To: J3                                                     J3/18-265
From: Bill Long
Subject: Protected components
Date: 2018-October-14
References: J3/18-146


Introduction
------------

Traditionally Fortran programers who wanted to encapsulate data and
associated procedures that operated on that data used a module.  More
recently, the alternate approach of using a derived type has become
increasingly popular.  This is partly due to the ability to more
easily extend a derived type and have polymorphic data objects.
However, this new approach lacks one of the facilities available to
the module approach - the ability to limit modification of specific
data objects using the PROTECTED attribute.


Use Cases
---------

Consider a set of data in a file.  For the programmer, the whole
description of this data, to be encapsulated in a derived type,
includes characterizing information such as, possibly, the name of the
file, descriptive information, and the size of the data set. This
auxiliary information would be stored in components of an object of
the derived type, but should be defined only by the associated module
procedure that sets up the values as part of reading the data from the
file.  These characterization components should be protected from
modification by actions outside the module.



Feature suggestion and possible issues
--------------------------------------

Allow PROTECTED as an attribute on data components of a derived type
definition that appears in a module, with the same general semantics
of the existing PROTECTED attribute for data objects.

Possible issue: What happens if a component added by type extension is
declared with the PROTECTED attribute?  The simplest rule would be to
permit definition of such components only in a procedure contained in
the module where the extended type is defined.

Possible issue: Can a value be specified for a component with the
PROTECTED attribute in a type constructor expression that appears
outside the module? I would propose that this be disallowed.

Possible issue: Can PROTECTED components be defined as part of
intrinsic assignment to an object of a type with PROTECTED components?
I would propose that this be allowed.  But, if the assignment is
defined assignment, then the defined assignment procedure has to be
contained in the module where the type is defined.


(Rough) Requirements
--------------------

Allow the PROTECTED attribute to be specified in the declaration of a
data component in a type definition that appears in a module. Any
object of that type will have components with the PROTECTED attribute
for components declared with that attribute, independent of whether
the whole object has the PROTECTED attribute.  Allow explicit
modification of the PROTECTED data component only in procedures
contained in the module where the derived type definition that
includes the component declaration appears.

Specify semantics for intrinsic and defined assignment where the
variable is a derived type with a component with the PROTECTED
attribute.

Specify whether a type value constructor for such a type may specify a
value for the PROTECTED component.