c ++依赖注入模式和策略

Question

我正在尝试应用此处讨论的想法： http://www.drdobbs.com/cpp/dependency-injection-and-policy-based-de/228701135

代码或多或少如下：

class MyOtherClass 
{
  int m;
public:
  MyOtherClass(int x) : m(x) { }
  int DoSomething() { return m + 1; }
}; 

template<typename T>
    struct DefaultFactory 
    {
      T* Create() { return new T(); }
      template<typename U>
      T* Create(U x) { return new T(x); }
      // ...
      // add more versions of Create as needed
    };

typedef MyOtherClass DefaultField;
typedef DefaultFactory<DefaultField> DefaultFact;

template<typename Field = DefaultField, typename Factory = DefaultFact>
    class MyClass 
    {
    public:
      boost::scoped_ptr<Field> m;
      MyClass() : m(Factory().Create(42)) { }
      int DoSomething() { return m->DoSomething(); }
    };

int main()
{
    MyClass<DefaultField,DefaultFact> a;
    cout<<"result="<<a.DoSomething()<<endl;

    return 0;
}

它工作正常。然而，最后，通过将不同的策略分组到一个结构中，这个人用c ++ 11提出了一个更好的方法：

struct DefaultPolicy 
{
      
    typedef MyOtherClass Field;
      
    typedef DefaultFactory<Field> Factory;

    };

template<typename Policy = DefaultPolicy>
    
    class MyClass 
    {
       
    public:
      
          boost::scoped_ptr<Field> m;
      
          MyClass() : m(Factory().Create(42)) { }
      
          int DoSomething() { return m.DoSomething(); }
    
    };

但它对我不起作用。我尝试过以下方法：

struct DefaultPolicy 
{
    typedef MyOtherClass Field;
    typedef DefaultFactory<Field> Factory;
};

template<typename Policy = DefaultPolicy>
    class MyClass 
    {
    public:
      boost::scoped_ptr<Policy::Field> m;
      MyClass() : m(Policy::Factory().Create(42)) { }
      int DoSomething() { return m->DoSomething(); }
    };


int main()
{
    MyClass<DefaultPolicy> a;
    a.DoSomething();
    return 0;
}

我收到错误：

main.cxx:37:25: error: template argument for template type parameter must be a type; did you forget 'typename'?
      boost::scoped_ptr<Policy::Field> m;
                        ^
/include/boost/smart_ptr/scoped_ptr.hpp:41:16: note: template parameter is declared here
template<class T> class scoped_ptr // noncopyable

我在做什么？谢谢你的回复