Is there any reason that a heap pointer would not get assigned to an array?

Question

The structure of Node is the following:

struct Node {
    int data;
    Node *next;
    bool someBool;
};

I have the following couple of lines:

    Node *hello = new Node{data, cur, !new_head}; // line A
    array[new_head_index] = hello;                // line B
}                                                 // line C

Value of new_head_index is confirmed to be 1 by GDB on all 3 lines.

GDB confirms, on lines A, B, and C, when I do p *hello (print the contents of hello), I get:

(gdb) p *hello
$7 = {data = 888, next = 0x8414e70, someBool = false}

But printing the contents of array@2 (array has length 2, declared in main as Node *heads[numHeads] = {new Node{0, nullptr, false}, nullptr};) has this on lines B and C (before line is actually executed):

(gdb) p **array@2
$8 = {{data = 777, next = 0x8414e70, someBool = true}, {data = 33, next = 0x378, someBool = 112}}

(The 777 node is expected, I filled it in before).

While on line A, it has:

(gdb) p **array@2
$9 = {{data = 777, next = 0x8414e70, someBool = true}, {data = 61265, next = 0x0, someBool = false}}

Essentially, hello is not being assigned to array[1]. What could be a possible reason for this?

Here is a minimal reproducible example: main.cc

#include <iostream>
#include <string>
#include "new_mod.h"
using namespace std;

int len(Node **array) {
    int i = 0, count = 0;
    while (array[i++]) { ++count; }
    return count;
}

void attach(Node **array, int head, int index, int data) {
    Node *hello = new Node{data, cur};
    array[new_head_index] = hello;
}

int main() {
    string command;
    int head;
    Node *array[numHeads] = {new Node{0, nullptr}, nullptr};

    while (cin >> command) {
        if (command == "a") {
            int m, x;
            cin >> head >> m >> x;

            attach(array, head, m, x);
        }
    }

}

mod.h

#ifndef MOD_H
#define MOD_H
#include <ostream>

struct Node {
    int data;
    Node *next;
    bool someBool = false;
};

const int numHeads = 2;

void attach(Node **array, int head, int index, int data);
#endif

Try putting in this input (I've named this file a.in)

a 0 0 777
a 0 1 888

Compile with g++ -Wall -g main.cc -o newe so you can do things with gdb!

By the way, here's what I did in gdb to get the above problem:

gdb newe
b attach(Node**, int, int, int)
run <a.in
layout n
c
n
n
n
n
n
n
n          (comment: I did n until line Node *hello = new Node{data, cur};)
p **array@2
n
n
p **array@2 (the problem is shown)

Answer 1

The problem is that Node *array[2] and Node **array are not the same thing, and when you ask GDB to p **array@2, you are asking to interpret array as if array was a double pointer, not an array.

You can observe this before starting the input loop:

(gdb) p *array@2
$1 = {0x55555556ae70, 0x0}
(gdb) p *array[0]
$2 = {data = 0, next = 0x0, someBool = false}

Here, you can see that array is in expected state: first node is all zeros, second is NULL.

But when you do:

(gdb) p **array@2
$3 = {{data = 0, next = 0x0, someBool = false}, {data = 4113, next = 0x3737203020302061, someBool = 55}}

you can immediately see that you are not looking at the right data: you know that array[1] is NULL, so it can't be pointing to a Node with 4113 in it.

I don't believe there is a GDB built-in syntax to print *array[0 .. N]. One possible solution is to define a helper function (which can easily be generalized to take the array name and size as parameters, but is kept trivial here for simplicity):

(gdb) def parray
Type commands for definition of "parray".
End with a line saying just "end".
>print *array[0]
>print *array[1]
>end
(gdb) parray
$4 = {data = 0, next = 0x0, someBool = false}
Cannot access memory at address 0x0
(gdb) c
Continuing.

Breakpoint 1, attach (array=0x7fffffffdb70, head=0, index=1, data=888) at foo.cc:34
34          array[new_head_index] = hello;
(gdb) n
35      }
(gdb) parray
$5 = {data = 777, next = 0x55555556ae70, someBool = false}
$6 = {data = 888, next = 0x55555556ae70, someBool = false}