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Class Table

Introduction

The class table stores information pertaining to all the classes declared in an ExpL program. For a class it stores member fields, member functions, name of the class and parent class pointer.

Structure

The structure of Class Table(CT) is as follows:

struct Classtable {
    char *Name;                           //name of the class
    struct Fieldlist *Memberfield;        //pointer to Fieldlist
    struct Memberfunclist *Vfuncptr;      //pointer to Memberfunclist
    struct Classtable *Parentptr;         //pointer to the parent's class table
    int Class_index;                      //position of the class in the virtual function table
    int Fieldcount;                       //count of fields
    int Methodcount;                      //count of methods
    struct Classtable *Next;              //pointer to next class table entry
};
Memberfield list is used to store the information regarding the type, name, fieldindex and type of class of all the member fields of that class. 
struct Fieldlist{
    char *Name;         //name of the field
    int Fieldindex;         //position of the field
    struct Typetable *Type;     //pointer to typetable
    struct Classtable *Ctype;   //pointer to the class containing the field
    struct Fieldlist *Next;     //pointer to next fieldlist entry
};

Memberfunc list is used to store the information regarding the type, name of the function, argument list, it's flabel and it's position. 

struct Memberfunclist {
    char *Name;                      //name of the member function in the class
    struct Typetable *Type;          //pointer to typetable
    struct Paramstruct *paramlist;   //pointer to the head of the formal parameter list
    int Funcposition;                //position of the function in the class table
    int Flabel;                      //A label for identifying the starting address of the function's code in the memory
    struct Memberfunclist *Next;     //pointer to next Memberfunclist entry
};

Associated Methods

  • struct Classtable* CInstall(char *name,char *parent_class_name)

    Creates a class table entry of given 'name' and extends the fields and the methods of parent class.

  • struct Classtable* CLookup(char *name)

    Search for a class table entry with the given 'name', if exists, return pointer to class table entry else return NULL.

  • void Class_Finstall(struct Classtable *cptr, char *typename, char *name)

    Installs the field into the given class table entry which is given as an argument.

  • void Class_Minstall(struct Classtable *cptr, char *name, struct Typetable *type, struct Paramstruct *arglist)

    Installs the method into the given class table entry which is given as an argument.

Illustration

Here is an example illustrating it. 

class
Person{
    decl
        str name;
        int age;
        int printDetails();
        str findName();
        int createPerson(str name, int age);
    enddecl
    int printDetails(){
        decl
        enddecl
        begin
            write(self.name);
            write(self.age);
            return 1;
        end
    }
    str findName(){
        decl
        enddecl
        begin
            return self.name;
        end
    }
    int createPerson(str name, int age){
        decl
        enddecl
        begin
            self.name=name;
            self.age=age;
            return 1;
        end
    }
}     /*end of Person class */
Student extends Person{

    decl
        int rollnumber;               /*  The members name and age are inherited from the parent class */
        str dept;
        int printDetails();
        int createStudent(str name, int age,int rollNo, str dept);
    enddecl
    int createStudent(str name, int age,int rollNo, str dept){
        decl
        enddecl
        begin
            self.name =name;
                self.age = age;
                    self.rollnumber = rollNo;
                    self.dept = dept;
                    return 1;
        end
    }
    int printDetails(){  /* This function is also overridden in the derived class */
        decl
        enddecl
        begin
            write(self.name);
            write(self.age);
            write(self.rollnumber);
            write(self.dept);
            return 1;
        end
    }         /**  The derived class inherits the findName() function from the parent **/
}  /* end of student class */
endclass

  1. As soon as the compiler encounters the class name , it installs the class name and the parent class name if present into the class table. Subsequently, If there is an extension to the parent class, all the member fields and methods of parent class are inherited. Following is how class table looks when person is installed.

  2. Following is how class table looks when student class is installed.

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