C++ passing std::unique_ptr as an argument

Question

I tried to implement binary tree using std::unique_ptr but errors come up and I don't understand the output error.

The code is the following:

#include <iostream>
#include <memory>
#include <functional>
#include <utility>

template <typename T>
class BinTreeNode {
public: 
    BinTreeNode(T key): data {key}, left {nullptr}, right {nullptr} {}
    ~BinTreeNode() {}
    T data;
    std::unique_ptr<BinTreeNode<T>> left;
    std::unique_ptr<BinTreeNode<T>> right;
};

template <typename T>
class BinTree {
public:
    BinTree() : root {nullptr} {} 
    ~BinTree() {}
    std::unique_ptr<BinTreeNode<T>> root;

    void insert(std::unique_ptr<BinTreeNode<T>> node, T key);
};

template <typename T>
void BinTree<T>::insert(
    std::unique_ptr<BinTreeNode<T>> node, 
    T key)
{
    if(node){ // != nullptr
        if(node->data < key) insert(node->right, key);
        else insert(node->left, key);
    } 
    else{
        std::unique_ptr<BinTreeNode<T>> u_ptr(new BinTreeNode<T>(key));
        node = std::move(u_ptr);
    }
}

int main(){
    BinTree<int> tree();
    tree.insert(tree.root, 10);
}

I assume the error is in insert function and is related to argument initialisation.

BinTree.cpp:65:27: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinTreeNode; _Dp = std::default_delete >]’ tree.insert(tree.root, 10); ^

In file included from /usr/include/c++/4.9/memory:81:0, from BinTree.cpp:2: /usr/include/c++/4.9/bits/unique_ptr.h:356:7: note: declared here unique_ptr(const unique_ptr&) = delete; ^

BinTree.cpp:35:6: error: initializing argument 1 of ‘void BinTree::insert(std::unique_ptr >, T) [with T = int]’ void BinTree::insert( ^

BinTree.cpp: In instantiation of ‘void BinTree::insert(std::unique_ptr >, T) [with T = int]’: BinTree.cpp:65:27: required from here BinTree.cpp:40:47: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinTreeNode; _Dp = std::default_delete >]’
if(node->data < key) insert(node->right, key); ^

In file included from /usr/include/c++/4.9/memory:81:0, from BinTree.cpp:2: /usr/include/c++/4.9/bits/unique_ptr.h:356:7: note: declared here unique_ptr(const unique_ptr&) = delete; ^

BinTree.cpp:35:6: error: initializing argument 1 of ‘void BinTree::insert(std::unique_ptr >, T) [with T = int]’ void BinTree::insert( ^

BinTree.cpp:41:30: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = BinTreeNode; _Dp = std::default_delete >]’ else insert(node->left, key); ^

In file included from /usr/include/c++/4.9/memory:81:0, from BinTree.cpp:2: /usr/include/c++/4.9/bits/unique_ptr.h:356:7: note: declared here unique_ptr(const unique_ptr&) = delete; ^

BinTree.cpp:35:6: error: initializing argument 1 of ‘void BinTree::insert(std::unique_ptr >, T) [with T = int]’ void BinTree::insert(

You forgot to tell us the error. What's happening ? Copy paste any output/error so everyone can see it. — nos
– nos, Commented Dec 24, 2015 at 15:13

Alexis Wilke · Accepted Answer · 2018-06-06 23:59:03Z

6

The error is caused by you trying to copy-construct the argument of BinTree::insert from tree.root. std::unique_ptr is move-only.

My guess is that node in BinTree::insert should be passed by reference. Reasons:

You'd have to std::move the tree.root into it (if passed by value), which would steal the ownership
You're move-assigning to it in BinTree::insert, these changes are not made to pass tree.root

edited Jun 6, 2018 at 23:59

Alexis Wilke

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answered Dec 24, 2015 at 15:17

LogicStuff

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1 Comment

SergeyA Over a year ago

@tomtom, make sure you understand what is actually happening there. While LogicStuff is likely correct in their guess, I am not sure if you fully understand the reasons for that.

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