Generic Interfaces (Visual C+)

Article
11/16/2012

The restrictions that apply to type parameters on classes are the same as those that apply to type parameters on interfaces (see Generic Classes (Visual C+)).

The rules that control function overloading are the same for functions within generic classes or generic interfaces.

Explicit interface member implementations work with constructed interface types in the same way as with simple interface types (see the following examples).

For more information on interfaces, see interface class.

[attributes] generic <class-key type-parameter-identifier[, ...]>
[type-parameter-constraints-clauses][accesibility-modifiers] interface class identifier [: base-list] {   interface-body} [declarators] ;

Remarks

attributes (optional)
Additional declarative information. For more information on attributes and attribute classes, see Attributes.
class-key
class or typename
type-parameter-identifier(s)
Comma-separated identifiers list.
type-parameter-constraints-clauses
Takes the form specified in Constraints
accessibility-modifiers (optional)
Accessibility modifiers (e.g. public, private).
identifier
The interface name.
base-list (optional)
A list that contains one or more explicit base interfaces separated by commas.
interface-body
Declarations of the interface members.
declarators (optional)
Declarations of variables based on this type.

Example

The following example demonstrates how to declare and instantiate a generic interface. In the example, the generic interface IList<ItemType> is declared. It is then implemented by two generic classes, List1<ItemType> and List2<ItemType>, with different implementations.

// generic_interface.cpp
// compile with: /clr
using namespace System;

// An exception to be thrown by the List when
// attempting to access elements beyond the
// end of the list.
ref class ElementNotFoundException : Exception {};

// A generic List interface
generic <typename ItemType>
public interface class IList {
   ItemType MoveFirst();
   bool Add(ItemType item);
   bool AtEnd();
   ItemType Current();
   void MoveNext();
};

// A linked list implementation of IList
generic <typename ItemType>
public ref class List1 : public IList<ItemType> {
   ref class Node {
      ItemType m_item;

   public:
      ItemType get_Item() { return m_item; };
      void set_Item(ItemType value) { m_item = value; };
      
      Node^ next;

      Node(ItemType item) {
         m_item = item;
         next = nullptr;
      }
   };

   Node^ first;
   Node^ last;
   Node^ current;

   public:
   List1() {
      first = nullptr;
      last = first;
      current = first;
   }

   virtual ItemType MoveFirst() {
      current = first;
      if (first != nullptr)
        return first->get_Item();
      else
         return ItemType();
   }

   virtual bool Add(ItemType item) {
      if (last != nullptr) { 
         last->next = gcnew Node(item);
         last = last->next;
      }
      else {
         first = gcnew Node(item);
         last = first;
         current = first;
      }
      return true;
   }

   virtual bool AtEnd() {
      if (current == nullptr )
        return true;
      else 
        return false;
   }

   virtual ItemType Current() {
       if (current != nullptr)
         return current->get_Item();
       else
         throw gcnew ElementNotFoundException();
   }

   virtual void MoveNext() {
      if (current != nullptr)
       current = current->next;
      else
        throw gcnew ElementNotFoundException();
   }
};

// An array implementation of IList
generic <typename ItemType>
ref class List2 : public IList<ItemType> {
   array<ItemType>^ item_array;
   int count;
   int current;

   public:

   List2() {
      // not yet possible to declare an
      // array of a generic type parameter
      item_array = gcnew array<ItemType>(256);
      count = current = 0;
   }

   virtual ItemType MoveFirst() {
      current = 0;
      return item_array[0];
   }

   virtual bool Add(ItemType item) {
      if (count < 256)
         item_array[count++] = item;
      else
        return false;
      return true;
   }

   virtual bool AtEnd() {
      if (current >= count)
        return true;
      else
        return false;
   }
   
   virtual ItemType Current() {
      if (current < count)
        return item_array[current];
      else
        throw gcnew ElementNotFoundException();
   }

   virtual void MoveNext() {
      if (current < count) 
         ++current;
      else
         throw gcnew ElementNotFoundException();
   }
};

// Add elements to the list and display them.
generic <typename ItemType>
void AddStringsAndDisplay(IList<ItemType>^ list, ItemType item1, ItemType item2) {
   list->Add(item1);
   list->Add(item2);
   for (list->MoveFirst(); ! list->AtEnd(); list->MoveNext())
     Console::WriteLine(list->Current());
}

int main() {
   // Instantiate both types of list.

   List1<String^>^ list1 = gcnew List1<String^>();
   List2<String^>^ list2 = gcnew List2<String^>();

   // Use the linked list implementation of IList.
   AddStringsAndDisplay<String^>(list1, "Linked List", "List1");
      
   // Use the array implementation of the IList.
   AddStringsAndDisplay<String^>(list2, "Array List", "List2");
}

Linked List List1 Array List List2

This example declares a generic interface, IMyGenIface, and two non-generic interfaces, IMySpecializedInt and ImySpecializedString, that specialize IMyGenIface. The two specialized interfaces are then implemented by two classes, MyIntClass and MyStringClass. The example shows how to specialize generic interfaces, instantiate generic and non-generic interfaces, and call the explicitly implemented members on the interfaces.

// generic_interface2.cpp
// compile with: /clr
// Specializing and implementing generic interfaces.
using namespace System;

generic <class ItemType>
public interface class IMyGenIface {
   void Initialize(ItemType f);
};

public interface class IMySpecializedInt: public IMyGenIface<int> {
   void Display();
};

public interface class IMySpecializedString: public IMyGenIface<String^> {
   void Display();
};

public ref class MyIntClass: public IMySpecializedInt {
   int myField;

public: 
   virtual void Initialize(int f) {
      myField = f;
   }

   virtual void Display() {
      Console::WriteLine("The integer field contains: {0}", myField);
   }    
};

public ref struct MyStringClass: IMySpecializedString {    
   String^ myField;

public:
   virtual void Initialize(String^ f) {
      myField = f;
    }

   virtual void Display() {
      Console::WriteLine("The String field contains: {0}", myField);
   }
};

int main() {
   // Instantiate the generic interface.
   IMyGenIface<int>^ myIntObj = gcnew MyIntClass();

   // Instantiate the specialized interface "IMySpecializedInt."
   IMySpecializedInt^ mySpIntObj = (IMySpecializedInt^) myIntObj;

   // Instantiate the generic interface.
   IMyGenIface<String^>^ myStringObj = gcnew MyStringClass();

   // Instantiate the specialized interface "IMySpecializedString."
   IMySpecializedString^ mySpStringObj = 
            (IMySpecializedString^) myStringObj;

   // Call the explicitly implemented interface members.
   myIntObj->Initialize(1234);
   mySpIntObj->Display();

   myStringObj->Initialize("My string");
   mySpStringObj->Display();
}

The integer field contains: 1234 The String field contains: My string

Generic Interfaces (Visual C+)

Remarks

Example

See Also

Other Resources

Additional resources