| Description |
The Nil constant is a pointer value that is defined as undetermined.
Use of a Nil pointer will result in an exception.
Nil is mostly used as a substitute for a Pointer parameter - it tells the routine that no Pointer value is available for this parameter.
Pointer variables are not set to Nil except in special circumstances, such as when creating a new object that contains pointers. This is because Delphi initialises the storage taken by a new object to 0's. A Nil pointer is one that has the value 0.
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| Related commands |
| Assigned |
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Returns true if a reference is not nil |
| Null |
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A variable that has no value |
| Pointer |
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Defines a general use Pointer to any memory based data |
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| Example code : A simple example |
// Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.
unit Unit1;
interface
uses
// The System unit does not need to be defined
Forms, Dialogs;
type
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
end;
var
Form1: TForm1;
implementation
{$R *.dfm} // Include form definitions
procedure TForm1.FormCreate(Sender: TObject);
var
myPtr : PChar;
begin
// Pointer variables are not set to nil by default
if myPtr = Nil
then ShowMessage('myPtr is nil')
else ShowMessage('myPtr is not nil');
// So we must set them to nil to be sure that they are undefined
myPtr := Nil;
if myPtr = Nil
then ShowMessage('myPtr is nil')
else ShowMessage('myPtr is still not nil');
end;
end.
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| Hide full unit code |
myPtr is not nil
myPtr is nil
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