C++ pointer problem : How to fix bugs in these codes?

Question

#include <vector>
struct node{
    int x;
};
node* temp = new node();
std::vector<node*> ptrs;
for (int i = 0; i < 10; i++){
    temp->x = i;
    ptrs.push_back(temp);
}
std::vector<node*>:: iterator j;
for (j = ptrs.begin(); j != ptrs.end(); j++){
    std::cout << (*j)->x;
}
delete temp;

This prints 999999999 as output. I want to have 123456789.

What is the best way to modify this? I mean, what is the best fashion?

Answer 1

You are allocating only 1 node object and then storing 10 node* pointers into your vector that all point at that 1 node object. So, in your 1st loop, all of the assignments to x are on that 1 node. When that 1st loop is finished, whatever last value was assigned to the 1 node's x, that is what your 2nd loop prints out 10 times.

For what you are attempting, you need to allocate and free 10 separate node objects, eg:

#include <vector>
struct node{
    int x;
};
node* temp;
std::vector<node*> ptrs;
for (int i = 0; i < 10; ++i){
    temp = new node; // <-- MOVED HERE!!!
    temp->x = i;
    ptrs.push_back(temp);
}
std::vector<node*>::iterator j;
for (j = ptrs.begin(); j != ptrs.end(); ++j){
    std::cout << (*j)->x;
}
for (j = ptrs.begin(); j != ptrs.end(); ++j){ // <-- ADD THIS LOOP!!!
    temp = *j;
    delete temp;
}

But, why are you using pointers at all? This is not a good example that warrants the use of pointers at all. You should create a vector of node objects, not a vector of pointers to node objects, eg:

#include <vector>
struct node{
    int x;
};
std::vector<node> nodes;
for (int i = 0; i < 10; ++i){
    node temp;
    temp.x = i;
    nodes.push_back(temp);
}
std::vector<node>::iterator j;
for (j = nodes.begin(); j != nodes.end(); ++j){
    std::cout << j->x; // or (*j).x
}

Alternatively:

#include <vector>
struct node{
    int x;
};
std::vector<node> nodes(10);
std::vector<node>::iterator j;
for (j = nodes.begin(); j != nodes.end(); ++j){
    j->x = i; // or (*j).x
}
for (j = nodes.begin(); j != nodes.end(); ++j){
    std::cout << j->x; // or (*j).x
}