Semantics. Names. Binding Time

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1 /24/ CSE 3302 Programming Languages Semantics Chengkai Li, Weimin He Spring Names Names: identif language entities variables, procedures, functions, constants, data tpes, Attributes: properties of names Eamples of attributes: Data tpe: int n = 5; (datatpe:integer) tpe: int itself is a name Value: ( value: 5) Location: int* ; = new int; Parameters, return value: int f(int n) {... Chengkai Li, Weimin He, Chengkai Li, Weimin He, 2 Binding Binding: associating attributes to names declarations assignments declarations (prototpe) and definition of a function The bindings can be eplicit or implicit e.g. int ; Eplicit binding: the data tpe of Implicit binding: the location of (static or dnamic, depending on where the declaration is) Binding Time Binding Time: the time when an attribute is bound to a name. Static binding (static attribute): occurs before eecution Language definition/implementation time: Therangeofdatatpeint tpe translation time (parsing/semantic analsis): The data tpe of a variable link time: The bod of eternal function load time: Location of global variable Dnamic binding (dnamic attribute): occurs during eecution entr/eit from procedure or program: the value of local variables Chengkai Li, Weimin He, 3 Chengkai Li, Weimin He, 4 Where can declarations happen? Blocks ({, begin-end, Algol descendants: C/C++, Java, Pascal, Ada, ) e.g., C Function bod Anwhere a statement can appear (compound statement) Eternal/global Structured data tpe Class Chengkai Li, Weimin He, 5 C++ Eample const int Maimum = 00; struct FullName {string Lastname, string FirstName; class Student { private: struct FullName name; int Age; public: void setvalue(const int a, struct FullName name); int TStudent(); ; void Student::setAge(const int a, string lname, string fname) { int i; Age = a; { int j; name.lastname = lname; name.firstname = fname; Chengkai Li, Weimin He, 6

2 /24/ Scope of Binding Scope of Binding: the region of the program where the binding is maintained (is valid, applies). Block-structured language leical scope (static scope): from the declaration to the end of the block containing the declaration. dnamic scope : introduced later. Chengkai Li, Weimin He, 7 int ; char ; { int i; double z; int w[0]; w i z Eample p q main Chengkai Li, Weimin He, 8 int ; char ; Declaration before Use Eception in OO languages: Scope of local declarations inside a class declaration includes the whole class. public class { public int getvalue() { return value; int value; Chengkai Li, Weimin He, value 9 Scope Hole Scope Hole: Declarations in nested blocks take precedence over the previous declarations. That is, binding becomes invisible/hidden. int ; char ; = a ; = 2; (bound with character data tpe) Chengkai Li, Weimin He, (bound with integer data tpe) 0 Access Hidden Declarations Hide a Declaration scope resolution operator :: (C++) int ; File : File 2: etern int ; int ; char ; = a ; (bound with ::=42; (bound with character data tpe) integer data tpe) the hidden integer variable = 2; Chengkai Li, Weimin He, File : File 2: etern int ; static int ; CSE3302 Programming Languages, UT-Arlington Chengkai Li, Weimin He, 2 2

3 /24/ Smbol Table Smbol Table: maintain bindings. Can be viewed as functions that map names to their attributes. Names SmbolTable Chengkai Li, Weimin He, Attributes 3 Static vs. Dnamic Scope Static scope (leical scope): scope maintained staticall (during compilation) follow the laout of source codes used in most languages Dnamic scope: scope maintained dnamicall (during eecution) follow the eecution path few languages use it (The bindings cannot be determined staticall, ma depend on user input). Lisp: considered a bug b its inventor. Perl: can choose leical or dnamic scope Chengkai Li, Weimin He, 4 int = ; char = a ; double =2.5; printf( %c\n,); int = 42; printf( %d\n,); int = ; char = a ; double =2.5; printf( %c\n,); int = 42; printf( %d\n,); The smbol table in p: the bindings available in p double, local to p char = b ; Chengkai Li, Weimin He, 5 char = b ; Character, global Chengkai Li, Weimin He, 6 int = ; char = a ; double =2.5; printf( %c\n,); int = 42; printf( %d\n,); The smbol table in q: integer, local to q char = b ; Chengkai Li, Weimin He, 7 int = ; char = a ; double =2.5; printf( %c\n,); int = 42; printf( %d\n,); The smbol table in main: the bindings available in main character, local to main char = b ; Chengkai Li, Weimin He, 8 3

4 /24/ The smbol table in previous slides are built during compilation The bindings are used in generating the machine code Result: a E.g., semantics of q int = 42; printf( %d\n,); Chengkai Li, Weimin He, The smbol table in q: integer, local to q 9 Practice for Point Point 2 Point 3 int,; void g(void) { = + ; = + ; void f(void) { int ; = + ; = + ; = ; = 2; f(); printf("=%d,=%d\n",,); Question : Draw the smbol table at the given points in the program, using static Question 2: What does the program print, using static 20 Chengkai Li, Weimin He, What if dnamic scope is used? int = ; char = a ; double =2.5; printf( %c\n,); int = 42; printf( %d\n,); char = b ; Chengkai Li, Weimin He, 2 What if dnamic scope is used? int = ; char = a ; double =2.5; printf( %c\n,); int = 42; printf( %d\n,); The smbol table in main: the bindings available in main char = b ; Chengkai Li, Weimin He, 22 What if dnamic scope is used? int = ; char = a ; double =2.5; printf( %c\n,); int = 42; printf( %d\n,); 98 The smbol table in q: integer, 42, local to q char = b ; Chengkai Li, Weimin He, 23 What if dnamic scope is used? int = ; char = a ; double =2.5; printf( %c\n,); int = 42; printf( %d\n,); 98 * The smbol table in p: the bindings available in p double, 2.5, local to p integer, 42, local to q char = b ; Chengkai Li, Weimin He, 24 4

5 /24/ Practice for Dnamic Scope Point Point 2 Point 3 int,; void g(void) { = + ; = + ; void f(void) { int ; = + ; = + ; = ; = 2; f(); printf("=%d,=%d\n",,); Question : Draw the smbol table at the given points in the program, using dnamic Question 2: What does the program print, using dnamic 25 Chengkai Li, Weimin He, 5

Semantics (cont.) Symbol Table. Static Scope. Static Scope. Static Scope. CSE 3302 Programming Languages. Static vs. Dynamic Scope

Semantics (cont.) Symbol Table. Static Scope. Static Scope. Static Scope. CSE 3302 Programming Languages. Static vs. Dynamic Scope -2-1 CSE 3302 Programming Languages Semantics (cont.) Smbol Table Smbol Table: maintain bindings. Can be viewed as functions that map names to their attributes. Names SmbolTable Attributes Chengkai Li,