How does the new keyword make a difference?

Question

class sortedList
{
    class Node
    {
        public:
            int value;
            Node* next;

            Node()
            {
                this->value = 0;
                this->next = NULL;
            }
            Node(int value)
            {
                this->value = value;
                this->next = NULL;
            } 
    }* head = NULL, * tail = NULL;

    public:
        void addNode(int value)
        {
            Node* n = Node(value);
            //More code...
        }
};

Why does adding new before Node(value) make the code correct? I think the code above should stay wrong because pointer doesn't equal a node.

adding the `new` makes it more like `(new Node)(value)`, not `new (Node(value))` like you seem to think. It's a completely different expression, not a unary operand applied to the same sub-expression. — , Nov 11 '19 at 16:35
`Node(value)` returns a `Node`, but `new Node(value)` returns a pointer to a `Node`. — HolyBlackCat, Nov 11 '19 at 16:35
[Prefer](https://arne-mertz.de/2015/12/modern-c-features-nullptr/) `nullptr` to `NULL`.. — Håkon Hægland, Nov 11 '19 at 16:53

score 4 · Answer 1 · answered Nov 11 '19 at 16:37

Node(value) is a temporary value of type Node. Node* n is a variable of type Node*. You're attempting to initialise an object of type Node* with an argument of type Node. These types are not implicitly convertible to one another, so the program is ill-formed.

The result of new T is T*. So, result of new Node(value) is a Node*. An object of type Node* can be initialised with an argument of type Node*.

Cory Kramer · Accepted Answer · 2019-11-11T18:10:58.550

3

This is not valid code:

Node* n = Node(value);

The reason new makes it work is that new returns a pointer.

You can either declare your Node using dynamic storage:

Node* n = new Node(value);

or automatic storage:

Node n(value);

edited Nov 11 '19 at 18:10

answered Nov 11 '19 at 16:36

Cory Kramer

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Please don't peddle this "heap"/"stack" stuff – Lightness Races in Orbit Nov 11 '19 at 17:26
1

Do you prefer "dynamic" and "automatic" instead? If so I'll edit accordingly – Cory Kramer Nov 11 '19 at 17:54
1

@CoryKramer yes, that would be better. Those are the terms the standard uses. The stack and heap are how compiler vendors *implement* those terms – Remy Lebeau Nov 11 '19 at 18:05
@RemyLebeau Possibly :) – Lightness Races in Orbit Nov 11 '19 at 18:15
@CoryKramer Ta :P – Lightness Races in Orbit Nov 11 '19 at 18:16

How does the new keyword make a difference?

2 Answers2