7.1 De�nition

7.1 Denition

As of version 1.1, FPC supports interfaces. Interfaces are an alternative to multiple inheritance (where a class can have multiple parent classes) as implemented for instance in C++. An interface is basically a named set of methods and properties: A class that implements the interface provides all the methods as they are enumerated in the Interface denition. It is not possible for a class to implement only part of the interface: it is all or nothing.

Interfaces can also be ordered in a hierarchy, exactly as classes: An interface denition that inherits from another interface denition contains all the methods from the parent interface, as well as the methods explicitly named in the interface denition. A class implementing an interface must then implement all members of the interface as well as the methods of the parent interface(s).

An interface can be uniquely identied by a GUID (GUID is an acronym for Globally Unique Identier, a 128-bit integer guaranteed always to be unique¹ . Especially on Windows systems, the GUID of an interface can and must be used when using COM.

The denition of an Interface has the following form:

_________________________________________________________________________________________________________ Interface type
-- --Interface -|--------|------------|---------------end------------
heritage- -['-GUID ']-- - component list-

-- --heritage -(- interface type identi er -)-------------------------------

-- -- ---- ---------------------------------
component list 6 meptrohpoedrt dye de nnititioionn -||
-------------------|
___________________________________________________________________

Along with this denition the following must be noted:

Interfaces can only be used in DELPHI mode or in OBJFPC mode.
There are no visibility speciers. All members are public (indeed, it would make little sense to make them private or protected).
The properties declared in an interface can only have methods as read and write speciers.
There are no constructors or destructors. Instances of interfaces cannot be created directly: instead, an instance of a class implementing the interface must be created.
Only calling convention modiers may be present in the denition of a method. Modiers as virtual, abstract or dynamic, and hence also override cannot be present in the denition of a interface denition.

The following are examples of interfaces:

IUnknown = interface ['{00000000-0000-0000-C000-000000000046}']
  function QueryInterface(const iid : tguid;out obj) : longint;
  function _AddRef : longint;
  function _Release : longint;
end;
IInterface = IUnknown;

IMyInterface = Interface
  Function MyFunc : Integer;
  Function MySecondFunc : Integer;
end;

As can be seen, the GUID identifying the interface is optional.

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