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Table of Contents
2. TMT Pascal Language Description
2.2. Pascal Language Structure
2.2.13. OOP Extensions
2.2.13.14. Interface Type
2.2.13.14. Interface Type
Interfaces are similar to objects, however they can be declared only in
the outermost scope of a program or unit, and they do not implement their
own methods. Thus the interface methods cannot be declared as virtual or
override. An interface type declaration has the following format:
type InterfaceName = interface (AncestorInterface) [ '{GUID}' ]
memberList
end;
where AncestorInterface and ['{GUID}'] are optional. The
MemberList can include only methods. Unlike objects, fields are not
allowed in the interface declaration. All members of an interface are always
public (viability modifiers are not allowed).
Also interfaces have no constructors or destructors.
They cannot be instantiated, except through objects that implement their
methods. If no ancestor is specified, the interface is a direct descendant
of IUnknown , which is defined in the System unit as follows:
type
IUnknown = interface
function QueryInterface (const iid: TIID;
var obj):LongInt; stdcall;
function AddRef: Longint; stdcall;
function Release: Longint; stdcall;
end;
To be compatible with Delphi, an interface declaration can specify a globally
unique identifier (GUID) which is represented by a string literal enclosed in
brackets immediately preceding the member list.
The GUID part of the declaration must have the following format:
[' {xxxxxxxx -xxxx -xxxx -xxxx -xxxxxxxxxxxx}']
The TGUID and PGUID types are declared in the System unit as follows:
type
PGUID = ^TGUID;
TGUID = packed record
D1: Longint;
D2: Word;
D3: Word;
D4: array [0..7] of Byte;
end;
TIID = TGUID;
To declare a typed constant of type TGUID, one can use a string literal to
specify its value. For example,
const
IID_IMYINTERFACE: TGUID = '{3C2A92C2-8AE5-43AC-BEB8-1957FA47C30F}';
In function calls, either a GUID or an interface identifier can serve as a
value or constant parameter of type TGUID.
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