Why we use "->" in this code? [question about an example in "Teach yourself C++" book
Hi fellas
I'm reading "Teach yourself C++" book by Herbert Schildt to learn C++. there is an example in chapter 10 which is tough to me.:frown: this is the code I'm talking about:
Code:
#include <iostream>
#include <cstdlib>
#include <cctype>
using namespace std;
class list {
public:
list *head; // pointer to start of list
list *tail; //pointer to end of list
list *next; //pointer to next item
int num; // value to be stored
list() { head = tail = next = null;}
virtual void store(int i) = 0;
virtual void retrieve() = 0;
};
//create a queue-type list
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;
}
// remove from start of list
i = head->num;
p = head;
head = head->next;
delete p;
return i;
}
// create a stack-type list
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;
}
int main()
{
list *p;
// demonstrate queue
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';
//demonstrate stack
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';
return 0;
}
This is alone tough code that I've seen so far in this book. now the questions:
1- He has defined three class pointer named head,tail and next as "list" object. are we allowed to define these pointers within class "list"? it a bit made me confused.
2- In some parts of the code you can see something e.g. this:
"tail->next = item;"
We use "->" to refer a data-object of a class. am I right? but here "tail" and "next" are both pointer to an object of "list". then why does he use "->"? totally these parts e.g. "tail->next" make me confused.
3- Would you please tell me how "store" and "retrive" functions work?
Notice this part of the book talks about polymorphism by virtual functions.
