delphi assign to const?

Question

this site : http://www.drbob42.com/delphi/wizards.htm

showed a very puzzling code at the bottom

 unit ShareMem;
 { (c) 1997 by Bob Swart (aka Dr.Bob - http://www.drbob42.com }
 interface

 const
...
 uses
   Windows;

 const
   Handle: THandle = 0;
...
 function GetCommandLine: PChar; stdcall;
   external 'kernel32.dll' name 'GetCommandLineA';
...
   begin
     Handle := LoadLibrary('BCBMM.DLL');
 end.

how could this be ?

Answer 1

Delphi has something called assignable consts which allows a const value to be assigned. This can be turned on/off through compiler directives and switches. For a longer answer see here.

It sometimes comes in handy in times before class properties were possible. Even if the const is declared inside a function, it keeps its value between calls.

procedure Test;
{$WRITEABLECONST ON}
const
  AssignableConst: Integer = 0;
{$WRITEABLECONST OFF}
begin
  AssignableConst := AssignableConst + 1; 
  WriteLn('Test is called ' + IntToStr(AssignableConst) + ' times'); 
end;

Answer 2

A typed const, by default (Edit: as noted by Rob in comments, this was changed to no longer be the default years ago), is more like a static variable. You can turn this behavior off with a compiler directive.

This was commonly used as a substitute for class/static properties in old versions of Delphi. Now that Delphi actually has that feature, there is no good reason to do this IMHO.