[DUG] Variables stored

Fri Jan 21 09:43:32 NZDT 2011

Assignable typed constants are pointless.

If you are going to declare a constant and then use a compiler switch to
make it behave like a variable, then be honest and just use a variable!!

Typed constants cannot even be used where "normal" constants can be:

    const

       ONE: Integer = 1;

   procedure Foo(aIndex: Integer = ONE);   // ERROR: Constant expression
expected

or:

  case iValue of

    ONE: ;  // ERROR: Constant expression expected

  end;

Remove the type declaration from the ONE const, and both compile just fine
(note that this is independent of the Assigned Typed Constants compiler
setting).

A typed constant is to all intents and purposes a variable, and might as
well be declared as such (note that you can initialise a unit variable as
part of it's declaration):

  var

    ONE: Integer = 1;

One final comment - Delphi really has no concept of a "global" variable.
The highest visibility is "unit variable".  You still have to explicitly
"use" a unit to bring it into scope, and if two units declare variables of
the same name normal scoping rules apply but you can explicitly qualify a
reference using the unit name if you need to override the default scope.

Most of the time global/unit variable is close enough that the distinction
doesn't matter, but when it comes to dogma the old rules that "global
variables are the spawn of Satan himself" needs to be tempered in Delphi
with the fact that they are a bit better "behaved" than the sort of global
variable that those ritualistic rules were originally devised for.

However, it is still true that the occasions when a unit variable is called
for are relatively few, but they should not be simply outlawed for being
something they are not.

J

From: delphi-bounces at delphi.org.nz [mailto:delphi-bounces at delphi.org.nz] On
Behalf Of Robert martin
Sent: Friday, 21 January 2011 09:18
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] Variables stored

Hi John

While all you suggest may be true (I don't know about the compiler setting
stuff) I would strongly recommend anybody starting out with programming /
delphi avoids using global variables.  Its a nasty habit to get into :)  

Also the compiler setting you talk about sounds dangerous as someone who
didn't know about it could become highly confused looking at the code or
trying to compile it on a fresh install of Delphi !

Also note I changed the typo spelling of the subject because it was annoying
me :)

Cheers
Rob

On 21/01/2011 9:04 a.m., John Bird wrote: 

There is another way as well, you can declare simple global variables -
depending where you declare it determines it's scope - how visible it it is.

In this example string2 can be seen by any unit that uses this one, just as
Form11 (the particular instance of TForm11, and is also a global variable)
is visible.

String3 can be seen by all procedures in this unit, but not by anywhere
else.    If you have lots of simple variables to store and they don't need
to be inherited etc this is the simplest way to do it.

You can take this further:

If I have lots of constants to declare, or variables to store I create a
unit which is code only (no classes) eg called storeunit and declare all the
constants and variables I want there, any form or unit that wants access to
all these variables just has to add storeunit to its uses clause.   This
centralises all such declarations in one place away from a form and is very
tidy.   I will often put simple global functions and procedures in here too,
as they also become globally available, eg various standard ways for
formatting dates and strings.   Also this unit can be uses in different
projects as well.   For this just go to File/New/Unit   and the IDE gives
you a new blank unit already to add stuff to - a simpler unit with no form
or class stuff.

Here string4 string5 integer1 integer2 integer3 can all be seen from
anywhere that uses Storeunit

It depends on whether you like using global variables or not.   Also its a
good idea to use a clear naming convention for such variables.

There are other tricks you can do too -  you can alter compiler settings to
allow assignable constants for a procedure, then any values assigned here
will be preserved between calls to the procedure.   But that seems to be
confusing to me, as it really becomes a variable and not a constant.   I saw
that used in and example where the program wanted a counter of the number of
times the procedure was called, and the counter constant in the procedure
was assigned a new value each time the procedure was called, its quite a
tidy way to do that sort of thing.  In this case the scope (visibility) of
the variable is totally limited to the one procedure.

type
  TForm11 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    MyString : string;
  end;

var
  Form11: TForm11;

  string2: string;

implementation

uses storeunit;

{$R *.dfm}

var

string3: string;

procedure TForm11.Button1Click(Sender: TObject);
begin
  MyString := 'Hello, world!';

  string2:='Hello world2';

  string5:='Hello world5';
end;

unit Storeunit;

interface

var

string4:string;

string5:string;

integer1:integer;

integer2:integer;

integer3:integer;

implementation

end.

John

From: Marshland Engineering <mailto:marshland at marshland.co.nz>  

Sent: Thursday, January 20, 2011 3:45 PM

To: delphi at delphi.org.nz 

Subject: [DUG] Variabels stored

Is there a way to store variables so I can use them from one procedure to
another?  

I have been currently storing them in hidden edit.text boxes on the form but
there must be a better way. 

Cheers Wallace

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