CODEWITHSUNDEEP
delphipointers
KingKong
KingKong

Reputation: 455

Accessing values from pointer problem

I have a class of type TObject called CModelItem

I want to have a list of these objects and be able to modify the values of each one

So, I created a class

CQueueList = class(TList)

private

public

end;

and I make

QueueList : CQueueList;

in var

Now, I can add the CModelItem to this list, like so:

QueueList := CQueueList.Create;
for idx := 0 to ndx - 1 do
  begin
    MyItem := CModelItem.Create;
    MyItem.CopyHead(CModelItem(RunList.Objects[idx]));
    MyItem.ReadData;
    MyItem.NumOfIterations := NumRepEdit.Value;
    MyItem.IsInQueue := True;
    MyItem.LogEvents := EventsCheckBox.Checked;
    MyItem.LogMatch := MatchCheckBox.Checked;
    MyItem.LogUpdates := UpdatesCheckBox.Checked; 

    QueueList.Add(MyItem);
  end;

I can also use it, so I can do:

DefForm := TRunDefForm.Create(Self, QueueList.Items[idx]);

with DefForm taking in a component and a CModelItem

But I'm running into problems trying to modify the values of an object in QueueL

First, I can't access something like MyItem.IsInQueue by doing

QueueList.Items[idx].IsInQueue := blah;

because it tells me IsInQueue is 'an undeclared identifier'

I've also tried making a new CModelItem and copying the information over, like this:

idx := QueueListBox.ItemIndex;
MyItem := QueueList.Items[idx];

and this compiles fine, but throws up an 'access violation error' when it goes into that function

I noticed that QueueList.Items[idx] is a pointer, but I'm really not sure how I should be accessing it

Upvotes: 1

Views: 172

Answers (1)

Ondrej Kelle
Ondrej Kelle

Reputation: 37211

The compiler complains because TList.Items returns an untyped pointer.

You can use a typecast:

  CModelItem(QueueL.Items[idx]).IsInQueue := blah;

You can also reimplement the Items property in your CQueueList class:

private
  function GetItems(Index: Integer): CModelItem;
public
  property Items[Index: Integer]: CModelItem read GetItems; default;
end;

function CQueueList.GetItems(Index: Integer): CModelItem;
begin
  Result := inherited Items[Index];
end;

As you've seen, using a local variable works; although the access violation is a bug probably somewhere else in your code.

Upvotes: 3

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