CIS 4OO LECTURE 8

Declarations, (continued):

· Choice representations

· Storage management

· Generic operators, (i.e. overloading operators)

· Type checking

Constants:

· Literal (i.e. "23")

· Programmer defined

v Can only simply defined variables be constants?

v Can expression assign values to constants?

v When is the value assigned?

v Does language specify pre-defined constants, (pi, e, 'maxint')?

Instances of simple type:

· Integer

· Sub-range*, (i.e. x := 0…100;)

· Floating point

· Fixed point*

· Rational numbers*

· Complex numbers*

· Enumeration*

· Boolean*

( * = not likely to have hardware support)

Issues:

· Initialization

· Conversion/ coercion, (compiler attempting to convert data type)

· Static and dynamic checking

®Structure type, (None of these are likely to have hardware support):

· Vector

· Multi-dimensional arrays

· Records, ('structs')

· Strings

· Files

· Pointers

Issues of structure type:

· Specification

· Operation

· Implementation

v Storage representation, (sequential vs. linked)

v Garbage/ dangling reference

v Efficient selection of components

· Type checking

v Component existence

v Component type

Data control:

· Language features affecting…

v Visibility, (scope)

v Accessibility, (extent of variable º lifetime, i.e. from allocation to deallocation)

v Binding between an identifier and data operator or sub-projects

Data control concepts:

· Reference environments

v Local

v Non-local, (i.e. 'friend', 'static', or header in C++. 'common block' in Fortran-SEE BELOW)

v Global

v Pre-defined, (pi, e, etc.)

· Visibility

· Dynamic scope

· Reference operations-given an identifier and referencing environment, return associated data object, (pointers, et al)

EXAMPLE OF NON-LOCAL REFERENCE ENVIRONMENT:

Data control issues:

· Aliasing, (more than one name or access path)

· Dynamic scope-most recent association rule, (run time)

· Static scope, (program text)

"Block structure" language scope rules, (ALGOL):

· Declarations at the head of block define the local referencing environment for the block

· References outside block are assumed to be in next larger block

· References made to inner block identifiers are hidden from outer block

· Block may be named, reference becomes part of outer block referencing environment

2 choices upon block exit:

1) Deletion

2) Retention, (history sensitivity)

Non-local referencing environments:

Explicit common:

· Shared block of data

· Declarations added to compiler table

Import/ export, (retention):

Procedure P(variables);

Define x, y, z; // w, y, and z are made available for export

x, y, z : real

u, v : integer

begin

…

call Q

…

end

procedure Q(variables)

uses P.x, P.z // looks at procedure P and imports x and z

y : integer

begin

…

something happens

…

end