使用不同的类存储对象成员变量的引用

Question

我正在尝试创建一个Container类,我可以通过使用该对象成员变量作为其标识符从容器中检索对象.但我得到一个编译错误,因为我试图存储指针(？)/引用对象成员变量

template <typename Object>
class Container
{
     private:
         template <typename dataType>
         dataType Object::* memberVariable; //  error here "data member 'memberVariable' cannot be a member template"

         template <typename dataType>
         std::map <dataType, Object*>     instanceVarMap;  // what would be more efficient; a Map or unordered_map? I heard that 
         std::map <unsigned int, Object*> instanceIntMap;  // ...unordered_maps use more memory & Maps are better when integers are the keys

    public;
        template <typename dataType>
        Collection( dataType Object::*nMemberVariable )
        {
            memberVariable = nMemberVariable;
        }

        template <typename dataType>
        Object* operator[] ( dataType nParam )
        {
             // do I need to check whether the element already exists or does
             // stl already do this for me?
             if ( instanceVarMap.find(nParam) == instanceVarMap.end() )
             {
                  return NULL;
             } 

             return instanceVarMap[ nParam ];
        }

        Object* operator[] ( unsigned int nParam )
         {
             if ( instanceIntMap.find(nParam) == instanceIntMap.end() )
             {
                  return NULL;
             } 

             return instanceIntMap[ nParam ];
         }

         void store( Object* o )
         {
               if ( o==NULL  ||  instanceMap.contains(o->memeberVariable) != instanceMap.end() ) { return; }

               instanceIntMap.insert( o->ID, o );
               instanceVarMap.insert( o->memberVariable, o ); // is this the correct way I get the objects member variable? o->memberVariable
         }
};


// I am doing this so I can use the class like so
struct FoodItem
{
    unsigned int ID;
    string name;
    double price;
};

Collection <FoodItem*> foodCol( &FoodItem::name );   

// after storing some FoodItems in foodCol, I can retreive a FoodItem either 
// by their ID or their name
FoodItem* f = foodCol["coffee"];  // find FoodItem by their member variable 'name'
FoodItem* g = foodCol[1];         // find FoodItem by their ID var

Answer 1

C++ 中不允许声明模板数据成员（不要与定义静态成员时使用的模板语法混淆）。实现这一目标的最佳方法是，

template <typename Object, typename dataType>  // <-- Add dataType here
class Container
{
     private:
         dataType Object::* memberVariable; // use 'dataType' simply
// ...
};