Delphi Basics : Nil command

DelphiBasics

Nil
Constant

A pointer value that is defined as undetermined

System unit

const Nil = Pointer(0);

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.

Related commands

Assigned		Returns true if a reference is not nil
Null		A variable that has no value
Pointer		Defines a general use Pointer to any memory based data

Download this web site as a Windows program.

Example code : A simple example

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;

Show full unit code

myPtr is not nil
myPtr is nil