How to pass arguments to a Policy's constructor?

Question

In code:

template<class T>
struct FactorPolicy
{
    T factor_;
    FactorPolicy(T value):factor_(value)
    {
    }
};

template<class T, template<class> class Policy = FactorPolicy>
struct Map
{
};

int _tmain(int argc, _TCHAR* argv[])
{
        Map<int,FactorPolicy> m;//in here I would like to pass a double value to a  
 //FactorPolicy but I do not know how.  
        return 0;
    }

Edited [for Mark H]

template<class T, template<class> class Policy = FactorPolicy>
struct Map : Policy<double>
{
    Map(double value):Policy<double>(value)
    {
    }
};

Answer 1

One way is to provide member function templates that take a template arg to use with the policy. For example:

template<class T, template<class> class Policy = FactorPolicy> 
struct Map 
{
  template <typename V>
  void foo(const Policy<V> &p)
  {
  }
}; 

Then, in main:

Map<int,FactorPolicy> m;

m.foo(FactorPolicy<double>(5.0));

Another possibility is to specify it as part of Map template instantiation, by adding a third template arg to Map:

template<class T, template<class> class Policy = FactorPolicy, 
         class V = double> 
struct Map 
{
  void foo(const V &value)
  {
    Policy<V> policy(value);
  }
}; 

Then:

Map<int,FactorPolicy,double> m;

m.foo(5.0);

Answer 2

If you were to pass a double, it would require that the type argument of FactorPolicy be double inside Map, unless you make the FactorPolicy constructor to accept a double. I don't think that is what you want though. You must inform Map that Policy needs a double, so first add that type argument. Secondly, I think you will need to forward the actual double value from the Map constructor.

template<class T, typename U, template<class> class Policy = FactorPolicy >
struct Map {
    Map(U val) {
        policy_ = new Policy<U>(val);
    }

    Policy<U>* policy_;
};

int main()
{
    Map<int, double, FactorPolicy> m(5.63);
    return 0;
}