Defining template in header and trying to use it in the cpp file

Question

I am making a game in SFML 2.0 where I have to store different assets like textures, sounds. So I used templates to achieve this.

I have a header file called AssetManager.hpp:

#ifndef ENGINE_ASSETMANAGER_HPP
#define ENGINE_ASSETMANAGER_HPP

#include <SFML/Audio.hpp>
#include <SFML/Graphics.hpp>

#include <string>
#include <map>

namespace Engine
{
    template <class asset> class AssetManager
    {
        std::map<std::string, asset> data;

        public:
            void add(std::string name, std::string path);

            void remove(std::string name);

            asset &get(std::string name);
    };
}

#endif // ENGINE_ASSETMANAGER_HPP

The type of the template is a class because I'm needing to store class type in a map.

Now the problem is when I try to define the functions in the AssetManager.cpp file:

#include "AssetManager.hpp"

namespace Engine
{
    template <class asset>
    void AssetManager<asset>::add(std::string name, std::string path)
    {
        asset file;
        file.loadFromFile(path);
        this->data[name] = file;
    }

    template <class asset>
    void AssetManager<asset>::remove(std::string name)
    {
        this->data.erase(name);
    }

    template <class asset>
    asset &AssetManager<asset>::get(std::string name)
    {
        return this->data.at(name);
    }
}

I get this one single error that doesn't make sense to me:

error LNK2019: unresolved external symbol "public: void __cdecl Engine::AssetManager<class sf::Texture>::add(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (?add@?$AssetManager@VTexture@sf@@@Engine@@QEAAXV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@0@Z) referenced in function main