Template class- unresolved externals c++ SFML

Question

I have a template class ResourceHolder which is supposed to carry "Resources". The function's declarations are in ResourceHolder.inl file. I have followed the book "SFML Game Development Book" and as I can see all that they have done in their ResourceHolder class is almost the same as mine. Here are links to their ResourceHolder.h: https://github.com/LaurentGomila/SFML-Game-Development-Book/blob/master/07_Gameplay/Include/Book/ResourceHolder.hpp

and ResourceHolder.inl: https://github.com/LaurentGomila/SFML-Game-Development-Book/blob/master/07_Gameplay/Include/Book/ResourceHolder.inl

But I get unresolved externals whenever I try to create an instance of that class.

Here's my ResourceHolder.h:

#pragma once
#include <map>
#include <memory>
#include <string>

template<typename Identifier, typename Resource>
class ResourceHolder
{
  public:

     ResourceHolder();

    ~ResourceHolder();

     void load(Identifier, std::string);

     Resource& get(Identifier);

  private:

     std::map<Identifier, std::unique_ptr<Resource>> mResources;

};


  #include "ResourceHolder.inl"

ResourceHolder.inl:

template<typename Identifier, typename Resource>
void ResourceHolder<Identifier, Resource>::load(Identifier id, std::string pathToFile)
{
std::unique_ptr<Resource> resource(new Resource());

if (!resource->loadFromFile(pathToFile))
{
    //Didn't find any "Resource" at that location!

}

mResources.insert(std::pair<Identifier, Resource>(id, std::move(resource)));
}


 template<typename Identifier, typename Resource>
 Resource& ResourceHolder<Identifier, Resource>::get(Identifier id)
 {
   auto found = mResources.find(id);

   return *found->second;
 }

And also I have ResourceIdentifiers.h file where I keep some typedefs...:

#ifndef RESOURCEIDENTIFIERS_H
#define RESOURCEIDENTIFIERS_H

namespace sf
{
   class Texture;
   class Font;
}

namespace Textures
{
    enum ID
    {
        Eagle,
        Raptor,
        Desert,
        TextureCount
    };
}

 namespace Fonts
 {
    enum ID
    {
        Arial
    }; 
 }

 template<typename Identifier, typename Resource>
 class ResourceHolder;

 typedef ResourceHolder<Textures::ID, sf::Texture> TextureHolder;
 typedef ResourceHolder<Fonts::ID, sf::Font> FontHolder;

 #endif

I use enums for Identifier and sf::Texture or sf::Font for Resource.

Answer 1

You havent implemented the constructor nor the destructor. Therefor the linker tells you that he is lacking the definition of "external symbols" (ctor and dtor in this case).