Using an unique_ptr as a member of a struct

Question

So I have a struct like this:

struct node {
    node_type type;
    StaticTokensContainer token_sc;
    std::unique_ptr<node> left;    // here
    std::unique_ptr<node> right;   // and here
};

When I tried to write a recursive function to convert a node to string for printing like this:

std::string node_to_string(node n) {
    return "<other members converted to strings>" + node_to_string(*(n.left));
}

It gives me: function "node::node(const node &)" (declared implicitly) cannot be referenced -- it is a deleted function

I don't understand why this error is showing up, I had no problem when I did (*n) when I tried passing n as std::unique_ptr<node>, so I don't see why doing *(n.left) is not allowed.

Answer 1

you pass node by value, and you cannot set one std::unique_ptr equal to another:

int main() {
    std::unique_ptr<node> a = std::make_unique<node>();
    std::unique_ptr<node> b = a; //Error C2280
}

because who is now the unique owner of the data?

---

To fix your error, simple take a const reference in your function. This way, you pass the reference and that won't create copies:

std::string node_to_string(const node& n) {
    return "<other members converted to strings>" + node_to_string(*(n.left));
}

(you should also think about exiting this function, otherwise you will get either get a stack overflow or a dereferenced nullptr, whichever comes first. For example, add if (!n.left) return "";)