simple_storage.h

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#pragma once
 
#include <map>
#include <optional>
#include <stdexcept>
#include <string>
#include <variant>
 
class SimpleStorage {
 public:
  using StorageKey = std::string;
  using StorageValue = std::variant<int, float, std::string>;
 
  template <class... TSelector>
  struct overloaded : TSelector... {
    using TSelector::operator()...;
  };
 
  template <typename T>
  T get_value_or_default(const std::string &key, T default_value) {
    // So we provide the overloads for the () operator here. Every version of it
    // takes in x and returns T or throws...
    return std::visit(overloaded{
                          [](const T value) -> T { return value; },
                          [](auto) -> T {
                            throw std::invalid_argument{
                                "invalid value for T in settings::value_of"};
                          },
                      },
                      instance().try_get(key).value_or(default_value));
  }
 
  static SimpleStorage &instance();
 
  std::optional<StorageValue> try_get(const StorageKey &key_name);
 
  void set_value(const StorageKey &key_name, const StorageValue &value);
 
  void delete_all();
 
  void delete_key(const StorageKey &key_name);
 
  void save();
 
 protected:
  std::map<StorageKey, StorageValue> session_storage_;
 
  SimpleStorage() = default;
 
  friend class Engine;
 
  void load();
};