Author: Tomas Rutkauskas

How to work with a stack of interfaces? Should I use TObjectStack or simply TStack?

Answer:

Using a storage class that uses elements typed as pointer or TObject is very risky 
with interfaces, since hard typecasts are needed, and they mess up the reference 
counting. So it is better to write your own stack class for interfaces, perhaps 
based on an internal TInterfaceList as storage mechanism. Something like this 
(untested!):

1   { ... }
2   type
3     TInterfaceStack = class
4     private
5       FList: TInterfacelist;
6       FCurrent: IInterface;
7       function GetTop: IInterface;
8     public
9       constructor Create; virtual;
10      destructor Destroy; override;
11      procedure Push(aIntf: IInterface);
12      procedure Pop;
13      function IsEmpty: boolean;
14      property Top: IInterface read GetTop;
15    end;
16  
17    { TInterfaceStack }
18  
19  constructor TInterfaceStack.Create;
20  begin
21    inherited;
22    FList := TInterfacelist.Create;
23    FList.Capacity := 32;
24  end;
25  
26  destructor TInterfaceStack.Destroy;
27  begin
28    FList.Free;
29    inherited;
30  end;
31  
32  function TInterfaceStack.GetTop: IInterface;
33  begin
34    Result := FCurrent;
35  end;
36  
37  function TInterfaceStack.IsEmpty: boolean;
38  begin
39    Result := not Assigned(FCurrent);
40  end;
41  
42  procedure TInterfaceStack.Pop;
43  begin
44    if Flist.Count > 0 then
45    begin
46      FCurrent := FList[FList.count - 1];
47      FList.Delete(Flist.Count - 1);
48    end
49    else
50      FCurrent := nil;
51  end;
52  
53  procedure TInterfaceStack.Push(aIntf: IInterface);
54  begin
55    if not IsEmpty then
56      FList.Add(FCurrent);
57    FCurrent := aIntf;
58  end;