Hummingbird Hummingbird - 11 days ago 8
C++ Question

linking error in friend function of a template class

I have template class node and in order to follow the copy-and-swap idiom i am tryig to write the

swap
function which is a
friend
of class
Node


my code is :

Node.h


#ifndef NODE_H
#define NODE_H

#include<memory>
template<typename type>
class Node
{
type data_;
std::unique_ptr<Node> next_;
public:
Node(type data);
Node(const Node& other);
Node& operator=(Node other);
friend void swap(Node& lhs, Node& rhs);
};

#include "Node.tpp"
#endif


and the
Node.tpp


template<typename type>
Node<type>::Node(type data):data_(data),next_(nullptr){
}

template<typename type>
Node<type>::Node(const Node& other)
{
data_ = other.data_;
next_ = nullptr;
if(other.next_ != nullptr)
{
next_.reset(new Node(other.next_->data_));
}
}
template<typename type>
void swap(Node<type>& lhs, Node<type>& rhs)
{
using std::swap;
swap(lhs.data_,rhs.data_);
swap(lhs.next_,rhs.next_);
}

template<typename type>
Node<type>& Node<type>::operator=(Node other)
{
swap(*this,other);
return *this;
}


when i test it from
Source.cpp


#include "Node.h"
int main()
{
Node<int> n(3);
Node<int> n2(n);
Node<int> n3(5);
n3 = n2;
}


i am getting the following linking error :

LNK2019: unresolved external symbol "void __cdecl swap(class Node<int> &,class Node<int> &)

Answer

The friend declaration doesn't "inherit" the template parameter, but needs its own:

template<typename type>
class Node
{
    // ...
public:
    // ...
    template<typename U> // <<<<<<<<<<<
    friend void swap(Node<U>& lhs, Node<U>& rhs);
                      // ^^^           ^^^
    // ...
};