engine/include/resource_manager.h

#ifndef ENGINE_INCLUDE_RESOURCE_MANAGER_H_
#define ENGINE_INCLUDE_RESOURCE_MANAGER_H_

#include <memory>
#include <stdexcept>
#include <string>
#include <unordered_map>
#include <vector>

namespace engine {

class IResourceManager {
 public:
  virtual ~IResourceManager() = default;
};

template <class T>
class ResourceManager : public IResourceManager {
 public:
  std::shared_ptr<T> Add(const std::string& name,
                         std::unique_ptr<T>&& resource) {
    if (resources_.contains(name) == false) {
      std::shared_ptr<T> resource_shared(std::move(resource));
      resources_.emplace(name, resource_shared);
      return resource_shared;
    } else {
      throw std::runtime_error("Cannot add a resource which already exists");
    }
  }

  void AddPersistent(const std::string& name, std::unique_ptr<T>&& resource) {
    persistent_resources_.push_back(Add(name, std::move(resource)));
  }

  std::shared_ptr<T> Get(const std::string& name) {
    if (resources_.contains(name)) {
      std::weak_ptr<T> ptr = resources_.at(name);
      if (ptr.expired() == false) {
        return ptr.lock();
      } else {
        resources_.erase(name);
      }
    }
    // resource doesn't exist:
    throw std::runtime_error("Resource doesn't exist: " + name);
    return {};
  }

 private:
  std::unordered_map<std::string, std::weak_ptr<T>> resources_{};
  // This array owns persistent resources
  std::vector<std::shared_ptr<T>> persistent_resources_{};
};

}  // namespace engine

#endif