C++ SFML Gamedev Book - Unresolved External Symbol from ResourceHolder class - c++

Question

I have the following three files, of which I cannot find the source of an error that it is producing:
Main.cpp
#include <SFML/Graphics.hpp>
#include <iostream>
#include "ResourceHolder.h"
namespace Textures
{
enum ID { Landscape, Airplane, Missile };
}
int main()
{
//...
try
{
ResourceHolder<sf::Texture, Textures::ID> textures;
textures.load(Textures::Airplane, "Airplane.png");
}
catch (std::runtime_error& e)
{
std::cout << "Exception: " << e.what() << std::endl;
}
//...
}
ResourceHolder.h
#pragma once
#include <map>
#include <string>
#include <memory>
#include <stdexcept>
#include <cassert>
template <typename Resource, typename Identifier>
class ResourceHolder
{
public:
void load(Identifier id, const std::string& fileName);
Resource& get(Identifier id);
const Resource& get(Identifier id) const;
private:
void insertResource(Identifier id, std::unique_ptr<Resource> resource);
std::map<Identifier, std::unique_ptr<Resource>> mResourceMap;
};
ResourceHolder.cpp
#include "ResourceHolder.h"
template <typename Resource, typename Identifier>
void ResourceHolder<Resource, Identifier>::load(Identifier id, const std::string& fileName)
{
//Create and load resource
std::unique_ptr<Resource> resource(new Resource());
if (!resource->loadFromFile(fileName)) {
throw std::runtime_error("ResourceHolder::load - Failed to load " + fileName);
}
//If loading was successful, insert resource to map
insertResource(id, std::move(resource));
}
template <typename Resource, typename Identifier>
Resource& ResourceHolder<Resource, Identifier>::get(Identifier id)
{
auto found = mResourcemap.find(id);
assert(found != mResourceMap.end());
return *found->second();
}
template <typename Resource, typename Identifier>
void ResourceHolder<Resource, Identifier>::insertResource(Identifier id, std::unique_ptr<Resource> resource)
{
//Insert and check success
auto inserted = mResourceMap.insert(std::make_pair(id, std::move(resource)));
assert(inserted.second);
}
If I were to remove the try-catch combination in main.cpp, the code compiles fine; However, if I leave it there it gives me an LNK2019 (Unresolved external symbol) Error.
What is the source of this error, and how would I fix it?

Answer 1

You can't define templates inside .cpp files. They have to be defined in the header so the compiler can see the implementation and generate the specific classes.
Here's a better question/answer on why it is so Why can templates only be implemented in the header file?.
EDIT: What's wrong in the get function
Two things.
First is this auto found = mResourcemap.find(id);. Your map name is incorrect, m should be upper case -> mResourceMap.
Then the line return *found->second();. The map iterator contains a pair, and the first and second members are not functions but data members. You should write return *found->second;.
I would advise you to understand the structures you're working with before using templates. The compile errors with templates are pretty messy and harder to read. Also you could make a separate test program and make a resource manager with no templates to understand your errors more easily, then build the template on top of your working resource manager.

Answer 2

With all the other answers providing you with enough information to why your code don't compile and might not be valid, this is a resource manager i wrote for SFML some time ago, might be of use to you:
HPP FILE:
#ifndef RESOURCEMANAGER_HPP
#define RESOURCEMANAGER_HPP
/************ INCLUDES ***********/
#include <iostream>
#include <map>
#include <vector>
#include <string>
#include <memory>
#include "SFML/Graphics.hpp"
#include "SFML/Audio.hpp"
class ResourceManager
{
private:
std::map<std::string,std::unique_ptr<sf::Texture>> listImageContainer;
std::map<std::string,std::pair<std::unique_ptr<sf::SoundBuffer>,std::unique_ptr<sf::Sound>>> listSoundContainer;
std::map<std::string,std::unique_ptr<sf::Font>> listFontContainer;
public:
ResourceManager();
std::unique_ptr<sf::Sound>& LoadSound(const std::string);
std::unique_ptr<sf::Font>& LoadFont(const std::string);
std::unique_ptr<sf::Texture>& LoadImage(const std::string);
~ResourceManager();
};
#endif
CPP FILE:
#include "ResourceManager.hpp"
ResourceManager::ResourceManager()
{
}
std::unique_ptr<sf::Sound>& ResourceManager::LoadSound(const std::string _fileName)
{
if (listSoundContainer.find(_fileName) == listSoundContainer.end())
{
std::unique_ptr<sf::SoundBuffer> soundBuffer(new sf::SoundBuffer());
if (soundBuffer->loadFromFile("assets/sound/" + _fileName) != false)
{
std::unique_ptr<sf::Sound> sound(new sf::Sound(*soundBuffer));
listSoundContainer[_fileName] = std::make_pair(std::move(soundBuffer), std::move(sound));
return listSoundContainer[_fileName].second;
}
else
{
std::cerr << "Error loading sound..." << std::endl;
}
}
else
{
return listSoundContainer[_fileName].second;
}
}
std::unique_ptr<sf::Font>& ResourceManager::LoadFont(const std::string _fileName)
{
if (listFontContainer.find(_fileName) == listFontContainer.end())
{
std::unique_ptr<sf::Font> font(new sf::Font());
if (font->loadFromFile("assets/font/" + _fileName)!=false)
{
listFontContainer[_fileName] = std::move(font);
return listFontContainer[_fileName];
}
else
{
std::cerr << "Error loading font..." << std::endl;
}
}
else
{
return listFontContainer[_fileName];
}
}
std::unique_ptr<sf::Texture>& ResourceManager::LoadImage(const std::string _fileName)
{
if (listImageContainer.find(_fileName) == listImageContainer.end())
{
std::unique_ptr<sf::Texture> texture(new sf::Texture);
if (texture->loadFromFile("assets/image/" + _fileName)!=false)
{
listImageContainer[_fileName] = std::move(texture);
return listImageContainer[_fileName];
}
else
{
std::cerr << "Error loading image: " << _fileName << std::endl;
}
}
else
{
return listImageContainer[_fileName];
}
}
ResourceManager::~ResourceManager(){}
How to use:
ResourceManager resourceManager;
auto& sound = resourceManager.LoadSound("nice.wav");
auto& image = resourceManager.LoadImage("head.png");
auto& sound2 = resourceManager.LoadSound("nice.wav"); //<--- already loaded
sound.play();
etc...