Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

Constructors and Class References
A constructor can be called using a variable of a class-reference type. This allows construction of objects whose
type isn't known at compile time. For example,
type TControlClass = class of TControl; 
function CreateControl(ControlClass: TControlClass; 
const ControlName: string; X, Y, W, H: Integer): TControl; 
begin 
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---

     Result := ControlClass.Create(MainForm); 
     with Result do 
      begin 
        Parent := MainForm; 
        Name := ControlName; 
        SetBounds(X, Y, W, H); 
        Visible := True; 
      end; 
end;
The 
CreateControl
 function requires a class-reference parameter to tell it what kind of control to create. It uses
this parameter to call the class's constructor. Because class-type identifiers denote class-reference values, a call to
CreateControl
 can specify the identifier of the class to create an instance of. For example,
CreateControl(TEdit, 'Edit1', 10, 10, 100, 20);
Constructors called using class references are usually virtual. The constructor implementation activated by the call
depends on the runtime type of the class reference.