7.1 Definition

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:

Along with this definition the following must be noted:

• Interfaces can only be used in DELPHI mode or in OBJFPC mode.
• There are no visibility specifiers. 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 specifiers.
• 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 modifiers may be present in the definition of a method. Modifiers as virtual, abstract or dynamic, and hence also override cannot be present in the definition of a interface definition.