Search code examples
delphiinterfacereference-countingtinterfacedobject

Reference-counting for objects

In my code I use a small data-storing class, which is created in different places. To avoid memory leaks and simplify things, I want to use reference counting, so I did

type TFileInfo = class (TInterfacedObject, IInterface)

and removed all my manual calls to TFileInfo.Free. Unfortunately Delphi reported a lot of memory leaks. Searching on SO I found the following question explaining why this doesn't work:

Why aren't descendants of TInterfacedObject garbage collected?

There is a workaround presented there but it requires me (at least if i get it right) to write a custom interface IFileInfo and provide it with a lot of getters and setters, which I want to avoid.

EDIT I should add that I insert the create FileInfo objects into two different kinds of hash tables: one descending from TBucketList and another one is a hash map implementation from the Codegear forum. Internally they both user pointers, so the situation is just like in the other question.

Is there any other possibility to make objects in Delphi use reference-counting?

Solution

  • Unfortunately, the Delphi compiler generates the necessary code to inc/dec reference count only when you use interfaces (in your case custom interface IFileInfo). Moreover, if interfaces are cast to pointer (or TObject for that matter), again no reference counting is possible. For example, assumming global variable list : TList: 

    var ifi : IFileInfo;
begin
  ifi := TFileInfo.Create;
  list.Add(TFileInfo(ifi));
end;

    after the method returns list[list.Count - 1] will contain dangling pointer.

    So interfaces cannot be used in a hashmap that casts them to pointers, the hashmap implementation must keep them as IInterface.