1. Name Spaces: ALL versus OOL
Maryam Gholamalitabar
Spring 2017

2. The Procedure as a Name Space
Each Scope (procedure or block) creates its own name space
Any name (almost) can be declared locally
Local names obscure identical non-local names
Local names cannot be seen outside the procedure
We call this set of rules & conventions “lexical scoping”
Examples
Algol and Pascal are the classic examples
Nested Procedures, often with deep nesting
C has global, static, local, and block scopes
Blocks can be nested, procedures cannot

3. The Procedure as a Name Space
Why introduce lexical scoping?
Simplifies rules for naming & resolves conflicts
Lets the programmer introduce “local” names with impunity
Provide a compile-time mechanism for binding “free” variables
The Questions
At point p, which declaration of x is active?
At run-time, where is x found?
The Answer
The compiler needs structures that model both the creation and the destruction of scopes
Compilers use lexically-scoped symbol tables for this purpose

4. Lexically-Scoped Symbol Tables
High-level idea
Create a new table for each scope
Chain them together for lookup
The symbol table interface
enter() - enter a new scope level
insert(name) – creates entry for name in current scope
lookup(name) – lookup a name, return an entry
exit() – leave scope, remove all names declared there

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20. Name Resolution in an OOL
What names can an executing method access?
Names defined by the method
Instance variables
Class variables and method
Any object defined in the global name space
An OOL resembles an ALL, with a different name space
Scoping is relative to hierarchy in the code of an ALL
Scoping is relative to hierarchy in both the code & the data of an OOL

21. Concrete Example: The Java Name Space
Code within a method M for object O of class C can see:
Local variables declared within M
All instance variables of O & class variables of C
All public and protected variables of any superclass of C
Classes defined in the same package as C or in any explicitly imported package
Public class variables and public class instance variables of imported classes
package class and instance variables in the package containing C
Class declarations can be nested!
Member declarations hide outer class declarations of the same name
Accessibility options: public, private, protected, package

22. Java Symbol Tables
To compiler method M of object O in class C, the compiler needs:
Lexically scoped symbol table for the current block and its surrounding scopes
Just like ALL– inner declarations hide outer declarations
Chain of symbol tables for inheritance
Class C and all of its superclasses
Need to find methods and instance variables in any superclass
Symbol tables for all global classes (package scope)
Entries for all members with visibility
Need to construct symbol tables for imported packages and link them into the structure in appropriate places
Three sets of tables for name resolution
In an ALL, we could combine 1 & 3 for a single unified set of tables.

23. The Java Name Space
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { // inherits x, y, & draw() from Point Color c; // local data public void draw() {…} // override (hide) Point’s draw public void test() { y = x; draw(); } // local code Class C { // independent of Point & ColorPoint int x, y; // local data public void m() // local code { Point p = new ColorPoint(); // uses ColorPoint, and, by inheritance y = p.x; // the definitions from Point p.draw();

24. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

25. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

26. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

27. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

28. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

29. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

30. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

31. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

32. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

33. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

34. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

35. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

36. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

37. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

38. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

39. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

40. Example
Class Point { public int x, y; public void draw(); } Class ColorPoint extends Point { Color c; public void draw() {…} public void test() { y = x; draw(); } Class C { int x, y; public void m() { Point p = new ColorPoint(); y = p.x; p.draw();

41. Java Symbol Tables
To find the address for a reference to x in method M for an object O of class C, the compiler must:
For an unqualified use (i.e., x):
Search the symbol table for the method’s lexical hierarchy
Search the symbol tables for the receiver’s class hierarchy
Search global symbol table (current package and imported)
In each case check visibility attribute of x

42. Java Symbol Tables
To find the address for a reference to x in method M for an object O of class C, the compiler must:
For a qualified use (i.e., Q.x):
Find Q by the method above
Search from Q for x
Must be a class or instance variable of Q or some class it extends
Check visibility attribute of x

43. Reference
Cooper, Keith, and Linda Torczon. Engineering a compiler. Elsevier, 2011.