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Sort on a field in an inherited TObjectList

Posted on 2010-11-18
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Last Modified: 2012-05-10
When sorting a generic TObjectList inherited from another I get a compilation error
“There is no overload version of Sort that can be called with these arguments”

To explain with an example -

TPerson = class
  FFirstName : string;
  FLastName : string;
end;

TPersonList = class(TObjectList<TPerson>)
 end;

I can sort TPerson on a name field  using

procedure TForm1.Button3Click(Sender: TObject);
begin
  PersonsList.Sort(TComparer<TPerson>.Construct(
   function(const Item1,Item2:TPerson): Integer
   begin
      Result := CompareText(Item1.LastName, Item2.LastName);
   end));
end;

Now, if we have two inherited classes

 TChild = class(TPerson)
 private
  fAge: integer;
and
TChildList = class(TPersonList)
 end;

I get a compilation error (“There is no overload version of Sort that can be called with these arguments”) with

procedure TForm1.Button4Click(Sender: TObject);
begin
  ChildList.Sort(TComparer<TChild>.Construct(
   function(const Item1,Item2:TChild): Integer
   begin
      Result := Item1.Age - Item2.Age;
   end));
end;

Any suggestions on how to rectify this.

0
Comment
Question by:Dennis_Treacy
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8 Comments
 
LVL 32

Expert Comment

by:ewangoya
ID: 34169947
Thats because ChildList is a list of TPerson

You can declare it as

  TChildList = class(TObjectList<TChild>)
  end;
0
 
LVL 25

Expert Comment

by:epasquier
ID: 34169966
declare
 
TChildList = class(TObjectList<TChild>) end;

Open in new window

otherwise you inherit the parameter types of all functions from TPersonList, so TPerson instead of the TChild that you want.
0
 
LVL 25

Expert Comment

by:epasquier
ID: 34169974
Ok ewangoya, you got me ;o)
are you Speedy Gonzales cousin ?
0
 
LVL 32

Expert Comment

by:ewangoya
ID: 34169998
:) I have to try and stay a step ahead otherwise you give me a good whipping
0
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LVL 25

Expert Comment

by:epasquier
ID: 34170073
I know, take that as an honorific gesture. It's because you annoy me, as JimyX, and aflarin before, being on all questions around while at the same time I have had a lot of work and couldn't quite respond as quickly and often as before...
I have the impression it will take forever to get those 20k points that will let me enter top 5... I hoped to achieve it in less than a year, but that day has passed and I still see no end of it.

So because of that frustration, I make you pay whenever you don't give your best

That is all the more futile from me that I annoyed Geert_Gruwez pretty much the same way a year ago.

Sorry again for being a bit caught in that game ;o)
0
 

Author Comment

by:Dennis_Treacy
ID: 34170124
ewangoya:

Thanks, but I actually want to sort TChildList without foregoing access to the methods in TPersonList, that's why TChildList is inherited from TPersonList. I made up this small example for the purposes of this question, but my real life case has a bunch of object lists inherited from some base object lists so I can share methods and this is where I want to use the solution

Is it possible to sort TChildList without loosing access tp methods in TPersonList - I tried, without luck

  TChildList(PersonsList).Sort(TComparer<TChild>.Construct(
   function(const Item1,Item2:TChild): Integer
   begin
      Result := Item1.Age - Item2.Age;
   end));

0
 
LVL 32

Accepted Solution

by:
ewangoya earned 500 total points
ID: 34170259
You can do a type cast, this will work but it is not clean

 ChildList.Sort(TComparer<TPerson>.Construct(
   function(const Item1, Item2: TPerson): Integer
   begin
      Result := TChild(Item1).Age - TChild(Item2).Age;
   end));
0
 

Author Closing Comment

by:Dennis_Treacy
ID: 34170306
Great! Thanks very much
0

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