Make rvalues persist as pointers in a recursive class

Question

I have a simple linked list node class which I can use to create a list 1->2->3 like so:

struct ListNode {
    int data_;
    ListNode *child_;
    ListNode(int data, ListNode *child) : data_{data}, child_{child} {}
};

ListNode node3(3, &node4);
ListNode node2(2, &node3);
ListNode head(1, &node2);

I want to make this construction less verbose by adding a new constructor, allowing something to the effect of:

ListNode head(1, (2, (3, nullptr)));

Essentially, I'd like the temporary objects created in the constructor to persist so I can point to them. How can I achieve this?

I've found an answer to a similar question here but I this doesn't handle the recursive structure in my class definition. I've been looking at move semantics but can't understand how to make it work with child being a pointer type.

You can't move `child_`, you can only move `data_` but moving an `int` is pointless. You have to copy the list if you want to do this recursively. A better way might be to support initializer lists so you can do `ListNode head{1,2,3};` You still have to allocate `ListNode`s for `2` and `3` but you skip having the temporary nodes. — Goswin von Brederlow, Jul 19 '22 at 01:45
You should have a `class List` that has a `ListNode *head = nullptr;` and a constructor for `List{1,2,3}`. Would be cleaner that way. You can also improve a lot of functions if `List` has a `ListNode **tail = &head;`. — Goswin von Brederlow, Jul 19 '22 at 01:49
This cannot be done, as described. C++ does not work this way. — Sam Varshavchik, Jul 19 '22 at 01:49
Unlike L value references, R value references cannot be be used to initialize via constructor arguments. Only via aggregate initialization. — doug, Jul 19 '22 at 01:54
Why not use `std::unique_ptr` instead of raw pointers, and create new nodes in the constructor? — Dmitry Kuzminov, Jul 19 '22 at 02:35

score 0 · Answer 1 · answered Jul 19 '22 at 03:26

You would probably like to have the concept of a List to supplement your Node. In combination with an initializer_list this can get quite neat.

Something like

#include <iostream>

struct ListNode {
    int data_;
    ListNode *child_;
    ListNode(int data, ListNode *child) : data_{data}, child_{child} {}
};

struct List{
    ListNode* head = nullptr;
    List(std::initializer_list<int> list){
        for(auto val:list){
            insert(val);
        }
    }
    void insert(int value){
        head = new ListNode(value, head);
    }
};

now you can create it with something like

int main() {
    List list = {3, 2, 1};
}

See https://godbolt.org/z/sn6orf5h9

Now, there are plenty of improvements to be made here of course, some mentioned in the comments. But that was not what you were asking for.

Make rvalues persist as pointers in a recursive class

1 Answers1