dynamic memory in c++

[aditya somani]

While working on dynamic memory in c++ I noticed that values of my previous variables were changing whenever i invoke a loop.

for the following code::

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace std;
 
 
template <class T>
class Node {
private:
 
public:
    T item;
    Node<T>* next;    
    Node(T a,Node<T>* c) {item = a;next = c;};
    void set_item(T a)  {item = a;};
    void set_item(Node<T>* c) {next = c;};    
};
 
 
template <class T>
class List {
private:
        Node<T>* head;
        Node<T>* tail;
        int length;
public:
        List() {head = NULL;tail = NULL;length=0;};
        int size();
        bool empty();
        void push_back(T);
        void push_front(T);
        void insert(int,T);
        void print();
        bool ismember(T);//pain
 
        // Member functions to
        // interface with list
};
 
template <class T>
void List<T>::push_front(T a){
    length++;                  //initially head->x1->null
    Node<T>b(a,head);          //now head->new entry->x1->null
    head = &b;
    if(length == 1) tail = &b;
    cout<<"look push front gives"<<(*head).item<<endl;
    }
 
template <class T>
void List<T>::print(){
        cout<<"here"<<(*head).item<<endl;
    Node<T> *mynode;
    mynode = head;
    cout<<"here   "<<(*head).item<<endl;
    for(int i = 0;i < length;i++,mynode = (*mynode).next)
    {cout<<i<<"th entry is "<<(*mynode).item<<endl;};
}
 
int main(){
List<int> l;
cout<<l.size()<<endl;
l.push_front(23);
l.print();
 
return 0;}
 
 

the out put after compiling is

look push front gives23
here23
here -1077380900
0th entry is -1077380900

but this works fine if the for loop in print() is quoted.
what is the problem?

[ilikepython]

While working on dynamic memory in c++ I noticed that values of my previous variables were changing whenever i invoke a loop.

for the following code::

template <class T>
class List {
private:
        Node<T>* head;
        Node<T>* tail;
        int length;
public:
        List() {head = NULL;tail = NULL;length=0;};
        int size();
        bool empty();
        void push_back(T);
        void push_front(T);
        void insert(int,T);
        void print();
        bool ismember(T);//pain
 
        // Member functions to
        // interface with list
};
 
template <class T>
void List<T>::push_front(T a){
    length++;                  //initially head->x1->null
    Node<T>b(a,head);          //now head->new entry->x1->null
    head = &b;
    if(length == 1) tail = &b;
    cout<<"look push front gives"<<(*head).item<<endl;
    }
 
template <class T>
void List<T>::print(){
        cout<<"here"<<(*head).item<<endl;
    Node<T> *mynode;
    mynode = head;
    cout<<"here   "<<(*head).item<<endl;
    for(int i = 0;i < length;i++,mynode = (*mynode).next)
    {cout<<i<<"th entry is "<<(*mynode).item<<endl;};
}
 

the out put after compiling is

look push front gives23
here23
here -1077380900
0th entry is -1077380900

but this works fine if the for loop in print() is quoted.
what is the problem?

I'm guessing that has something to do with undefined behaivior because otherwise I can't explain it. I couldn't believe it myself, the first time, but changing your push_front function seems to be working:

template <class T>
void List<T>::push_front(T a){
    length++;                  //initially head->x1->null
    Node<T> b(a, head);        //now head->new entry->x1->null
    head = &b;
    if(length == 1) tail = &b;
    cout<<"look push front gives"<<(*head).item<<endl;
    }

You create the b node as an object and then assign it to a pointer. Try declaring it like this:

Node<T> *b = new Node<T> (a, head);
 

And then assign to head like this:

head = b;
 

Try it and see if it works.

PS In your node classs you have item and next declared as a public variable and you have accesor functions. That's pointless. Either make them private or you don't need setitem.

[ilikepython]

Also, I think the problem with assigning the pointer to an adress of an object is that that object is local so is deleted when the function exits. Then you have a pointer that points to garbage (nothing).