I'm new to c++ can you explain pointers in this instance

user_5075778 0
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Why does line 18 Node* bob = new(); not give me an error while line 22 node actualValue = new Node; does.

#include <iostream>

using namespace std;

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

int main() {
    cout << "Hello, World!" << std::endl;
    int a = 200;
    int* ptr = &a;

    cout << a << " pointer is " << ptr << endl;

    Node* bob = new Node();
    bob->data = 1;


    Node actualValue = new Node;
    return 0;
}
pointers c++
4 Answers

Node* bob = new Node(); don't give error because new Node() returns a pointer Node* and the type of bob is also Node*. You can do assignments between pointers having same types.

Node actualValue = new Node; give error because the type of actualValue is Node and the class Node doesn't have any constructors that accept the pointer Node*.

user_40623540

In C++ when you declare a new instance of a class, you can skip the brackets if the constructor has no arguments.

However, this line

Node actualValue = new Node;

is incorrect as the actualValue variable is decalred in the stack, not in the heap. You can only use new operator with the pointer declarations.

user_39832130

You must come from another language or be a complete beginner, but that's not a problem. I'm here to help you.

In C++, you can create a stack, and a heap variable. The differences are here:

int stack_var = 5;
int* heap_var = new int;
*heap_var = 5;

The big difference is that a heap variable is allocated with the new keyword. You must also know, that stack variables are freed automatically, when the scope ends (basically when you reach the first '}' after the allocation). But you have to delete the heap variables like this:

delete heap_var;

This will free up the memory. The main problem you have is that calling new returns a pointer. A pointer is basically a memory address. A pointer is basically the declared the same as an object, but we add an asterix (*). Like this:

Someobject object;
Someobject* pointer;

So to wrap it up, you must modify the code like this:

Node* bob = new Node();
bob->data = 1;

Node actualValue;

//This is also important!!!
delete bob;

Or this:

Node* actualValue = new Node();

delete actualValue;

I didn't mention it, but it may not be trivial for a beginner: once you delete some memory, you can't access it anymore.

Another difference between stack and heap variables:

You access members of a stack object with a dot (.) operator (stack_obj.foo = 5), while in case of pointers, with the arrow (->) operator (pointer->foo = 5)

Hope it cleared it :D

user_120208460

Why does line 18 Node* bob = new(); not give me an error while line 22 node actualValue = new Node; does

The line in question is

Node actualValue = new Node;

new Node returns a pointer, you can't assign it to an instance of the Node class. And you forgot your parantheses after new Node to call the constructor. The correct code is

Node *actualValue = new Node();
user_1087960
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