Interface delegation + overriding - delphi

Question

Due to the lack of multiple inheritance in Delphi, I need to work with interface delegation. This is a very new topic to me and I have a problem with combining overridding with interface delegation.
The class TMyNode must inherit from TBaseClass and needs to implement IAddedStuff . I want to have the default implementation of all functions of IAddedStuff in TAddedStuffDefaultImplementation , so I don't need to have duplicate code for getters/setters everywhere. So, I have delegated those things using DefaultBehavior .
The problem is, that TAddedStuffDefaultImplementation is meant to have virtual methods, so I want to override them directly in TMyNode . This does work if I write FDefaultImplementation: TAddedStuffDefaultImplementation; instead of FDefaultImplementation: IAddedStuff; .
But now, for some reasons TAddedStuffDefaultImplementation will increase the Ref-Counter for x: TBaseClass;, so it cannot be freed. What should I do?
My simplified reproduction code is below:
program Project2;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils;
type
IAddedStuff = interface(IInterface)
['{9D5B00D0-E317-41A7-8CC7-3934DF785A39}']
function GetCaption: string; {virtual;}
end;
TAddedStuffDefaultImplementation = class(TInterfacedObject, IAddedStuff)
function GetCaption: string; virtual;
end;
TBaseClass = class(TInterfacedObject);
TMyNode = class(TBaseClass, IAddedStuff)
private
FDefaultImplementation: TAddedStuffDefaultImplementation;
public
property DefaultBehavior: TAddedStuffDefaultImplementation read FDefaultImplementation
write FDefaultImplementation implements IAddedStuff;
destructor Destroy; override;
// -- IAddedStuff
// Here are some functions which I want to "override" in TMyNode.
// All functions not declared here, should be taken from FDefaultImplementation .
function GetCaption: string; {override;}
end;
{ TAddedStuffDefaultImplementation }
function TAddedStuffDefaultImplementation.GetCaption: string;
begin
result := 'PROBLEM: CAPTION NOT OVERRIDDEN';
end;
{ TMyNode }
destructor TMyNode.Destroy;
begin
if Assigned(FDefaultImplementation) then
begin
FDefaultImplementation.Free;
FDefaultImplementation := nil;
end;
inherited;
end;
function TMyNode.GetCaption: string;
begin
Result := 'OK: Caption overridden';
end;
var
x: TBaseClass;
gn: IAddedStuff;
s: string;
begin
x := TMyNode.Create;
try
TMyNode(x).DefaultBehavior := TAddedStuffDefaultImplementation.Create;
Assert(Supports(x, IAddedStuff, gn));
WriteLn(gn.GetCaption);
finally
WriteLn('RefCount = ', x.RefCount);
// x.Free; // <-- FREE fails since FRefCount is 1
end;
ReadLn(s);
end.

Answer 1

If you are delegating the IAddedStuff then you should also implement non-default behavior on another class and pass it by constructor injection.
Also if you are mixing object and interface references, make sure the ref counting does not conflict. When using interface delegation the reference of the container object gets changed.
program Project1;
{$APPTYPE CONSOLE}
uses
Classes,
SysUtils;
type
IAddedStuff = interface(IInterface)
['{9D5B00D0-E317-41A7-8CC7-3934DF785A39}']
function GetCaption: string; {virtual;}
end;
TAddedStuffDefaultImplementation = class(TInterfacedObject, IAddedStuff)
function GetCaption: string; virtual;
end;
TAddedStuffOverriddenImplementation = class(TAddedStuffDefaultImplementation)
function GetCaption: string; override;
end;
TBaseClass = class(TInterfacedPersistent);
TMyNode = class(TBaseClass, IAddedStuff)
private
FAddedStuff: IAddedStuff;
property AddedStuff: IAddedStuff read FAddedStuff implements IAddedStuff;
public
constructor Create(const addedStuff: IAddedStuff);
end;
{ TAddedStuffDefaultImplementation }
function TAddedStuffDefaultImplementation.GetCaption: string;
begin
result := 'PROBLEM: CAPTION NOT OVERRIDDEN';
end;
{ TAddedStuffOverriddenImplementation }
function TAddedStuffOverriddenImplementation.GetCaption: string;
begin
Result := 'OK: Caption overridden';
end;
{ TMyNode }
constructor TMyNode.Create;
begin
FAddedStuff := addedStuff;
end;
var
x: TBaseClass;
gn: IAddedStuff;
begin
x := TMyNode.Create(TAddedStuffOverriddenImplementation.Create);
try
Assert(Supports(x, IAddedStuff, gn));
WriteLn(gn.GetCaption);
finally
x.Free;
end;
Readln;
ReportMemoryLeaksOnShutdown := True;
end.
Edit:
After the discussion in the comments I would suggest the following:
program Project1;
{$APPTYPE CONSOLE}
uses
Classes,
SysUtils;
type
IAddedStuff = interface(IInterface)
['{9D5B00D0-E317-41A7-8CC7-3934DF785A39}']
function GetCaption: string;
end;
TAddedStuffDefaultImplementation = class(TInterfacedObject, IAddedStuff)
function GetCaption: string; virtual;
end;
TBaseClass = class(TInterfacedPersistent);
TMyNode = class(TBaseClass, IAddedStuff)
private
FAddedStuff: IAddedStuff;
property AddedStuff: IAddedStuff read FAddedStuff implements IAddedStuff;
public
constructor Create;
end;
TAddedStuffOverriddenImplementation = class(TAddedStuffDefaultImplementation)
private
FMyNode: TMyNode;
public
constructor Create(AMyNode: TMyNode);
function GetCaption: string; override;
end;
{ TAddedStuffDefaultImplementation }
function TAddedStuffDefaultImplementation.GetCaption: string;
begin
result := 'PROBLEM: CAPTION NOT OVERRIDDEN';
end;
{ TMyNode }
constructor TMyNode.Create;
begin
FAddedStuff := TAddedStuffOverriddenImplementation.Create(Self);
end;
{ TAddedStuffOverriddenImplementation }
constructor TAddedStuffOverriddenImplementation.Create(AMyNode: TMyNode);
begin
FMyNode := AMyNode;
end;
function TAddedStuffOverriddenImplementation.GetCaption: string;
begin
Result := 'OK: Caption overridden';
end;
var
x: TBaseClass;
gn: IAddedStuff;
begin
x := TMyNode.Create;
try
Assert(Supports(x, IAddedStuff, gn));
WriteLn(gn.GetCaption);
finally
x.Free;
end;
ReadLn;
ReportMemoryLeaksOnShutdown := True;
end.