Sometimes it happens that I would like to convert a Delphi object to an interfaces. Interfaces offer some advantages.
\nOn the one hand you can handle different objects that are not derived from a common hierarchy but share the same methods.
\nOn the other hand interfaces are reference counted. You can use the reference counting as a poor man’s garbage collection.<\/p>\n
Normally objects that are interfaced are derived from TInterfacedObject.
\nLet’s have a look at the implementation of TInterfacedObject in Delphi XE3.<\/p>\n
...\r\n TInterfacedObject = class(TObject, IInterface)\r\n protected\r\n{$IFNDEF AUTOREFCOUNT}\r\n FRefCount: Integer;\r\n{$ENDIF}\r\n function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;\r\n function _AddRef: Integer; stdcall;\r\n function _Release: Integer; stdcall;\r\n...\r\n\r\nfunction TInterfacedObject._AddRef: Integer;\r\nbegin\r\n{$IFNDEF AUTOREFCOUNT}\r\n Result := AtomicIncrement(FRefCount);\r\n{$ELSE}\r\n Result := __ObjAddRef;\r\n{$ENDIF}\r\nend;\r\n\r\nfunction TInterfacedObject._Release: Integer;\r\nbegin\r\n{$IFNDEF AUTOREFCOUNT}\r\n Result := AtomicDecrement(FRefCount);\r\n if Result = 0 then\r\n Destroy;\r\n{$ELSE}\r\n Result := __ObjRelease;\r\n{$ENDIF}\r\nend;\r\n...<\/pre>\nYou can see two things:<\/p>\n
\n- Every interfaced object has to implement IInterface.<\/li>\n
- EMBT will introduce a real garbage collection but currently a TInterfacedObject is simply reference counted.<\/li>\n<\/ol>\n
Normally you have to decide whether an object is interfaced or not if you are implementing<\/strong> it, not if you are using<\/strong> it.
\nThat’s why I introduced a small wrapper class for an object. The wrapper class is interfaced and offers full access to the underlying object.<\/p>\nunit ObjGuard;\r\n\r\ninterface\r\n\r\ntype\r\n IObjGuard<T: class> = interface(IInterface)\r\n ['{7FBC48DC-1ED4-450F-8418-64E0D942F115}']\r\n function get_Obj: T;\r\n property Obj: T read get_Obj;\r\n end;\r\n\r\n TObjGuard<T: class> = class(TInterfacedObject, IObjGuard<T>)\r\n strict private\r\n FObj: T;\r\n function get_Obj: T; inline;\r\n strict protected\r\n property Obj: T read get_Obj;\r\n public\r\n constructor Create(AObj: T);\r\n destructor Destroy; override;\r\n end;\r\n\r\nimplementation\r\n\r\nuses\r\n System.Classes;\r\n\r\n{ TObjGuard<T> }\r\n\r\nconstructor TObjGuard<T>.Create(AObj: T);\r\nbegin\r\n inherited Create;\r\n FObj := AObj;\r\nend;\r\n\r\ndestructor TObjGuard<T>.Destroy;\r\nbegin\r\n FObj.Free;\r\n inherited Destroy;\r\nend;\r\n\r\nfunction TObjGuard<T>.get_Obj: T;\r\nbegin\r\n Result := FObj;\r\nend;\r\n\r\nend.<\/pre>\nThis makes the use of the object very simple. Just create an instance of the ObjGuard and the resulting interface offers access to the underlying object.
\nYou can use it for example in records which don’t offer a destructor so that there must be a way to destroy object fields.<\/p>\n
...\r\ntype\r\n TMyRecord = record\r\n strict private\r\n FStream: IObjGuard<TFileStream>;\r\n function get_Stream: TStream; inline;\r\n public\r\n constructor Create(const AFileName: string);\r\n property Stream: TFileStream read get_Stream;\r\n end;\r\n\r\n...\r\nconstructor TMyRecord.Create(const AFileName: string);\r\nbegin\r\n FStream := TObjGuard<TFileStream>.Create(TFileStream.Create(AFileName, fmCreate));\r\nend;\r\n\r\nfunction TMyRecord.get_Stream: TStream;\r\nbegin\r\n Result := FStream.Obj;\r\nend;\r\n\r\nend.<\/pre>\n","protected":false},"excerpt":{"rendered":"Sometimes it happens that I would like to convert a Delphi object to an interfaces. Interfaces offer some advantages. On the one hand you can handle different objects that are not derived from a common hierarchy but share the same … Continue reading