Languages Focus: Variables

A variable holds a value that you can use and change throughout your code so long as the variable is within scope. With variable declaration, you not only want to know the syntax of how you declare a variable but you also want to know where. Are you allowed to declare a variable inline? What are the available scopes: local vs. global. Can you assign a value at the same time you declare a variable?

procedure SomeProcedure;

var
  Fullname: String;
  Age: Integer;

  X, Y, Z: Double;

  MyArray: array [1..100] of Char;

begin

end;

//You can initialize global variables.

var

  ClickCounter: Integer = 0;

Interface versus Implementation

Variables declared in the interface section of a unit are truly global and you should limit the number of variables you declare in the interface section especially for reusable units (units that contain classes). Variables declared in the implementation section of a unit have a scope limited to the unit.

Initializing Local Variables

You cannot initialize local variables. The following commented out code, does not work:

procedure TForm2.Button6Click(Sender: TObject);
var
//  ButtonClicks: Integer = 0;  //Does not work!
begin

end;

Initializing Global Variables

You can initialize global variables but not local variables. Suppose you wish to allow a user to click a button up to 3 times. You can initialize a global variable to track clicks.

The following code does work:

var
  ClickCounter: Integer = 0;  //Does work!
procedure TForm2.Button7Click(Sender: TObject);
begin
  If ButtonClicks >= 3 then
    ShowMessage('Stop clicking the button.')
  Else
  begin
    ButtonClicks := ButtonClicks + 1;
    Form2.Caption := IntToStr(ButtonClicks);
  end;
end;

Instance Counter

Initialized Global Variables and Static Data: Initialized global variables are important for many reasons but I will discuss it's relation to static class data here. Static class data is data of a class that retains state (it's value) whether or not there is an instance of a class. Suppose you wish to have an instance counter. If no classes are currently created, the current value of your instance counter needs to be 0. For each class created, you add 1. For each destroyed, you subtract 1.

• Delphi Instance Counter