Extra snippets for Stack overflow question: https://stackoverflow.com/questions/46202087/tree-elements-initialized-with-references-to-emplaced-deque-members-results-in-n

more_node_examples.cpp

// Like solution 4, but uses a dummy Node object for leaf nodes instead of *this
// The implicitly declared constructor is fine because we don't need to fix up *this

class Node
{
public:
    Node(int d, Pool& pool)
        : level{d}
        , l{d > 0 ? pool.emplace_back(Node(d - 1, pool)) : dummy}
        , r{d > 0 ? pool.emplace_back(Node(d - 1, pool)) : dummy}
    {
    }

    int level;
    const Node& l;
    const Node& r;

    int check() const
    {
        if(!(&l == dummy))
            return l.check() + 1 + r.check();
        else
            return 1;
    }
private:
    static Node dummy;
    Node(): level(0), l(dummy), r(dummy) {}
};

// Need to *define* dummy as well as declare 
Node Node::dummy; 


// Like solution 4, but uses pointers, and uses nullptr for leaf nodes
// The implicitly declared constructor is fine because we don't need to fix up *this

struct Node {
    Node(int d, Pool& pool)
        : level{d}
        , l{d > 0 ? &pool.emplace_back(Node(d - 1, pool)) : nullptr}
        , r{d > 0 ? &pool.emplace_back(Node(d - 1, pool)) : nullptr}
    {
    }

    int level;
    const Node* const l;
    const Node* const r;

    int check() const
    {
        if(l)
            return l->check() + 1 + r->check();
        else
            return 1;
    }
};


// If you're prepared to mix solution 2 with using nullptr instead of this, 
// you get the simplest solution of all; the Node code is totally separated
// from the code defining the place we choose to store instances of it.

struct Node {
    int level;
    const Node* const l;
    const Node* const r;

    int check() const
    {
        if(l)
            return l->check() + 1 + r->check();
        else
            return 1;
    }
};


using Pool = std::deque<Node>;

Node* make(int d, Pool& pool)
{
	Node* l = d > 0 ? make(d - 1, pool) : nullptr;
	Node* r = d > 0 ? make(d - 1, pool) : nullptr;
	return &pool.emplace_back(d, l, r);
}