ini configuration reader

ini.cpp

#include <algorithm>
#include <optional>
#include <type_traits>
#include <unordered_map>
#include <vector>

#include <cstdio>

// http://tristanbrindle.com/posts/a-quicker-study-on-tokenising/
template <typename Iterator, typename Lambda>
void for_each_token(Iterator&& first, Iterator&& last, char separator, Lambda&& lambda) {
    while (first != last) {
        const auto pos = std::find(first, last, separator);

        lambda(first, pos);

        if (pos == last) {
            break;
        }

        first = std::next(pos);
    }
}

template<class T> struct always_false : std::false_type {};

struct Config {
  std::unordered_map<std::string, std::string> config_values = {
    {"foo", "bar"},
    {"bar", "4242"},
    {"array", "one,two,three,four"}
  };

  template <typename T>
  std::optional<T> get(std::string const& key, std::optional<T> default_value = {}) {
    if (this->config_values.find(key) != std::end(this->config_values)) {
      if constexpr(std::is_same_v<T, int>) {
        return std::stoi(this->config_values.at(key));
      } else if constexpr(std::is_same_v<T, std::string>) {
        return this->config_values.at(key);
      } else if constexpr(std::is_same_v<T, std::vector<std::string>>) {
        auto v = this->config_values.at(key);
        std::vector<std::string> ret;
        for_each_token(std::begin(v), std::end(v), ',', [&ret](auto begin, auto end) {
          ret.emplace_back(begin, end);
        });
        return ret;
      } else {
        static_assert(always_false<T>::value, "supported types are int, std::string, std::vector<std::string>");
      }
    } else {
      return default_value;
    }
  }
};

int main() {
  Config c;

  auto v = c.get<std::string>("foo");
  printf("foo:%15s\n", v.value_or("No value found for this key").c_str());

  auto v2 = c.get<int>("bar");
  printf("bar:%15i\n", v2.value_or(0));

  auto v3 = c.get<std::string>("doesnotexist", "defaultvalue");
  printf("doesnotexist:%15s\n", v3.value_or("No value found for this key").c_str());

  auto v4 = c.get<std::vector<std::string>>("array");
  for (auto e : v4.value_or(std::vector<std::string>{})) {
    printf("%s ", e.c_str());
  } printf("\n");
}