C++编程中使用设计模式中的policy策略模式的实例讲解

//policy模式的常见使用实例smartptr，
template
<
  class T,
  template <class> class CheckingPolicy,
  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_;
};

template <class T>
struct OpNewCreator
{
  static T* Create()
  {
   return new T;
  }
};

template <class T>
struct MallocCreator
{
  static T* Create()
  {
   void* buf = std::malloc(sizeof(T));
   if (!buf) return 0;
   return new(buf) T;
  }
};

template <class T>
struct PrototypeCreator
{
  PrototypeCreator(T* pObj = 0)
   :pPrototype_(pObj)
  {}
  T* Create()
  {
   return pPrototype_ ? pPrototype_->Clone() : 0;
  }
  T* GetPrototype() { return pPrototype_; }
  void SetPrototype(T* pObj) { pPrototype_ = pObj; }
private:
  T* pPrototype_;
};

//test class
class Widget
{
};

//调用方法一：
template <class CreationPolicy>
class WidgetManager : public CreationPolicy
{  
};
void main()
{

typedef WidgetManager< OpNewCreator<Widget> > MyWidgetMgr;


}

//调用方法二：因为一般Manager是特定于某一类的class，所以在Manager中就指定要处理的class类型。
template <template <class Created> class CreationPolicy>
class WidgetManager : public CreationPolicy<Widget>
{  
};
void main()
{
  // Application code
typedef WidgetManager<OpNewCreator> MyWidgetMgr;
}

template <class T>
struct OpNewCreator
{
  static T* Create()
  {
  return new T;
  }
protected:
  ~OpNewCreator() {}
};

template <template <class> class CreationPolicy>
class WidgetManager : public CreationPolicy<Widget>
{ 
  void SwitchPrototype(Widget* pNewPrototype)
  {
   CreationPolicy<Widget>& myPolicy = *this;
   delete myPolicy.GetPrototype();
   myPolicy.SetPrototype(pNewPrototype);
  }
};