D Programming Language
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# Interfaces

    InterfaceDeclaration:
        interface Identifier ;
        interface Identifier BaseInterfaceListopt AggregateBody
        InterfaceTemplateDeclaration

    BaseInterfaceList:
        : Interfaces

Interfaces describe a list of functions that a class that inherits from the interface must implement. A class that implements an interface can be converted to a reference to that interface.

Some operating system objects, like COM/OLE/ActiveX for Win32, have specialized interfaces. D interfaces that are compatible with COM/OLE/ActiveX are called COM Interfaces.

C++ Interfaces are another form of interfaces, meant to be binary compatible with C++.

Interfaces cannot derive from classes; only from other interfaces. Classes cannot derive from an interface multiple times.

    interface D
    {
        void foo();
    }

    class A : D, D	// error, duplicate interface
    {
    }

An instance of an interface cannot be created.

    interface D
    {
        void foo();
    }

    ...

    D d = new D();		// error, cannot create instance of interface

Virtual interface member functions do not have implementations. Interfaces are expected to implement static or final functions.

    interface D
    {
        void bar() { }	// error, implementation not allowed
        static void foo() { } // ok
        final void abc() { } // ok
    }

Classes that inherit from an interface may not override final or static interface member functions.

    interface D {
        void bar();
        static void foo() { }
        final void abc() { }
    }

    class C : D {
        void bar() { } // ok
        void foo() { } // error, cannot override static D.foo()
        void abc() { } // error, cannot override final D.abc()
    }

All interface functions must be defined in a class that inherits from that interface:

    interface D
    {
        void foo();
    }

    class A : D
    {
        void foo() { }	// ok, provides implementation
    }

    class B : D
    {
        int foo() { }	// error, no void foo() implementation
    }

Interfaces can be inherited and functions overridden:

    interface D
    {
        int foo();
    }

    class A : D
    {
        int foo() { return 1; }
    }

    class B : A
    {
        int foo() { return 2; }
    }

    ...

    B b = new B();
    b.foo();		// returns 2
    D d = cast(D) b;	// ok since B inherits A's D implementation
    d.foo();		// returns 2;

Interfaces can be reimplemented in derived classes:

    interface D
    {
        int foo();
    }

    class A : D
    {
        int foo() { return 1; }
    }

    class B : A, D
    {
        int foo() { return 2; }
    }

    ...

    B b = new B();
    b.foo();		// returns 2
    D d = cast(D) b;
    d.foo();		// returns 2
    A a = cast(A) b;
    D d2 = cast(D) a;
    d2.foo();		// returns 2, even though it is A's D, not B's D

A reimplemented interface must implement all the interface functions, it does not inherit them from a super class:

    interface D
    {
        int foo();
    }

    class A : D
    {
        int foo() { return 1; }
    }

    class B : A, D
    {
    }		// error, no foo() for interface D

## Interfaces with Contracts

Interface member functions can have contracts even though there is no body for the function. The contracts are inherited by any class member function that implements that interface member function.

    interface I
    {
        int foo(int i)
          in { assert(i > 7); }
          out (result) { assert(result & 1); }

        void bar();
    }

## Const and Immutable Interfaces

If an interface has const or immutable storage class, then all members of the interface are const or immutable. This storage class is not inherited.

## COM Interfaces

A variant on interfaces is the COM interface. A COM interface is designed to map directly onto a Windows COM object. Any COM object can be represented by a COM interface, and any D object with a COM interface can be used by external COM clients.

A COM interface is defined as one that derives from the interface std.c.win dows.com.IUnknown. A COM interface differs from a regular D interface in that:

- It derives from the interface std.c.windows.com.IUnknown.
- It cannot be the argument of a DeleteExpression.
- References cannot be upcast to the enclosing class object, nor can they be downcast to a derived interface. To accomplish this, an appropriate QueryInterface() would have to be implemented for that interface in standard COM fashion.
- Classes derived from COM interfaces are COM classes.
- The default linkage for member functions of COM classes is extern(System). Note that if you want to implement or override any base-class methods of D interfaces or classes (ones which do not inherit from IUnknown), you have to explicitly mark them as having the extern(D) linkage:

      import core.sys.windows.windows;
      import std.c.windows.com;

      interface IText
      {
          void write();
      }

      abstract class Printer : IText
      {
          void print() { }
      }

      class C : Printer, IUnknown
      {
          // Implements the IText $(D write) class method.
          extern(D) void write() { }

          // Overrides the Printer $(D print) class method.
          extern(D) override void print() { }

          // Overrides the Object base class $(D toString) method.
          extern(D) override string toString() { return "Class C"; }

          // Methods of class implementing the IUnknown interface have
          // the extern(System) calling convention by default.
          HRESULT QueryInterface(const(IID)*, void**);
          uint AddRef();
          uint Release();
      }

  The same applies to other Object methods such as opCmp, toHash, etc.
- The first member of the vtbl[] is not the pointer to the InterfaceInfo, but the first virtual function pointer.

For more information, see Modern COM Programming in D

## C++ Interfaces

C++ interfaces are interfaces declared with C++ linkage:

    extern (C++) interface Ifoo
    {
        void foo();
        void bar();
    }

which is meant to correspond with the following C++ declaration:

    class Ifoo
    {
        virtual void foo();
        virtual void bar();
    };

Any interface that derives from a C++ interface is also a C++ interface. A C++ interface differs from a D interface in that:

- It cannot be the argument of a DeleteExpression.
- References cannot be upcast to the enclosing class object, nor can they be downcast to a derived interface.
- The C++ calling convention is the default convention for its member functions, rather than the D calling convention.
- The first member of the vtbl[] is not the pointer to the Interface, but the first virtual function pointer.

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