Non-const value reference cannot bind to a temporal value - c++

Question

when I try to implement a Binary tree with a few essential functions, there were no error pop up. However, when I initialize the tree and try to use its functions, there's a compile error showing, and I don't know how to fix it, can you help me. The following is my code and the error that shows:
Edit: I fix the problem by changing the argument of addItem(T& item) to addItem(const T& item). Thank you for your help!
Node.h
#ifndef BINARYTREE_NODE_H
#define BINARYTREE_NODE_H
template <class T>
class Node {
public:
Node<T>* left;
Node<T>* right;
T data;
};
BinaryTree.h
#ifndef BINARYTREE_BINARYTREE_H
#define BINARYTREE_BINARYTREE_H
#include "Node.h"
enum Traversal
{
INORDER,
POSTORDER,
PREORDER
};
template <class T>
class BinaryTree {
private:
Node<T>* root;
int size = 0;
Traversal currentTraversal;
void inOrder(Node<T>* parent,void f(T&));
void preOrder(Node<T>* parent,void f(T&));
void postOrder(Node<T>* parent,void f(T&));
Node<T>* insert(Node<T>* parent, T item);
public:
void setTraversal(Traversal order);
void addItem(const T& item);
BinaryTree();
void print(T& item);
};
#include "BinaryTree.cpp"
#endif //BINARYTREE_BINARYTREE_H
BinaryTree.cpp
#ifndef BINARYTREE_BINARYTREE_CPP
#define BINARYTREE_BINARYTREE_CPP
#include <iostream>
#include "BinaryTree.h"
template <class T>
BinaryTree<T>::BinaryTree() {
root = nullptr;
}
template <class T>
Node<T>* BinaryTree<T>::insert(Node<T> *parent, T item) {
if(parent == nullptr)
{
Node<T>* temp = new Node<T>;
temp->data = item;
return temp;
}
else if(parent->data > item)
{
if (parent->left != nullptr)
insert(parent->left,item);
else{
parent->left = new Node<T>;
parent->left->data = item;
parent->left->left = nullptr;
parent->left->right = nullptr;
}
}
else if (parent->data <= item)
{
if (parent->right != nullptr)
insert(parent->right,item);
else{
parent->right = new Node<T>;
parent->right->data = item;
parent->right->left = nullptr;
parent->right->right = nullptr;
}
}
}
template <class T>
void BinaryTree<T>::addItem(constT &item) {
insert(root,item);
size++;
}
template <class T>
void BinaryTree<T>::inOrder(Node<T> *parent, void f(T&))
{
if (parent != nullptr)
{
inOrder(f, parent->left);
f(parent->data);
inOrder(f,parent->right);
}
}
template <class T>
void BinaryTree<T>::preOrder(Node<T> *parent, void f(T&)) {
if (parent != nullptr)
{
f(parent->data);
preOrder(f,parent->left);
preOrder(f,parent->right);
}
}
template <class T>
void BinaryTree<T>::postOrder(Node<T> *parent, void f(T&)) {
if (parent != nullptr)
{
postOrder(f,parent->left);
postOrder(f,parent->right);
f(parent->data);
}
}
template <class T>
void BinaryTree<T>::print(T& item)
{
std::cout << item <<std::endl;
if(currentTraversal == PREORDER)
preOrder(root,print(item));
if (currentTraversal == POSTORDER)
postOrder(root,print(item));
if(currentTraversal == INORDER)
inOrder(root,print(item));
}
template <class T>
void BinaryTree<T>::setTraversal(Traversal order) {
currentTraversal = order;
}
#endif
main.cpp
#include <iostream>
#include "BinaryTree.h"
int main() {
BinaryTree<int> tree;
tree.addItem(2); // Here's the error showing: "Non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'"
}

Answer 1

You have:
T &item
As the argument to addItem(). It should either be a const reference or a plain value. That is, const T & or T.
For more on why this is the case, see: How come a non-const reference cannot bind to a temporary object?