Undefined reference when using generics

[gabriel11]

Recently my class started to dig deeper into working with generics and templates for Java. Since I'm still learning C++, I thought it would be a nice learning experience to translate the Java file.

But for some reason whenever I compile the C++ program, it gives me this error:

gabriel@desktop:/media/gabriel/main/repositories/CPP Sandbox/nodes$ g++ main.cpp node.cpp -o main
/usr/bin/ld: /tmp/ccqvlp1s.o: in function `main':
main.cpp:(.text+0x33): undefined reference to `Node<int>::Node(int)'
/usr/bin/ld: main.cpp:(.text+0x43): undefined reference to `Node<Node<int>*>::Node(Node<int>*)'
/usr/bin/ld: main.cpp:(.text+0x63): undefined reference to `Node<char const*>::Node(char const*)'
/usr/bin/ld: main.cpp:(.text+0x73): undefined reference to `Node<Node<char const*>*>::Node(Node<char const*>*)'
/usr/bin/ld: main.cpp:(.text+0x98): undefined reference to `Node<double>::Node(double)'
/usr/bin/ld: main.cpp:(.text+0xa8): undefined reference to `Node<Node<double>*>::Node(Node<double>*)'
collect2: error: ld returned 1 exit status 

I'm not too sure on why it's giving me an undefined reference, but if someone could point me to the right direction that would be great.

node.h

#pragma once

#include <iostream>

template <typename T>
class Node {

    private:
        T data;
        Node *next_node;

    public:
        Node(T data);
        T get_data();
        void set_next_node(Node next);
        Node get_next_node(); 

};

node.cpp

#include "node.h"

template <typename T>

Node<T>::Node(T data) {
    this->data = data;
}

main.cpp

#include "node.h"

int main() {

    Node numb = new Node(7);
    Node str = new Node("foobar");
    Node dbl = new Node(7.5);

    return 0;
}

[JLBorges]

See: Why can’t I separate the definition of my templates class from its declaration and put it inside a .cpp file?
https://isocpp.org/wiki/faq/templates#templates-defn-vs-decl

[kbw]

Alternatively, you can pre-instantiate the template for each type you intend to use.

For example, your node.cpp might look like:

#include "node.h"

template <typename T>
Node<T>::Node(T data) : data(data) {
}

template class <int>;
template class <double>;
template class <const char*>;

However, this is used to generate the template code once only, so you can put it in a library, reduce code duplication/bloat ...

But I think you don't need any of that, you just want it to work. So @JLBorges post is likely more appropriate.

[IWishIKnew]

You're not using generics.
Generics are a different concept in other HLLs...

C++ has templates. The defining characteristic of templates is that a version of the template is not generated by the compiler for any type unless it is either a) explicitly specialized or b) used with a type argument that requires a version of that template for the type.

This is why you'll only ever see compiler errors when you use the template object/function/struct, but not when you define them.

To answer your question -- kbw's answer is it. That would result in an explicitly specialized template though.