Delphi - Passing pointers to records

Question

In Delphi 2010, I have a TreeView. The treeview has a data object defined as a record.

// Define the Data Object that will be associated with each Node
type
  TNodeRec = record
    rId: Integer;
    rDate: TDateTime;
    rSite: string;
    rURL: string;
    rRedirURL: string;
    rTitle: string;
    rCategory: String;
    rURL_RtnCode: Integer;
    rURL_RtnText: String
  end;

I associate this data object with EACH node in the treeview, then I fill in the values. This works fine....

var
  BuffData: ^TNodeRec; // The data object associated with each node;
  URLNode: TTreeNode;
begin
...
          // Create our New Data Object
          new(BuffData);
          BuffData^.rId := 0;
          BuffData^.rDate := EDate;
          BuffData^.rURL := ElString;
          BuffData^.rRedirURL := '';
          BuffData^.rTitle := ElText;
          URLNode := TV1.Items.AddChildObject(nil, BuffData^.rTitle, BuffData);

My challenge is that from a buttonClick event, I need to pass this data object to another routine (in another UNIT) to modify it, and have the modified values available to the calling routine... This would normally be done via a VAR declaration, but I don't know that I need that, since I have declared this as a POINTER to a record.

Anyway, my ButtonClick event looks like this...

procedure TURLListForm.btnEvalSelectedClick(Sender: TObject);
var
SelNode : TTreeNode;
BuffData : TNodeRec;
begin

//  verify there is a node selected
...


BuffData := TNodeRec(TV1.Selected.Data^);
WebEvaluate(BuffData);
ShowMessage(TNodeRec(TV1.Selected.Data^).rRedirURL);

end;

The problem is that the ShowMessage box is always empty, aka rRedirURL is not getting passed back like it would as a VAR declaration.

WebEvaluate is defined as

procedure WebEvaluate(Data1: TNodeRec);
begin
TNodeRec(Data1).rRedirURL := 'Test value'
ShowMessage('In WebEvaluate Routine:' + Data1.rRedirURL);
end

Within the WebEvaluate routine, Data1.rRedirURL has a value assigned. When I get back to the calling routine, the value is lost. I believe it has to do with how I am passing the Data1 record, but I don't know how to correct it... What am I doing wrong?

Thanks

David Heffernan David Heffernan · Accepted Answer · 2014-01-29T15:14:02

If you are going to ask WebEvaluate to modify its parameter it must be passed as var. The way you write the procedure, the procedure takes a copy of the parameters value, and modifies that. Remember that modification to a parameter passed by value cannot be seen outside the function.

Change it to be like so:

procedure WebEvaluate(var Data1: TNodeRec);
begin
  Data1.rRedirURL := 'Test value';
  ShowMessage('In WebEvaluate Routine:' + Data1.rRedirURL);
end;

On top of that you are actually passing a copy of the record to WebEvaluate. Look at this code:

var
  BuffData : TNodeRec;
....
BuffData := TNodeRec(TV1.Selected.Data^);

Since BuffData is a record, assignment to BuffData takes a copy. But you need to modify the record stored in the tree view and so must pass that record, and not a copy.

So, remove the local variable BuffData that serves no purpose. And call the procedure like this, passing the record stored in the tree view:

WebEvaluate(TNodeRec(TV1.Selected.Data^));

To summarise you made the same mistake twice. You made a copy of the record in a local variable. You then passed that to your procedure which made another copy.

Delphi - Passing pointers to records

1 Answers