Global variables in Delphi

Nope, a unit is not equivalent to a form.

A unit is a module which contains part of your program. Each form is a separate unit but a unit does not have to contain a form.

Each unit has an interface section and a implementation section. The declarations in the interface section are visible to all units that use the unit:

unit A;

interface

  type
    TMyClass = class
    end;


implementation

end.


unit B;

interface

uses
  A;  // I can now see and use TMyClass.

You can declare global variables by declaring them in a unit:

unit A;

interface

  var
    GVar1 : Integer;

implementation

  var 
    GVar2 : Integer;

end.

GVar1 is visible and can be modified by all units using unit A. GVar2 is only visisble by the code of unit A because it is defined in the implementation section.

I strongly advice against using globals in the interface section because you have no control over them (because anybody can change them). If you really need a global, you better define it in the implementations section and provide access functions.

By the way, you can see a unit as a kind of a class (with a single instance). It even has a way to construct and destruct:

unit A;

interface

  type
    TMyClass = class
    end;


implementation

initialization
  // Initialize the unit
finalization
  // Free resources etc. You can olny have a finalization if you have an initialization.
end.