Unused interface reference is not destroyed

Question

i had another bug in my app caused by careless usage of Delphi interfaces. When i pass an interface to a procedure which ignores that argument, the instance is never freed. See the following simple example:

ITest = interface
    procedure Test;
end;

Tester = class(TInterfacedObject, ITest)
public
    procedure Test;
end;

Base = class
public
    procedure UseTestOrNot(test : ITest); virtual; abstract;
end;

A = class(Base)
public
    procedure UseTestOrNot(test : ITest); override;
end;

B = class(Base)
public
    procedure UseTestOrNot(test : ITest); override;
end;

{ A }

procedure A.UseTestOrNot(test: ITest);
begin
    test.Test();
end;

{ B }

procedure B.UseTestOrNot(test: ITest);
begin
    WriteLn('No test here');
end;

// -------- Test ---------------------------------------
var
    list : TObjectList<Base>;
    x : Base;
    t : ITest;
begin
    ReportMemoryLeaksOnShutdown := true;

    list := TObjectList<Base>.Create;
    list.Add(A.Create);
    list.Add(B.Create);

    // 1 x Tester leak for each B in list:
    for x in list do
        x.UseTestOrNot(Tester.Create);

    // this is ok
    for x in list do
    begin
        t := Tester.Create;
        x.UseTestOrNot(t);
    end;

    list.Free;
end.

Can you please explain what goes wrong with the reference counter? Can you give any best practice/ guideline (like "Never create an interfaced instance inside a function call [if you don't know what happens inside]).

The best solution i can think of for this example is to write a template method in class Base that saves the passed test instance and calls an abstract DoUseTestOrNot method.

EDIT Delphi 2010

Does not matter, it is a bug regardless of version. wait until delphi ex 2? Cheers — APZ28, Jan 13 '11 at 18:09
http://qc.embarcadero.com/wc/qcmain.aspx?d=75036 http://qc.embarcadero.com/wc/qcmain.aspx?d=71015 — Krystian Bigaj, Jan 13 '11 at 19:55
@kibab: those reports are for `const` parameters, this is for a regular parameter. — Jeroen Wiert Pluimers, Jan 13 '11 at 23:25

score 8 · Answer 1 · answered Jan 13 '11 at 15:03

8

Add a guid to you ITest declaration

ITest = interface
['{DB6637F9-FAD3-4765-9EC1-0A374AAC7469}']
    procedure Test;
end;

Change the loop to this

for x in list do
    x.UseTestOrNot(Tester.Create as ITest);

The GUID is neccesary to be able to use as

Test.Create as ITest makes the compiler to add the release where the created object goes out of scope.

answered Jan 13 '11 at 15:03

Mikael Eriksson

136,425
22
210
281

Thank your for this workaround. Is there any menu entry or button to insert the GUID? I always forget the shortcut, as i never use GUIDs (i always try to avoid as and is etc). – hansmaad Jan 14 '11 at 06:57

score 8 · Accepted Answer · edited May 23 '17 at 09:58

8

It is a different manifestation of the bugs here.
I will add this to the QC report.

This does not reproduce in Delphi XE update 1 any more.

--jeroen

edited May 23 '17 at 09:58

Community

1
1

answered Jan 13 '11 at 16:09

Jeroen Wiert Pluimers

23,965
9
74
154

For regression: can you mail me a fully functional console program (.dpr) showing the memory leak in Delphi 2010 - preferably with FastMM? Anything at pluimers dot com works, just start with my first name. That way I can rule out any typos I made from making your above code compile. – Jeroen Wiert Pluimers Jan 14 '11 at 11:00

Unused interface reference is not destroyed

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