I have a program like below, when I try to access a struct member using pointer to pointer it says the expression must have a pointer to class type. Please tell me how can I access data element of the struct object using a pointer to pointer

#include "stdafx.h"
#include<iostream>
struct Node{
    int data;
    Node* next;
};

void addnode(Node** node, int val)
{
    if (node == NULL){
        *node = new Node();
        *node->data = val;

    }
}



int _tmain(int argc, _TCHAR* argv[])
{
    Node* node;
    addnode(&node, 10);
    return 0;
}

CodePudding user response：

In *node->data = val; there are 3 operators involved.

• operator* to dereference
• operator-> to access a member
• operator= to do the assignment

In which order will these happen? According to operator precedence which states:

Precedence	Operator
2	operator->
3	operator*
16	operator=

node->data will happen first and then the result will be dereferenced - so you have *(node->data) on the left hand side.

You need to dereference node first and can use (... ) to override the precedence rules:

(*node)->data = val;

Also note that your original Node* is uninitialized and reading it (like you do in if(node == NULL) ) will cause the program to have undefined behavior. Initialize it to nullptr:

Node* node = nullptr; // here
addnode(&node, 10);

CodePudding user response：

*node->data = val; is equivalent to *(node->data) = val;, but what you want is (*node)->data = val; (so you need to add the parens).

CodePudding user response：

The problem is that due to operator precedence the statement *node->data = val; is equivalent to:

*(node->data) = val;

The above statement doesn't work because dereferencing node will give us Node* which does not have any member called data as it is a non-class type. To solve this you need to override this behavior by adding parenthesis around *node as shown below:

(*node)->data = val;

Additionally, the variable node inside main is uninitialized. You should initialize it as shown below:

 Node* node = nullptr;

CodePudding user response：

It's an order of operations thing. It needs to be (*node)->data = val. Also note that the if statement is doing the opposite of what you want: *node to not be an error you need to test that node is not null not that it is null.

CodePudding user response：

For starters you need to initialize the pointer node in main

Node* node = nullptr;

Secondly this expression

*node->data = val;

is equivalent to

* ( node->data ) = val;

due to the precedence of operators because postfix operators including the operator -> have a higher precedence than unary operators like the dereference operator *.

At least you need to write

( *node )->data = val;

But in any case the function does not make a sense.

If you need to add a new node to the beginning of the singly linked list then the function will look the following way

void addnode(Node** node, int val)
{
    *node = new Node { val, *node };
}

Or without using double pointers like

void addnode( Node* &node, int val)
{
    node = new Node { val, node };
}

In this case the function is called like

addnode(node, 10);

If you want to append a new node to the end of the singly-linked list then in this case the function can look like

void addnode(Node** node, int val)
{
    while ( *node ) node = &( *node )->next;
    *node = new Node { val, *node };
}