Unsatisfied forward or external declaration

Question

unit Undo;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;type
  TForm1 = class(TForm)
    Prebtn: TButton;
    Memo1: TMemo;
    ListBox1: TListBox;
    Nextbtn: TButton;
    procedure PrebtnClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;  TbaseExecuteAction=class
      procedure Execute;virtual;//这行报错
  end;  MemoAction=class(TbaseExecuteAction);  ListAction=class(TbaseExecuteAction);var
  Form1: TForm1;implementation
var
  list:array[1..100] of TbaseExecuteAction;
{$R *.dfm}procedure TForm1.PrebtnClick(Sender: TObject);beginend;end.[Error] Undo.pas(23): Unsatisfied forward or external declaration: 'TbaseExecuteAction.Execute'
[Fatal Error] UnDoApp.dpr(5): Could not compile used unit 'Undo.pas'
到底什么错误！！

Answer 1

修改为：    TbaseExecuteAction=class
      procedure Execute;virtual;abstract;
  end;