Delphi in a Nutshell

Name

Default Directive

Syntax

property Declaration; default;
property Declaration default Ordinal constant;

Description

The default directive has two uses in property declarations:

• To make an array property the class’s default property

• To supply a default value for an ordinal-type property

A class can declare one array property as the default property for the class. The default property lets you refer to an object reference as an array without mentioning the property name. For example, TList declares the Items property as its default array property. If List is a variable of type TList, you can refer to a list element as List[Index], which is a shorthand for List.Items[Index]. A class can have only one default property. Derived classes can define a different default property from that of an ancestor class.

An ordinal-type property (integer, enumeration, set, or character type) can list a default value. The default value has no effect on the property’s initial value. Instead, the default value is stored in the class’s RTTI, and Delphi’s design-time and runtime environments use the default value to reduce the size of a .dfm file or resource. Non-ordinal properties (e.g., floating-point, string, class) always use zero (empty string, nil, etc.) as the default value.

Tips and Tricks

• If you do not specify a default directive, Delphi assumes you mean nodefault and stores the smallest integer (-2,147,483,648) as the property’s default value. A side effect is that you cannot use the smallest integer ...