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D Programming Language 2.0


Last update Mon Apr 6 02:40:08 2009

Interfaces

InterfaceDeclaration:
	interface Identifier InterfaceBody
	interface Identifier : SuperInterfaces InterfaceBody

SuperInterfaces
	Identifier
	Identifier , SuperInterfaces

InterfaceBody:
	{ DeclDefs }

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
Interface member functions do not have implementations.
interface D
{
    void bar() { }	// error, implementation not allowed
}
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

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.windows.com.IUnknown. A COM interface differs from a regular D interface in that:

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: