C++ linked list values changing retroactively - c++

Question

EDIT: Solution Code included at the end.
I am trying to implement a linked list class that utilizes a node class as defined in the assignment. The below code block prints output as expected:
#include <iostream>
using namespace std;
// Node class as provided
class node {
void *info;
node *next;
public:
node (void *v) {info = v; next = 0; }
void put_next (node *n) {next = n;}
node *get_next ( ) {return next;}
void *get_info ( ) {return info;}
};
// Linked list class
class list {
//Start of the linked list
node *start;
public:
list (int v) {
start = new node (&v);
}
void insert (int value, int place=-1) {
node *temp = new node (&value);
if (place == 0) {
temp->put_next(start);
start = temp;
} else {
node *before = start;
for (int i = 1; before->get_next() != 0; i++) {
if (i == place) {
break;
}
before = before->get_next();
}
temp->put_next(before->get_next());
before->put_next(temp);
}
}
void remove(int place) {
if (place == 0) {
start = start->get_next();
} else {
node *curr = start;
for (int i = 1; curr != 0; i ++) {
if (i == place) {
curr->put_next(curr->get_next()->get_next());
break;
}
curr = curr->get_next();
}
}
}
void display() {
for (node *current = start; current != 0; current = current->get_next()) {
cout << *(static_cast<int*>(current->get_info())) << endl;
}
}
};
int main() {
list *tst = new list(10);
tst->display();
cout << "Prepending 9" << endl;
tst->insert(9,0);
tst->display();
cout << "Inserting 8" << endl;
tst->insert(8,1);
tst->display();
cout << "Prepending 7" << endl;
tst->insert(7,0);
tst->display();
tst->remove(0);
cout << "Removed the first element:" << endl;
tst->display();
cout << endl;
// cout << "Prepending 6" << endl;
// tst->insert(6,0);
// tst->display();
}
Creates this output:
10
Prepending 9
9
10
Inserting 8
9
8
10
Prepending 7
7
9
8
10
Removed the first element:
9
8
10
However, when I add this last statement to the end of the program flow in main:
tst->insert(6,0);
My output changes to this:
10
Prepending 9
9
10
Inserting 8
8
8
10
Prepending 7
7
7
7
10
Removed the first element:
134515798
134515798
10
What am I missing? How can adding a value later in execution change the output that happens before I even get to that point in the program flow?
I am using ideone.com as my IDE/to run the program, I've never had an issue before, but is that the issue?
Solution
#include <iostream>
using namespace std;
// Provided node class
class node {
void *info;
node *next;
public:
node (void *v) {info = v; next = 0; }
void put_next (node *n) {next = n;}
node *get_next ( ) {return next;}
void *get_info ( ) {return info;}
};
// List class template
template <class T>
class list {
node *start;
public:
list (T v) {
start = new node (&v);
}
// Insert method
void insert (T *value, int place=-1) {
node *temp = new node (value);
// If we're putting it at the beginning, then change the reference to start
if (place == 0) {
temp->put_next(start);
start = temp;
}
// We're inserting it somewhere other than the beginning, handle appropriately
else {
node *before = start;
// Loop to find preceeding node
for (int i = 1; before->get_next() != 0; i++) {
if (i == place) {
break;
}
before = before->get_next();
}
// Insert after preceeding node, and point at subsequent node
temp->put_next(before->get_next());
before->put_next(temp);
}
}
// Remove function
void remove(int place) {
// If we're removing hte beginning, then change start pointer
if (place == 0) {
start = start->get_next();
}
// Find node to remove
else {
node *curr = start;
for (int i = 1; curr != 0; i ++) {
if (i == place) {
// Cut target node out of list
curr->put_next(curr->get_next()->get_next());
break;
}
curr = curr->get_next();
}
}
}
// Print nodes
void display() {
for (node *current = start; current != 0; current = current->get_next()) {
cout << *(static_cast<T*>(current->get_info())) << endl;
}
cout << endl;
}
};
int main() {
int nine = 9;
int eight = 8;
int seven = 7;
int six = 6;
int five = 5;
cout << "Create list holding '10'" << endl;
list<int> *tst = new list<int>(10);
cout << "Prepending 9" << endl;
tst->insert(&nine,0);
cout << "Inserting 8 at 2nd place" << endl;
tst->insert(&eight,1);
cout << "Appending 7" << endl;
tst->insert(&seven);
cout << "Prepending 6" << endl;
tst->insert(&six,0);
cout << "Inserting 5 at 3rd place" << endl;
tst->insert(&five,2);
cout << "Show completed list:" << endl;
tst->display();
cout << "Removing the first element:" << endl;
tst->remove(0);
tst->display();
cout << "Removing the last element:" << endl;
tst->remove(4);
tst->display();
cout << "Removing the second element:" << endl;
tst->remove(1);
tst->display();
}

Answer 1

You have undefined behavior in your code, because you save pointers to local variables, variables that goes out of scope once the function returns.
The variable I'm talking about is the argument value inside the insert function, once the insert function returns that pointer is no longer valid.
The quick solution? Don't store pointers, store a list of integers. Or maybe make the list (and node) a templated class and store by value.
If you truly want a list that can contain anything, the consider using e.g. Boost any.