How to access private struct in class?

Question

I want to access my Node struct in order to create a function that generates a new node. To do so i thought that the only way to do it is to create a constructor inside my struct and to call the constructor in my CreateNode function. However i get an error :[Error] 'struct BS::Node' is protected. An ovious sollution to the problem is to make struct Node public however i want to ask if there is another way to access my struct and keep it private.

Header file

class BS
{
public:
    BS();     //constructor
protected:
    struct Node{
        int data;
        Node* left;
        Node* right;
        Node(int d,Node* lf ,Node* ri)
            :data(d),left(lf),right(ri){}
    }; 
    Node* root;    
    Node* CreateNode(int data);
};

CPP

BS::BS(){
  root=NULL;
}  
BS::Node* CreateNode(int data){
    Node* new_node= new Node(0,NULL,NULL);
    return new_node;
}

Possibly relevant https://stackoverflow.com/questions/44366905/make-protected-inner-class-public — StoryTeller - Unslander Monica, Jun 14 '17 at 12:33
Why dont you define your Node structure outside of class BS, that should also solve your problem. Or is there a reason not to do that? — Aeonos, Jun 14 '17 at 12:38
@Aeonos - namespace pollution, for one. To avoid *that* the OP would have to give a more verbose name to the Node structure. — StoryTeller - Unslander Monica, Jun 14 '17 at 12:40
@ie the function can return a pointer. But it has to be a memberfunction. — , Jun 14 '17 at 12:41
@ierostsant - What marcinj meant was that you have a scope error. It should be: `BS::Node* BS::CreateNode(int data){` — StoryTeller - Unslander Monica, Jun 14 '17 at 12:42
@ierostsant Use the `auto` keyword to assign anything outside of your `BS` class. See [here](https://stackoverflow.com/a/44218371/1413395). — πάντα ῥεῖ, Jun 14 '17 at 12:42
@StoryTeller Oh!!! Excuse me i have just realized it!!! Thank you ! — brain_dead_cow, Jun 14 '17 at 12:44

score 0 · Answer 1 · answered Jun 14 '17 at 12:53

0

Use a public class with a private constructor, and make the outer class a friend:

class BS
{
public:
  BS();     //constructor

  class Node {
    // note: private constructor
    Node(int d,Node* lf ,Node* ri)
    :data(d),left(lf),right(ri){}

    int data;
    Node* left;
    Node* right;

    // befriends BS
    friend BS;
  }; 

  Node* CreateNode(int data);

private:
  Node* root = nullptr;
};

BS::Node* BS::CreateNode(int data){
    Node* new_node= new Node(0, nullptr, nullptr);
    return new_node;
}

int main()
{

}

ps. don't use raw pointers. Use std::unique_ptr

answered Jun 14 '17 at 12:53

Richard Hodges

68,278
7
90
142

A nested class needs no friend specifier for the outer one – Jun 14 '17 at 13:05
@StoryTeller the class in the OP worked perfect. Thre was no need to make all private and get access back with friend. – Jun 14 '17 at 14:38
@manni66 - Your statement was made very general. You said *"A nested class needs no friend specifier for the outer one"*. You did not say *"There is no need for this transformation in the OP's code"*. The later is true, while the former is false. – StoryTeller - Unslander Monica Jun 14 '17 at 14:41
@StoryTeller agree, could have been more elaborate. – Jun 14 '17 at 14:47

score -2 · Answer 2 · edited Jun 20 '18 at 15:35

No, you can not create your CreateNode function the way you would like, even with friendship, because this function returns a BS::Node outside BS, while Node is protected:

#include <BS.hh>

BS::BS()
{
  root=nullptr;
}

BS::Node* BS::CreateNode(int data)
{
    BS::Node* new_node = new BS::Node(0,nullptr,nullptr);
    return new_node;
}

int main()
{
  BS bs;
  BS::Node* = bs.CreateNode(1); // <- Error, returning a BS::Node outside BS!
  return 0;
}

But you could use CreateNode inside BS with:

class BS
{
public:
    BS();     //constructor
protected:
    struct Node{
        int data;
        Node* left;
        Node* right;
        Node(int d,Node* lf ,Node* ri)
            :data(d),left(lf),right(ri){}
    };
    BS::Node* CreateNode(int data);
    BS::Node* root = CreateNode(0); // inside BS it's OK
};

How to access private struct in class?

2 Answers2