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

class attribute {#class-attribute}

An attribute object is a mapping from a <-name to an <-value. Attributes are used by class sheet to form a list of attribute-value pairs and by class object to associate object ->attributes to objects (object-level programming).

An attribute is a subclass of class program_object, allowing the programmer to spy attribute assignment and attribute fetch operations. See program_object ->trace and friends.

@see object<-all_attributes @see object->attribute @see class interceptor @see class sheet

attribute<->name: any Name of the attribute. When used as attribute to an object, this may be any name that is not already the name of an instance variable defined at the class level for this object. It is advised to use a name that is not defined as a get- or send-method at the class level. For a sheet any name is valid, but it is advised not to use a name that conflicts with a get- or send-method defined at class sheet. See also sheet ->value.
attribute<->value: any The value of an attribute has type any. This implies that any PCE object may be used to fill the attribute, but an attempt to fill it with a function object such as @receiver, @arg1, ... or an obtainer will evaluate the special object or obtainer:
```
send(Attribute, value, @arg1)
```
Will fill Attribute with the value @arg1 refers to.

attribute->initialise: name=any, value=any Create an attribute object from its <-name and <-value. Attributes are normally used only in the context of sheet objects or `object ->attribute'.
attribute->send: context=object, value=any Set the value of the attribute (takes 1 argument). Compatibility to send_method ->send.

attribute<-argument_type: index=[int] -> type Type of the 1-based argument. For compatibility with methods and variables. Returns type(any) if index equals 1 or @default.
attribute<-convert: any -> attribute Convert a name into an attribute with this name and <-value @nil. Used to associate attributes without giving them a value:
```
send(Object, attribute, age).
send(Sheet, append, age).
```
@see object->attribute
attribute<-get: object -> any Fetch the value of the attribute. No arguments may be supplied. Compatibility with get_method <-get.