Chapter 5: Programming Practices - TOC - 5.4 DATA STRUCTURES

5.4.7 Access Discriminants

guideline

Use access discriminants to create self-referential data structures, i.e., a data structure one of whose components points to the enclosing structure.

example

See the examples in Guidelines 8.3.6 (using access discriminants to build an iterator) and 9.5.1 (using access discriminants in multiple inheritance).

rationale

The access discriminant is essentially a pointer of an anonymous type being used as a discriminant. Because the access discriminant is of an anonymous access type, you cannot declare other objects of the type. Thus, once you initialize the discriminant, you create a "permanent" (for the lifetime of the object) association between the discriminant and the object it accesses. When you create a self-referential structure, that is, a component of the structure is initialized to point to the enclosing object, the "constant" behavior of the access discriminant provides the right behavior to help you maintain the integrity of the structure.

See also Rationale (1995, §4.6.3) for a discussion of access discriminants to achieve multiple views of an object.

See also Guideline 6.1.3 for an example of an access discriminant for a task type.