class_variable.md
In rswipl: Embed 'SWI'-'Prolog'

class class_variable {#class-class_variable}

Class-variables represents constants for the class. The are commonly used to represent default values for the visual appearance of graphical classes: thickness, fonts, colours, etc.

Class variables are normally declared using class_variable/4.

If a class defines both a variable object and a class_variable object, the value of the class-variable is used as default (initial) value for the instance variable.

Defaults for class-variables may be defined externally using the Defaults file, which resides in the XPCE home directory.

See also:

object<-class_variable_value
@pce->load_defaults
The UserGuide

class_variable<-default: any Program default value. This may be any object that satisfies <-type.
class_variable<-type: type Type of the class-variable. A textual value from the Defaults file will be converted using this type. Default is the type of the corresponding instance-variable.
class_variable-value: any Current value. Send-access is guarded by ->value to verify the type, while get-access is handled by <-value to check the Defaults file, or use <-default. The initial value is the constant @not_obtained.

class_variable->initialise: class=class, name=name, default=any, type=[type], summary=[string]* Define a class-variable for the given class. Name is the name of the class-variable. If this name matches the name of an instance variable (see class variable), the class-variable will be used as a default for the instance variable. Default defines the value if no value is specified in the Defaults file (see @pce ->load_defaults). Type declares the type. If the class-variable acts as a default-variable for an instance-variable, using @default will copy the type of the instance-variable. Summary is used for documentation. When absent, the documentation from the corresponding instance variable is used, or the documentation from a class-variable defined in one of the super-classes.

Class-variables are normally defined using class_variable/4 in the definition of a user-defined class.
class_variable->value: any Set value of the class-variable. Should be used very carefully, as instances are not notified, and may not be aware of the possibility that the value of a class-variable can change.

class_variable<-convert_string: textual=char_array -> any|function Called to convert the textual value found in the defaults file. It calls the @object_parser to translate the textual representation into an object, after which <-type is used to verify the type.
class_variable<-get: object -> unchecked Invoke (read) class-variable. Required to make class-variables available as normal object-behaviour.
class_variable<-print_name: -> name Yields .. Used to print messages.
class_variable<-string_value: -> char_array Returns the textual value from the Defaults file. See also @pce ->load_defaults.
class_variable<-value: -> any Compute and return the value. Normally, the value is used as default for an instance-variable, or fetched using object<-class_variable_value.