CODEWITHSUNDEEP
c++pointersreference
Salman Ahmed
Salman Ahmed

Reputation: 121

C++ Pointers - When do they get reassigned?

I have the following code to demonstrate my question:

#include <iostream>
#include <queue>
using namespace std;

int main(int argc, char* argv[]){
    queue<int> numbers;

    for (int i = 0; i < 10; i++) {
        numbers.push(i);
    }

    int temp = 10;
    int *last = &temp;

    for(int j = 0; j < 10; j++) {
        cout << *last << endl;
        int i = numbers.front();
        cout << *last << endl << endl;

        numbers.pop();
        last = &i;
    }

    return 0;
}

Which will print out:

10
10

0
1

1
2

2
3

3
4

4
5

5
6

6
7

7
8

8
9

I have some confusion on why the pointer last is being reassigned whenever int i changes. What I want is for for last to hold a reference to the last element - but am having trouble. I would guess that the pointer is pointing to the value of i - and changes when i changes. But i is inside the scope of the for loop, so I'm not exactly sure whats going on here.

Upvotes: 0

Views: 74

Answers (1)

Vlad from Moscow
Vlad from Moscow

Reputation: 311048

Though the program works because the address of local variable i is not being changed nevertheless the program has undefined behaviour becuase in each iteration of the loop except the first iteration pointer last refers to non alive object in statement

cout << *last << endl;

due to the assignment

last = &i;

A correct loop could look the following way

   for(int j = 0; j < 10; j++) {
        cout << *last << endl;
        int i = numbers.front();
        cout << *last << endl << endl;

        numbers.pop();
        *last = i;
        ^^^^^^^^^^^
    }

Upvotes: 2

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