c++ - 如何将参数传递给策略的构造函数？

Question

在代码中:

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;
    }

已编辑 [for Mark H]

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

Answer 1

一种方法是提供成员函数模板，该模板采用模板 arg 与策略一起使用。例如:

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

然后，在主要部分:

Map<int,FactorPolicy> m;

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

另一种可能性是通过向 Map 添加第三个模板参数，将其指定为 Map 模板实例化的一部分:

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

然后:

Map<int,FactorPolicy,double> m;

m.foo(5.0);