1 / 79

Basic Semantics

Basic Semantics. Content. Names, attributes, and bindings Declarations, scope and the symbol table Overloading Allocation, lifetimes, and the environment Variables and constants Aliases, dangling references, and garbage. Names.

jafari
Download Presentation

Basic Semantics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Basic Semantics

  2. Content • Names, attributes, and bindings • Declarations, scope and the symbol table • Overloading • Allocation, lifetimes, and the environment • Variables and constants • Aliases, dangling references, and garbage

  3. Names • A fundamental abstraction mechanism in a programming language is the use of names or identifiers to denote language entities or constructs • In most languages, constants, variables, and procedures can have names assigned by the programmer

  4. Attributes • The meaning of a name is determined by the attributes associated with the name const int n = 5; /* n is a constant */ int x; /* x is a variable */ double f(int n) { … } /* f is a function */

  5. Binding • The process of associating an attribute to a name is called binding const int n = 2; /* static binding */ int x; /* static binding */ x = 2; /* dynamic binding */ int *y; /* static binding */ y = new int; /* dynamic binding */

  6. Binding Time • Language definition time: int, char, float • Language implementation time: sizeof(int) • Translation time: types of variables • Link time: code body of external functions • Load time: locations of global variables • Execution time: values of variables, locations of local variables

  7. Symbol Table • Bindings can be maintained by a data structure called the symbol table • For an interpreter, the symbol table is a function that maps names to attributes Symbol Table Names Attributes

  8. Symbol Table • For a compiler, the symbol table is a function that maps names to static attributes Symbol Table Names Static attributes

  9. Environment & Memory • The dynamic attributes locations and values can be maintained by two functions environment (memory allocation) and memory(assignment)environment Names Locationsmemory Locations Values

  10. Declarations • Declarations are a principal method for establishing bindings • Declarations are commonly associated with a block. These declarations are called local declarations of that block. • Declarations associated with an enclosing block are called nonlocal declarations • Declarations can also be external to any block. These declarations are called global declarations

  11. An Example int x; /* global */ main() { int i, n; /* nonlocal */ i = 1; n = 10; while (i < n) { int m; /* local */ m = i++; f(m); } }

  12. Scope • The scope of a binding is the region of the program over which the binding is maintained • We can also refer to the scope of a declaration if all the bindings established by the declaration have identical scopes • In a block-structured language, the scope of a binding is limited to the block in which its associated declaration appears. Such a scope rule is called static (or lexical) scope

  13. int x; void p(void) { int i; … } void q(void) { int j; … } main() { int k;… } i x p j q main k An Example

  14. Visibility • The local binding of a name will take precedence over the global and nonlocal binding of the name. This causes a scope hole for the global and nonlocal binding of the name • The visibility of a binding includes only those regions of a program where the binding applies

  15. An Example int i; void p(void) { int i; … } void q(void) { int j; … } main() { int i;… } i i scope hole i i j scope hole i scope visibility

  16. Scope Resolution Operators • Scope resolution operators can be used to access the bindings hidden by scope holes

  17. An Example /* An example in C++ */ int x; class ex { int x; void f() { int x; x = 1; /* local variable x */ ex::x = 2; /* x field of class ex */ ::x = 3; /* global variable x */ } void g() { x = 4; /* x field of class ex */ } };

  18. Scope Across Files • In C, global variable declarations can actually be accessed across filesFile 1: extern int x; /* use x in other files */File 2: int x; /* x can be used by other files */File 2: static int x; /* x can only be used in this file */

  19. The Symbol Table • A symbol table is like a dictionary for names: It must support insertion, lookup, and deletion of names with associated attributes • The maintenance of scope information in a statically scoped language with block structure requires that declarations be processed in a fashion like stack

  20. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x

  21. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x char global y

  22. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x char global y void function p

  23. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } double local to p int global x char global y void function p

  24. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } double local to p1 int global x int array local to p2 char global y void function p

  25. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x char global y void function p

  26. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x char global y void function p void function q

  27. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x int local to q char global y void function p void function q

  28. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x char global y void function p void function q

  29. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x char global y void function p void function q void function main

  30. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } char local to main int global x char global y void function p void function q void function main

  31. An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x;… } int global x char global y void function p void function q void function main

  32. Symbol Tables for Structures struct { int a; char b; double c; } x = {1,'a',2.5}; void p(void) { struct { double a; int b; char c; } y = {1.2,2,'b'}; printf("%d, %c, %g\n",x.a,x.b,x.c); printf("%f, %d, %c\n",y.a,y.b,y.c); } main() { p(); return 0; } struct global a int x char b c double void function p a double int b struct local to p y c char

  33. Nested Procedures procedure ex is x: integer := 1; y: character := ‘a’; procedure p is x: float := 2.5; begin put(y); new_line; A: declare y: array (1..10) of integer; begin y(1) := 2; put(y(1)); new_line; put(ex.y); new_line; end A; end p; procedure q is y: integer := 42; begin put(x); new_line; p; end q; begin B: declare x: character := ‘b’; begin q; put(ex.x); new_line; end B; end ex;

  34. Nested Procedures ex procedure x integer x float y character block A procedure p y array of integer procedure y q integer block x B character

  35. Dynamic Scope • Using dynamic scope, the binding of a nonlocal name is determined according to the calling ancestors during the execution instead of the static program text

  36. An Example: Static Scope int x = 1; char y = ‘a’; void p(void) { double x = 2.5; printf(“%c\n”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%d\n”, x); p(); } main() { char x = ‘b’;q(); return 0; } a 1

  37. An Example: Dynamic Scope int = 1 global int x = 1; char y = ‘a’; void p(void) { double x = 2.5; printf(“%c\n”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%d\n”, x); p(); } main() { char x = ‘b’;q(); return 0; } x char = ‘a’ global y void function p void function q void function main

  38. An Example char = ‘b’ in main int = 1 global int x = 1; char y = ‘a’; void p(void) { double x = 2.5; printf(“%c\n”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%d\n”, x); p(); } main() { char x = ‘b’;q(); return 0; } x char = ‘a’ global y void function p void function q void function main

  39. An Example char = ‘b’ in main int = 1 global int x = 1; char y = ‘a’; void p(void) { double x = 2.5; printf(“%c\n”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%d\n”, x); p(); } main() { char x = ‘b’;q(); return 0; } x int = 42 in q char = ‘a’ global y void function p void function q 98 void function main

  40. An Example double = 2.5 in p char = ‘b’ in main int = 1 global int x = 1; char y = ‘a’; void p(void) { double x = 2.5; printf(“%c\n”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%d\n”, x); p(); } main() { char x = ‘b’;q(); return 0; } x int = 42 in q char = ‘a’ global y * void function p void function q void function main

  41. Issue of Dynamic Scope • When a nonlocal name is used in an expression or statement, the declaration that applies to that name cannot be determined by simply reading the program • Static typing and dynamic scoping are inherently incompatible • Languages that use dynamic scope are Lisp, APL, Snobol, Perl

  42. Overloading • An operator or function name is overloaded if it is used to refer to two or more different things in the same scope 2 + 3 integer addition 2.1 + 3.2 floating-point addition 2 + 3.2 error or floating-point addition by automatically converting 2 to 2.0

  43. Overload Resolution • Overloading of operators or function names can be resolved by checking the number of parameters (or operands) and the data types of the parameters (or operands)int max(int x, int y); max(2, 3);double max(double x, double y); max(2.1, 3.2);int max(int x, int y, int z); max(1, 3, 2);

  44. Overloading & Automatic Conversion • Automatic conversions complicate the process of overload resolutionmax(2.1, 3); C++: error, both conversions are allowed Ada: error, no conversion is allowed Java: only int to double is allowed

  45. Overloading & Automatic Conversion • Since there are no automatic conversions in Ada, the return type of a function can also be used to resolve overloadingfunction max(x:integer; y:integer) return intfunction max(x:integer; y:integer) return floata: integer;b: float;a := max(2, 3); -- call to max #1b := max(2, 3); -- call to max #2

  46. Operator Overloading • Ada and C++ also allow built-in operators to be extended by overloading • We must accept the syntactic properties of the operator, i.e., we cannot change their associativity or precedence

  47. An Example typedef struct { int i; double d; } IntDouble; bool operator < (IntDouble x, IntDouble y) { return x.i < y.i && x.d < y.d; } IntDouble operator + (IntDouble x, IntDouble y) { IntDouble z; z.i = x.i + y.i; z.d = x.d + y.d; return z; } int main() { IntDouble x = {1, 2.1}, y = {5, 3.4}; if (x < y) x = x + y; else y = x + y; cout << x.i << " " << x.d << endl; return 0; }

  48. Name Overloading • A name can be used to refer to completely different things class A { AA(AA) { A: for(;;) { if (A.A(A) == A) break A; } return A; } }

  49. Environment • Depending on the language, the environment may be constructed statically (at load time), dynamically (at execution time), or a mixture of the two • FORTRAN uses a complete static environment • LISP uses a complete dynamic environment • C, C++, Ada, Java use both

  50. Location Allocation • Typically, global variables are allocated statically • Local variables are usually allocated automatically and dynamically in stack-based fashion • The lifetime or extent of an allocated location is the duration of its allocation in the environment

More Related