policy

template <class Apolicy>

class Host

{

  Apolicy direct_policy_use;

  Apolicy <SomeInternalType> InternalClone;  // Problem 1: Can't do this

};

template <class T, template <class T> class Apolicy>

class Host2

{

  Apolicy <T> common_use;  

  Apolicy <SomeInternalType> InternalClone;  

  // Can do this now but 

  // Problem 2: policies that require more than one type parameter can't participate.

};

Solution and Sample Code[edit]

A member template struct (called rebind) is used to pass a different type parameter to the policy class template. For example,

template <typename T>

class NiftyAlloc

{

  public:

    template <typename Other>

    struct rebind // The Policy Clone idiom

    { 

       typedef NiftyAlloc <Other> other;

    };

    //...

};

template <typename T, class Alloc = NiftyAlloc <T> >

class Vector 

{

  public:

    typedef typename Alloc::template rebind<long>::other ClonePolicy;

    // Here, Alloc may not be a template class or a parametrized instantiation of

    // a class that takes unknown number of type parameters.

}; 

#include <iostream>

#include <vector>

#include <list>

using namespace std;

class SliderWidget

{

public:

 SliderWidget()

 {

  std::cout<<"Slider Widget created"<<std::endl;

 }

};

class BoxWidget

{

public:

 BoxWidget()

 {

  std::cout<<"Box Widget created"<<std::endl;

 }

};

template <class T>

class OpNewCreateor

{

public:

 static T* create()

 {

  return new T;

 }

protected:

 ~OpNewCreateor(){}

};

template <class T>

class MallocCreator

{

public:

 static T* create()

 {

  void * buf = std::malloc(sizeof(T));

  if(!buf) return 0;

  return new(buf) T;

 }

protected:

 ~MallocCreator(){}

};

template <class T>

class PrototypeCreator

{

public:

 PrototypeCreator(T* pObj = 0)

  :pPrototype(pObj)

 {

 }

 T* create()

 {

  return pPrototype ? pPrototype->clone() : 0;

 }

 T* getPrototype(){return pPrototype;}

 void setPrototype(T*pObj){pPrototype = pObj;}

protected:

 ~PrototypeCreator(){}

private:

 T* pPrototype;

};

template<class T>

class ContainerVec

{

public:

 void push(T* widget)

 {

  mVecContainer.push_back(widget);

 }

 ~ContainerVec(){}

private:

 std::vector<T*> mVecContainer;

};

template <class T>

class ContainerList

{

public:

 void push(T* widget)

 {

  mListContainer.insert(widget);

 }

 

 ~ContainerList(){}

private:

 std::list<T*> mListContainer;

};

template <

 class T,

 template<class > class CreationPolicy = MallocCreator,

 template<class > class Container = ContainerVec

>

class WidgetManager :public CreationPolicy<T>     

{

public:

 typedef CreationPolicy<T> BaseClass;

 T* create()

 {

  T* tmp =  BaseClass::create();

  mContainer.push(tmp);

  return tmp;

 }

private:

 Container<T> mContainer;

};

typedef WidgetManager<BoxWidget,OpNewCreateor,ContainerVec> BoxWidgetManager;

typedef WidgetManager<SliderWidget,OpNewCreateor,ContainerList> SliderWidgetManager;

 

int main()

{

 BoxWidgetManager boxWidgetManager;

 BoxWidget * boxWidget = boxWidgetManager.create();

  cout << typeid(BoxWidgetManager).name() << endl; 

 

 system( "pause");

}