Delphi in a Nutshell

Name

Nil Keyword

Syntax

const nil = Pointer(0);

Description

The nil keyword is a special Pointer value that is guaranteed to be distinct from any real pointer.

The numeric value of nil is zero. When you create a new object, therefore, and Delphi initializes all the object’s fields to zero, you can rely on pointer, class, interface, and method type fields to be initialized to nil.

Tips and Tricks

• You can also assign nil to method-type variables or fields, in which case Delphi stores nil for the code and data pointers. When comparing a method with nil, Delphi checks only the code pointer. In other words, the following two examples are equivalent:

if Notify = nil then ...
if (TMethod(Notify).Code = nil then ...

• The most common use for nil is to mark pointer and method-type variables with a “not-a-pointer” value. For example, an event property that does not have an event handler assigned to it has the value nil.

• Assigning nil to an interface-type variable causes Delphi to call _Release on the old value of the variable (if the variable was not nil to start with).

• Delphi represents an empty dynamic array, long string, or wide string as a nil pointer.

• Many Delphi programmers prefer the Assigned function over a comparison with nil. It is primarily a style issue, but see the Assigned function for more information.

See Also

Assigned Function, Pointer Type