7.1 Definition

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 definition. 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 definition that inherits from another interface definition contains all the methods from the parent interface, as well as the methods explicitly named in the interface definition. 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 identified by a GUID. GUID is an acronym for Globally Unique Identifier, 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 definition of an Interface has the following form:

Interface type

--Interface -|-------------------------------------end------------
           |heritage| -[’-GUID ’]--|- component list|

--       - -                   - -------------------------------
  heritage  (  interface type identifier  )

--            ----              ---------------------------------
  component list | -method definition-||
               ---property definition--|

Along with this definition the following must be noted:

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;  
IInterface = IUnknown;  
IMyInterface = Interface  
  Function MyFunc : Integer;  
  Function MySecondFunc : Integer;  

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

1In theory, of course.