Pointer to uninitialized data member

Question

Is it safe to store reference/pointer on uninitialized data member?

Not use, but store at some moment of time.

I know that I have to use smart pointers. But for code simplicity I decide to give an example with owning raw pointers. Which is not very good in practice but I think good enough for example.

Here is the example of code:

struct Node {
    Node(Node*& node_ptr) : list_first_node_ptr{node_ptr} {}
    /* ... */
    Node*& list_first_node_ptr;
};

struct List {
    // Is this line - dangerous?
    List() : first_node_ptr{new Node{first_node_ptr}} {}
    /* ... */
    Node* first_node_ptr;

    ~List() {delete first_node_ptr;}
};

I initialize first_node_ptr with Node object, but pass in constructor of Node object still not uninitialized first_node_ptr.

One more question: is memory already allocated for first_node_ptr when I pass it, so will the reference address at Node constructor be valid?

I think that this example is about the same but simplified version. I'm right?

class Base {
 public:
    // var_ptr points to uninitialized va
    Base() : var_ptr{&var}, var{10} {}

    int* var_ptr;
    int var;
};

P.S. Why when I write (Node*)& instead of Node*& I get compilation errors about incomplete type?

P.P.S. Can the usage of smart pointers change the situation?

P.P.P.S. @JesperJuhl asked about use case. The idea is to create circular linked list where every Node has a reference on Head pointer to first sentinel Node. I get this idea from Herb Sutter in CppCon video 2016 43:00. In video he talked about unique pointers but I give example with raw pointers, it may be become a little bit confusing.

I've asked [a similar question](https://stackoverflow.com/questions/50028982/is-taking-the-address-of-a-member-of-an-uninitialized-object-well-defined), though it's not exactly a duplicate. — François Andrieux, Jun 21 '18 at 17:07
I think `std::shared_from_this` will handle this situation better, if I understood logic properly — Slava, Jun 21 '18 at 17:08
@Slava `std::shared_from_this` doesn't work in constructors. — François Andrieux, Jun 21 '18 at 17:09
@FrançoisAndrieux it does not have, idea is not to pass it to `Node` but return it when needed — Slava, Jun 21 '18 at 17:10
Anyway looks like AB problem - `Node` should not have this pointer — Slava, Jun 21 '18 at 17:11
Why would you ever want to store a `Node *&` as a member variable? — Jesper Juhl, Jun 21 '18 at 17:11
The question is about the legality of the code, not about the design of the example used to illustrate the question. Edit : Nevermind, I hadn't noticed the P.P.S. — François Andrieux, Jun 21 '18 at 17:14
@JesperJuhl your question is moot. It's about code conformance, not about the reason for the code. — SergeyA, Jun 21 '18 at 17:15
Possible dupe: https://stackoverflow.com/questions/11338601/can-you-bind-a-reference-to-an-uninitialized-member — Rakete1111, Jun 21 '18 at 17:19

score 1 · Accepted Answer · edited Jun 21 '18 at 20:43

1

Node::list_first_node_ptr is a reference. It doesn't care about the value of the referenced object, just that the referenced object itself actually exists. Since List::first_node_ptr is guaranteed to exist when Node::list_first_node_ptr is constructed, this is legal code.

To phrase it another way; a reference can never be null, but it can reference a pointer that is null.

It's worth noting, though, that List::first_node_ptr will not point to a valid object until Node has finished constructing. This means you can't dereference List::first_node_ptr or Node::list_first_node_ptr until the Node constructor has finished.

edited Jun 21 '18 at 20:43

Remy Lebeau

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answered Jun 21 '18 at 17:27

Fibbs

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Nitpicking: _you can reference a pointer which is null_ - the pointer has an indeterminate value initially. – Maxim Egorushkin Jun 21 '18 at 18:55
1

The one in his example does, I was just trying to keep the rephrasing simple to understand. Either way, dereferencing a null pointer or a pointer in an indeterminate state is undefined behaviour. – Fibbs Jun 21 '18 at 20:42

Pointer to uninitialized data member

Is it safe to store reference/pointer on uninitialized data member?

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