[fpc-pascal] various ways of passing a class instance as a parameter

[Graeme Geldenhuys]

Hi,

What exactly is the difference (if any) between the parameter modifier
when you pass a class instance to a procedure?

In the example below, I can define foo() as follows...

procedure foo(AClass: TStringList);
  or
procedure foo(var AClass: TStringList);
  or
procedure foo(const AClass: TStringList);


...and the program output is always the same. As in the first case
where I don't specify var or const, how does FPC treat the AClass
parameter?


-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
program a;

{$mode objfpc}{$H+}

uses
 Classes;

procedure foo(AClass: TStringList);
begin
  AClass.Add('inside foo');
end;

var
 sl: TStringList;
begin
  sl := TStringList.Create;
  try
    sl.Add('inside main');
    foo(sl);
    sl.Add('the end');
    writeln(sl.Text);
  finally
    sl.free;
  end;
end.
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-


Here is the program output:

$ ./a
inside main
inside foo
the end



-- 
Regards,
  - Graeme -


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