valgrind is displaying memory leakage in singly link list example

Question

Here is my code.

#include <bits/stdc++.h> 
using namespace std; 

class Node { 
public: 
    int data; 
    Node* next; 
}; 

void printList(Node* n) 
{ 
    while (n != NULL) { 
        cout << n->data << " "; 
        n = n->next; 
    } 
} 

void delete_list(Node *n)
{
    Node *tmp = NULL;
    tmp = new Node();
    while (n != NULL){
        tmp=n->next;
        delete (Node *)n;
        n=tmp;
    }
    delete (Node *)tmp;
}


int main() 
{ 
    Node* head = NULL; 
    Node* second = NULL; 
    Node* third = NULL; 

    head = new Node(); 
    second = new Node(); 
    third = new Node(); 

    head->data = 1; 
    head->next = second;

    second->data = 2;
    second->next = third; 

    third->data = 3;
    third->next = NULL; 

    printList(head); 

    // Delete the entire list
    delete_list(head);

    return 0; 
}

HEAP SUMMARY:enter code here in use at exit: 16 bytes in 1 blocks total heap usage: 6 allocs, 5 frees, 73,792 bytes allocated 16 bytes in 1 blocks are definitely lost in loss record 1 of 1 at 0x4C3017F: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) by 0x10895E: delete_list(Node*) (demo.cpp:24) by 0x108A87: main (demo.cpp:58) LEAK SUMMARY: definitely lost: 16 bytes in 1 blocks indirectly lost: 0 bytes in 0 blocks possibly lost: 0 bytes in 0 blocks still reachable: 0 bytes in 0 blocks suppressed: 0 bytes in 0 blocks For counts of detected and suppressed errors, rerun with: -v ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

Why do you create a node when deleting the list? And before doing anything else with the node, you assign another one, losing last pointer to the node just created – which is your memory leak (or at least one of). — Aconcagua
void delete_list(Node *n) { Node *tmp = NULL; // tmp = new Node(); while (n != NULL){ tmp=n->next; delete (Node *)n; n=tmp; } // delete (Node *)tmp; } — Rajeev

Aconcagua Aconcagua · Accepted Answer · 2019-12-24T08:33:14

void delete_list(Node *n)
{
    Node *tmp = NULL;

    // this is your memory leak:
    // you just created a new node ...
    tmp = new Node();

    while (n != NULL)
    {
        // ... and very first thing you do is assigning another node to the only
        // pointer holding the node created just before
        tmp = n->next;

        delete (Node *)n;
        n = tmp;
    }

    // apart from cast being obsolete (tmp HAS appropriate type!)
    // tmp is already nullptr and you don't delete anything at all
    delete (Node *)tmp;
}

This example is shows quite well why it's a good idea to keep variables as local as possible:

while (n != NULL)
{
    Node* tmp = n->next;
    delete (Node *)n;
    n = tmp;
}

You simply cannot get tempted to assign anything without need (or even create irrelevant objects) or trying to delete something that doesn't exist any more...

valgrind is displaying memory leakage in singly link list example

2 Answers