Modula-3: Reference types

# Reference types

A reference value is either `NIL` or the address of a variable, called the referent.

A reference type is either traced or untraced. When all traced references to a piece of allocated storage are gone, the implementation reclaims the storage. Two reference types are of the same reference class if they are both traced or both untraced. A general type is traced if it is a traced reference type, a record type any of whose field types is traced, an array type whose element type is traced, or a packed type whose underlying unpacked type is traced.

A declaration for a traced reference type has the form:

        TYPE T = REF Type

where `Type` is any type. The values of `T` are traced references to variables of type `Type`, which is called the referent type of `T`.

A declaration for an untraced reference type has the form:

        TYPE T = UNTRACED REF Type

where `Type` is any untraced type. (This restriction is lifted in unsafe modules.) The values of `T` are the untraced references to variables of type `Type`.

In both the traced and untraced cases, the keyword `REF` can optionally be preceded by "`BRANDED b`" where `b` is a text constant called the brand. Brands distinguish types that would otherwise be the same; they have no other semantic effect. All brands in a program must be distinct. If `BRANDED` is present and `b` is absent, the implementation automatically supplies a unique value for `b`. Explicit brands are useful for persistent data storage.

The following reference types are predeclared:

        REFANY    Contains all traced references
        ADDRESS   Contains all untraced references
        NULL      Contains only NIL

The `TYPECASE` statement can be used to test the referent type of a `REFANY` or object, but there is no such test for an `ADDRESS`.

Examples of reference types:

        TYPE TextLine = REF ARRAY OF CHAR;

        ControllerHandle = UNTRACED REF RECORD
          status: BITS 8 FOR [0..255];
          filler: BITS 12 FOR [0..0];
          pc: BITS 12 FOR [0..4095]
        END;

        T = BRANDED "ANSI-M3-040776" REF INTEGER;

        Apple  = BRANDED REF INTEGER;
        Orange = BRANDED REF INTEGER;

    Last modified on Wed Apr  3 09:38:58 PST 1996 by heydon
         modified on Mon Apr 18 15:32:31 PDT 1994 by kalsow