one or more functions operating on some common information

class ListElem { public: int value; // Integer ListElem *next; // Pointer used to make link }; void InsertAtHead(ListElem * & h, ListElem * elem) { elem->next = h; // Make new list element points // to the start of "old list" h = elem; // Make "h" points to // the **new** starting object } void DeleteAtHead(ListElem * & h) { ListElem *p2; if (h == NULL ) // If list is empty, return(NULL) return; p2 = h; // Delete p2 h = p2->next; // Unlink AND return new head delete p2; // Return memory !! } int main(int argc, char *argv[]) { ListElem *head; InsertAtHead(head, newElem); }

The functions InsertAtHead and DeleteAtHead operating on the common information given by head element "h"

The information and the functions and maintained separately: ListElem *head; InsertAtHead(head, newElem); This can lead to abuse: NotAListElem *x; InsertAtHead((ListElem *) x, newElem);

void InsertAtHead(ListElem * & h, ListElem * elem) { elem->next = h; h = elem; } void DeleteAtHead(ListElem * & h) { ListElem *p2; if (h == NULL ) return; p2 = h; h = p2->next; delete p2; } int main(int argc, char *argv[]) { ListElem *head; InsertAtHead(head, newElem); }

The InsertAtHead() and DeleteAtHead() functions operates on a list A list is represented by the variable: ListElem *head;

The InsertAtHead() and DeleteAtHead() functions always operate on a list

class List { public: ListElem *head; void InsertAtHead(List *x, ListElem * elem) { elem->next = x->head; x->head = elem; } void DeleteAtHead(List *x) { ListElem *p2; if (x->head == NULL ) return; p2 = x->head; x->head = p2->next; delete p2; } }

We defined a class to contain the functions and the data/variables that the functions operate on We named this new class List When we define a List variable, we will use: List myList; The myList variable will has a head componet variable accessed as follows: myList.head We will pass List variables to the InsertAtHead() and DeleteAtHead() functions: void InsertAtHead(List *x, ListElem * elem) void DeleteAtHead(List *x) To access the head variable in parameter variable x we must use: x->head That's why we need to make the changes in the program.

The parameter x is always needed (because it represents the list)

(An implicit parameter that is always passed of a function.

We will soon how such a parameter is passed)

Result:

class List { public: ListElem *head; void InsertAtHead(ListElem * elem) { elem->next = x->head; x->head = elem; } void DeleteAtHead() { ListElem *p2; if (x->head == NULL ) return; p2 = x->head; x->head = p2->next; delete p2; } }

The name x is not particularly meaningful A special keyword is designated to refer to the implicit parameter this

class List { public: ListElem *head; void InsertAtHead(ListElem * elem) { elem->next = this->head; this->head = elem; } void DeleteAtHead() { ListElem *p2; if (this->head == NULL ) return; p2 = this->head; this->head = p2->next; delete p2; } }

List myList; p = new ListElem; p->value = 1500; myList.InsertAtHead(p); myList.DeleteAtHead(p); Print(myList.head);

The implicit parameter is passed in as an variable reference before the function call.

• Prog file: click here

class List { public: ListElem *head; void InsertAtHead(ListElem * elem) { elem->next = x->head; x->head = elem; } void DeleteAtHead() { ListElem *p2; if (x->head == NULL ) return; p2 = x->head; x->head = p2->next; delete p2; } void Print() { ListElem *p; p = this->head; cout << "List ==> "; while ( p != NULL ) { cout << p->value << " "; p = p->next; } cout << "\n\n"; } }

List myList; myList.Print();

Again, the implicit parameter is passed in as an variable reference before the function call.

• Prog file: click here

class List { public: ListElem *head; void InsertAtHead(ListElem * elem) { elem->next = this->head; this->head = elem; } void DeleteAtHead() { ListElem *p2; if (this->head == NULL ) return; p2 = this->head; this->head = p2->next; delete p2; } }

class List { public: ListElem *head; void InsertAtHead(ListElem * elem) { elem->next = head; head = elem; } void DeleteAtHead() { ListElem *p2; if (head == NULL ) return; p2 = head; head = p2->next; delete p2; } }

A name X used in a scope but is not defined in that scope is resolved as follows: If the name is defined in the class then it refers to: this->X Otherwise it refers to a object (variable or function) defined the global scope

• Prog file: click here

int x = 99; global variable class myClass { public: double x; class variable void f1() { double x; local variable x = 1111.0; // x refers to the local variable } void f2() { double x; this->x = 2222.0; // this->x refers to the class variable } void f3() { x = 3333.0; // x refers to the class variable } void f4() { ::x = 4444.0; // ::x refers to the global variable } }; int main(int argc, char *argv[]) { myClass a; a.x = 9999; a.f1(); cout << "After a.f1(): x = " << x << ", a.x = " << a.x << endl << endl; a.f2(); cout << "After a.f2(): x = " << x << ", a.x = " << a.x << endl << endl; a.f3(); cout << "After a.f3(): x = " << x << ", a.x = " << a.x << endl << endl; a.f4(); cout << "After a.f4(): x = " << x << ", a.x = " << a.x << endl << endl; }

• Prog file: click here