:28:35: error: type/value mismatch at argument 1 in template parameter list for ‘template<class> class ThreadingModel’
main.cpp:28:35: error:   expected a type, got ‘SmartPtr’

推荐答案

您尝试将 SmartPtr 作为类型参数传递到 ThreadingModel 。 的SmartPtr 不过是一个模板，而不是一个具体的类型，的注入的类名是不是在继承列表中。

You are trying to pass SmartPtr as a template type argument to ThreadingModel. SmartPtr however is a template, not a concrete type, and the injected class-name is not available in the inheritance list.

另外请注意，你不能只使用默认参数在任意位置（§14.1/ 11 的）模板参数：

Also note that you can't just use default arguments for template parameters in arbitrary positions (§14.1/11):

如果一个模板参数有一个默认的模板参数，其随后所有的模板参数应提供一个默认的模板参数。

If a template-parameter has a default template-argument, all subsequent template-parameters shall have a default template-argument supplied.

修正这些问题的代码：

template
<
  class T,
  template <class> class ThreadingModel,
  template <class> class CheckingPolicy = EnsureNotNull
>
class SmartPtr
  : public CheckingPolicy<T>
  , public ThreadingModel<SmartPtr<T, ThreadingModel, CheckingPolicy> > 
//                         ^ .... now passing a concrete class .... ^
{
    T* operator->() {
        // the following use of SmartPtr is fine as it is the injected class-name:
        typename ThreadingModel<SmartPtr>::Lock guard(*this);
        // ...
    }
};

请注意，现代C ++设计是一本出色的书， t替换一本关于 Vandevoorde / Josuttis 等模板的好书。

Note that while Modern C++ Design is an excellent book, it can't replace a good basic book on templates like Vandevoorde/Josuttis.

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