X3J3/97-145

Date:    21 Feb 1997
To:      X3J3/WG5
From:    Malcolm Cohen <malcolm@fortran.demon.co.uk>
Subject: Derived Type Encapsulation

Problem:
--------
It is possible to define an opaque derived type OT that has a user-defined
assignment procedure.  However, the user-defined assignment procedure is not
invoked when assigning objects of another type T containing components of type
OT.  This applies both to pointer and non-pointer components.  Since the very
reason type OT has user-defined assignment is probably that the intrinsic
assignment was inappropriate, this means that an inappropriate assignment will
be performed unless the writer of type T takes special care (i.e. also has
user-defined assignment).

Example:
--------
	MODULE iso_varying_string
		TYPE varying_string
			PRIVATE
			CHARACTER, POINTER :: value(:)
		END TYPE
		INTERFACE ASSIGNMENT(=)
			MODULE PROCEDURE assign_vs_to_vs
		END INTERFACE
	CONTAINS
		SUBROUTINE assign_vs_to_vs(var,expr)
			TYPE(varying_string),INTENT(OUT) :: var
			TYPE(varying_string),INTENT(IN) :: expr
			ALLOCATE(var%value(SIZE(expr%value,1)))
			var%value = expr%value
		END SUBROUTINE
	END MODULE
	PROGRAM programme
		USE iso_varying_string
		TYPE mytype
			TYPE(varying_string) name
		END TYPE
		TYPE(varying_string) x, y
		TYPE(mytype) a, b
		x = y ! invokes "assign_vs_to_vs(x,y)"
		a%name = b%name ! invokes "assign_vs_to_vs(a%name,b%name)"
		a = b ! does intrinsic assignment on A%name and B%name
	END

Other Problems:
---------------
	(1) It is very easy to drop the user-defined assignment even for normal
variables - a simple "USE iso_varying_string,ONLY:varying_string" quietly loses
the appropriate assignment.
	(2) Deliberate subversion of the "opaque" data type is just as easy.

Solution:
---------
1.  Add "default assignment" that is used by the assignment statement for
derived types.  Default assignment will
	(i) do pointer-assignment on pointer components
	(ii) do intrinsic assignment on non-pointer components of intrinsic type
	(iii) do defined assignment on non-pointer components of derived type
if there is a defined assignment procedure for the component type; and
	(iv) do default assignment on other non-pointer components of derived
type.  And, add a note to section 1.x noting that this is a change from F95 to
F2002.