TQueue.Extract Method

Remove top queue item.

Pascal

function Extract: T;

C++

__fastcall T Extract();

Description

Extract removes and returns the top element of the queue. Count is decremented by 1. If Count is already 0, an error is raised.

An OnNotify event occurs indicating an item was removed from the queue. Extract is the same as Dequeue except for the event code indicating an element was extracted rather than removed.

Extract functions similarly to Peek except that Extract removes an element from the queue.

Extract is similar to Dequeue and is provided so items may be removed without freeing.

This is a O(1) operation.

Delphi Examples:

Copy Code

{
This example demonstrates the usage of the generic TQueue class.
}
procedure TForm3.Button1Click(Sender: TObject);
var
  Queue: TQueue<String>;
begin
  { Create a new Queue }
  Queue := TQueue<String>.Create();

  { Register a notification call-back }
  Queue.OnNotify := QueueChanged;

  { Enqueue some items up the Queue }
  Queue.Enqueue('John');
  Queue.Enqueue('Mary');
  Queue.Enqueue('Bob');
  Queue.Enqueue('Anna');
  Queue.Enqueue('Erica');

  { Show the last enqueued element without modifying the Queue }
  MessageDlg('First enqueued element is: "' + Queue.Peek() + '".', mtInformation, [mbOK], 0);

  { Extract the top element: "Erica" }
  Queue.Extract();

  { Reduce the capacity }
  Queue.TrimExcess();

  { The remaining count of elements }
  MessageDlg('The queue contains ' + IntToStr(Queue.Count) + ' elements.', mtInformation, [mbOK], 0);

  { Show the last enqueued element by modifying the Queue }
  MessageDlg('First enqueued element is: "' + Queue.Dequeue() + '".', mtInformation, [mbOK], 0);

  { Clear the Queue }
  Queue.Clear();

  { Destroy the Queue completely }
  Queue.Free;
end;

procedure TForm3.QueueChanged(Sender: TObject; const Item: String; Action: TCollectionNotification);
begin
  { This method is called by the List everytime a change occurs }
  if Action = cnAdded then
     MessageDlg('Element added: ' + Item, mtInformation, [mbOK], 0)
  else if Action = cnRemoved then
     MessageDlg('Element removed: ' + Item, mtInformation, [mbOK], 0)
  else if Action = cnExtracted then
     MessageDlg('Element extracted: ' + Item, mtInformation, [mbOK], 0)
end;