Why is TList freed in this code?

[John Kouraklis 94]

Hi,

I've got difficulties figuring out what is wrong with the following code:

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils,
  System.Generics.Collections;

var
  dic: TObjectDictionary<string, TList<string>>;
  list: TList<string>;
begin
  try
    dic:=TObjectDictionary<string, TList<string>>.Create([doOwnsValues]);
    list:=TList<string>.Create;
    list.Add('111');
    dic.AddOrSetValue('aaa', list);
    var gg: integer:=dic.Items['aaa'].Count;
    writeln(gg); // 1

    var l2: TList<string>:=dic.Items['aaa'];
    var a:integer:=l2.Count;
    writeln(a);        // 1

    l2.Add('222');
    a:=l2.Count;
    writeln(a);  // 2

    dic.Items['aaa']:=l2;

    a:=l2.Count;
    writeln(a);  // 0???

    a:=dic.Items['aaa'].count;
    writeln(a);  // 0???

    dic.Free;   // Invalid point?

    readln;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

When I assign the TList to the dictionary Value, it loses all the elements.

 

What am I doing wrong here? Can't see what is going on.

 

Thanks

[Henk Post 3]

The dic[‘aaa’] and l2 are the same object already. Hence, the assignment is not needed at all.

 

But the direct assignment (formerly known as “AddOrSetValue” in this case will set the value. This is done by removing (and destroying) the old value and after that add the new one... Thus the l2 is “cleared” (destroyed).

[pyscripter 689]

 dic.Items['aaa']:=l2;

dic.Items['aaa'] already contains l2 and l2 is freed before it is readded, since you set 

doOwnsValues

Also an issue would occur if you do

dic.Items['bbb']:=l2;

 

When the dictionary is destroyed the list will be freed twice.

Edited August 28, 2020 by pyscripter

[John Kouraklis 94]

So, the doOwnsValues frees the objects in assignments as well. I thought it only takes care of them when the dictionary is freed.

 

Thanks a lot guys

[David Heffernan 2345]

Also the owned objects are destroyed when items are removed from the dictionary. This is the same ownership model as the classic TObjectList. 

[Stefan Glienke 2002]

That's why IMultiMap<TKey,TValue> from Spring4d is so cool 

[David Heffernan 2345]

16 minutes ago, Stefan Glienke said:

That's why IMultiMap<TKey,TValue> from Spring4d is so cool 

Only cool for the users. Mind-bending if it's your job to implement it!

[John Kouraklis 94]

4 hours ago, Stefan Glienke said:

That's why IMultiMap<TKey,TValue> from Spring4d is so cool 

So, how does it behave? never used it

[Stefan Glienke 2002]

What do you mean how does it behave - like a multimap

[John Kouraklis 94]

Oh, good to know it honors its name