Virtual functions: interface class
#include <iostream>
#include <cstdlib>
using namespace std;
class list {
public:
list *head; // pointer to next item in list
list *tail;
list *next;
int num; // value to be stored
list() {
head = tail = next = NULL;
}
virtual void store(int i) = 0;
virtual int retrieve() = 0;
};
class queue : public list {
public:
void store(int i);
int retrieve();
};
void queue::store(int i)
{
list *item;
item = new queue;
if(!item) {
cout << "Allocation error.\n";
exit(1);
}
item->num = i;
// put on end of list
if(tail)
tail->next = item;
tail = item;
item->next = NULL;
if(!head)
head = tail;
}
int queue::retrieve()
{
int i;
list *p;
if(!head) {
cout << "List empty.\n";
return 0;
}
i = head->num;
p = head;
head = head->next;
delete p;
return i;
}
class stack : public list {
public:
void store(int i);
int retrieve();
};
void stack::store(int i)
{
list *item;
item = new stack;
if(!item) {
cout << "Allocation error.\n";
exit(1);
}
item->num = i;
// put on front of list for stack-like operation
if(head)
item->next = head;
head = item;
if(!tail)
tail = head;
}
int stack::retrieve()
{
int i;
list *p;
if(!head) {
cout << "List empty.\n";
return 0;
}
// remove from start of list
i = head->num;
p = head;
head = head->next;
delete p;
return i;
}
class sorted : public list {
public:
void store(int i);
int retrieve();
};
void sorted::store(int i)
{
list *item;
list *p, *p2;
item = new sorted;
if(!item) {
cout << "Allocation error.\n";
exit(1);
}
item->num = i;
// find where to put next item
p = head;
p2 = NULL;
while(p) { // goes in middle
if(p->num > i) {
item->next = p;
if(p2) p2->next = item; // not 1st element
if(p==head) head = item; // new 1st element
break;
}
p2 = p;
p = p->next;
}
if(!p) { // goes on end
if(tail) tail->next = item;
tail = item;
item->next = NULL;
}
if(!head) // is first element
head = item;
}
int sorted::retrieve()
{
int i;
list *p;
if(!head) {
cout << "List empty.\n";
return 0;
}
// remove from start of list
i = head->num;
p = head;
head = head->next;
delete p;
return i;
}
int main()
{
list *p;
queue q_ob;
p = &q_ob; // point to queue
p->store(1);
p->store(2);
p->store(3);
cout << "Queue: ";
cout << p->retrieve();
cout << p->retrieve();
cout << p->retrieve();
cout << '\n';
stack s_ob;
p = &s_ob; // point to stack
p->store(1);
p->store(2);
p->store(3);
cout << "Stack: ";
cout << p->retrieve();
cout << p->retrieve();
cout << p->retrieve();
cout << '\n';
sorted sorted_ob;
p = &sorted_ob;
p->store(4);
p->store(1);
p->store(3);
p->store(9);
p->store(5);
cout << "Sorted: ";
cout << p->retrieve();
cout << p->retrieve();
cout << p->retrieve();
cout << p->retrieve();
cout << p->retrieve();
cout << '\n';
return 0;
}
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