J3/13-214

To: J3
From: Van Snyder
Subject: Protected types
Date: 2013 December 19
Reference: 04-167; 2004 hate-dislike-like-love score 0-0-11-0

Title: Protected types

Submitted by: Van Snyder

Status: For consideration

Basic functionality: Define a PROTECTED attribute for types.

Rationale:  The PROTECTED attribute cannot protect a linked list that is
traversed using a pointer.  The PROTECTED attribute for a type would
improve performance and reduce labor cost by not requiring PRIVATE
components and trivial "read" functions.

Estimated impact: Minor

Markham M6 conformance: Remove simple deficiency and discrepancy.

Detailed specification: Define a PROTECTED attribute for types.  If a
type has the PROTECTED attribute and the type definition is accessed by
use association:

      o a nonpointer dummy argument of the type shall have INTENT(IN),
        { this isn't strictly necessary but might improve error
          reporting }
      o a variable of the type shall not be storage associated,
        { this is stronger than the PROTECTED attribute for variables,
          which prohibits common but admits equivalence }
      o a variable of the type or a subobject of a variable of the type
        shall not appear in a variable definition context (16.6.7), and
      o a subobject of a variable of the type shall not appear in a
        pointer association context (16.6.8).

This allows a pointer outside the module where the type is defined to
step through a linked list, but prohibits revising pointers or component
values within the list.

Draft edits: To estimate scope of project

Insert an alternative for <type-attr-spec>

R427 <type-attr-spec> <<is>> ABSTRACT
                      ...
                      <<or>> PROTECTED

Within 5.3.15:

Insert a <<5.3.15.1 General>> subclause, encompassing the introductory
blather of 5.3.15, C548 and C549.

Within C549, replace "procedure pointer or variable" by "procedure
pointer, variable, or derived type definition".

Insert a <<5.3.15.2 PROTECTED attribute for variables and procedure
pointers>> subclause heading after C549.

After 5.3.15p3 insert a <<PROTECTED attribute for types>> subclause

The PROTECTED attribute for a type imposes limitations on the usage of
variables of the type if the type is accessed by use association.

C552a If the definition of a type that has the PROTECTED attribute is
      accessed by use association

      o a nonpointer dummy argument of the type shall have INTENT(IN),
      o a variable of the type shall not be storage associated,
      o a variable of the type or a subobject of a variable of the type
        shall not appear in a variable definition context (16.6.7), and
      o a subobject of a variable of the type shall not appear in a
        pointer association context (16.6.8).

{It isn't necessary, and might be undesirable, to specify that a
variable of a type that has the PROTECTED attribute and is accessed by
use association has the PROTECTED attribute.  One can always specify it
explicitly.}

If the definition of a type that has the PROTECTED attribute is accessed
by use association, a nonpointer object of the type, and its subobjects,
are not definable.

NOTE 5.21a
  Unlike a variable with the PROTECTED attribute, a variable of a type
  with the PROTECTED attribute can be a <data-target> in a
  <pointer-assignment-stmt>.  This allows, for example, to traverse a
  linked list but prevents changing objects within the list.