# Section 6.6.3:
Attribute Definition

Defining an attribute is merely a short-hand way of defining one or two features - a reader routine and a writer routine. The way in which these are implemented in any conforming Sather implementation does not change this.

The difference between a shared entity and an attribute lies in the different internal visibility - captured in the invariants to the abstract syntax below.

#### Concrete Syntax

attribute definition = | [Private_SY | Readonly_SY], Attr_SY,
| declaration ;

#### Abstract Syntax

The two features (reader and writer) which go to make up the attribute short-hand need to be defined as a pair as follows.

##### Reader

This feature needs defining without an implementation, without arguments - just a value, giving the invariant shown below.

Feature ::
name : token
args : [ set of Feature ]
kind : [ Type_Specifier ]
value : [ Expression ]
impl : [ seq of Statement ]
vis : [ Visibility ]
ext_vis : Visibility
uses : [ Method_Kind ] ;
inv mk_Feature(-,args,-,val,imp,vis,-,use) ==
use = One_Shot
and (vis = {})
and (imp = nil)
and (val <> nil)
and (args = nil)

Note that the external visibility field needs testing against the concrete syntax specifications in the well-formedness checks.

##### Writer

This feature needs defining with a single statement as implementation, with a single argument, giving the invariant shown below.

Feature ::
name : token
args : [ set of Feature ]
kind : [ Type_Specifier ]
value : [ Expression ]
impl : [ seq of Statement ]
vis : [ Visibility ]
ext_vis : Visibility
uses : [ Method_Kind ] ;
inv mk_Feature(-,args,-,val,imp,vis,-,use) ==
use = One_Shot
and (vis = {})
and (len imp = 1)
and (hd imp = Assign_Call)
and (val = nil)
and (card args = 1)

Note that the external visibility field needs testing against the concrete syntax specifications in the well-formedness checks.

#### Static Semantics

TO BE DONE

#### Dynamic Semantics

TO BE DONE

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Comments or enquiries should be made to Keith Hopper . Page last modified: Friday, 26 May 2000. |