CS 342 Lecture -*- Outline -*-

* Data Structures
** Kinds of data structures
***	primitives
		no components (integers, enumeration types...)
***	structured types
		have components that are other data (arrays, records)
		structured types are second-class citizens

** Types designed for security and efficiency
		enumeration types, subrange types, set types, ...

** Problems with Pascal Types

***	feature interaction problem for arrays

****		cannot write programs that work on arrays of different sizes
			(in original Pascal)
			-dimensions of array part of type
			-actual parameters must match formals
			
			e.g. cannot write sort procedure that sorts
				arrays of integers with different index types

****		solution adopted in ISO Standard Pascal
			allow conformant array schemas as types for formals:
				---------------
				array[lwb..upb: integer] of real
				--------------
				is type of real arrays indexed by integers;
				binds lwb to lower bound and upb to upperbound

		still can't write sort that works on all arrays

***	Security of variant records
		useful when parts of record depend on data in other parts
			---------
			record s: sign;  {illustrates syntax}
				case op: opcode of
				0,8:
					(addrs: array[1..2] of data_address);
				1,2,3,4,5,6,7:
					(addrs: array[1..3] of data_address);
				9:	();
				end
			end
			---------
****		space efficiency
****		security benefit, cannot access fields that are meaningless
****		security loophole
			no need to synchronize changing of tag with fields
				of variant
			can have variant without tag represented
				case typename of ...
		=> can subvert type system, access uninitialized storage

*** Pointer Security
	can only refer to storage for values of given type
	can only refer to storage on the heap (dynamic alloc)
		not to the run-time stack.
			=> prevents dangling references
	but "dispose" (in impls only) permits dangling references!