C++17 带有元组的查找表。如果找不到给定值，我需要一个编译时错误

Question

我有一个包含两个键入值的表。在生产代码中，我在元组中搜索给定值。我没有得到的是，如果在元组中找不到该值，我想得到一个编译时错误。下面的例子：

我有这个功能：

template <typename TLookup, typename TValueToSearch, typename TSetting>
constexpr auto lookupNeededSetting(const TValueToSearch &valueToSearch)
{
  TSetting setting = {};
  typename TLookup::Type const table = typename TLookup::Type{};

  std::apply(
      [&valueToSearch, &setting](const auto &...entry) {
        // Iterate over each entry in the table
        return (((entry.fits(valueToSearch)) ? setting = entry.value() : false) || ...);
      },
      table);

  return setting;
}

这会像这样搜索给定的表：

template <typename... TEntries>
struct LookupTable
{
  using Type = std::tuple<TEntries...>; ///< Type representing the tuple of entries.
};

给定的条目有一个函数

fits

，它检查给定的值是否相同，如果是，则返回翻译后的值。不起作用的是，如果给定的值不在表中，我希望出现编译时错误，但我无法让它工作。这可能吗？

它的用法是这样的：

using TestValueLookup = LookupTable<Entry<TypedValue<uint16_t, 1>, TypedValue<uint8_t, 0x00>>,
                                    Entry<TypedValue<uint16_t, 2>, TypedValue<uint8_t, 0x01>>,
                                    Entry<TypedValue<uint16_t, 3>, TypedValue<uint8_t, 0x02>>,
                                    Entry<TypedValue<uint16_t, 4>, TypedValue<uint8_t, 0x03>>>;

constexpr auto settingValue = utilities::lookupNeededSetting<TestValueLookup, uint16_t, uint8_t>(1);

新示例：

template <typename TLookup, typename TValueToSearch, typename TSetting, auto VValueToSearch>
constexpr bool lookupNeededSetting2(TSetting& setting) {
    bool found = false;
    typename TLookup::Type const table = typename TLookup::Type{};

    std::apply(
        [&found, &setting](const auto&... entry) {
            // Iterate over each entry in the table
            ((entry.fits(VValueToSearch) ? (setting = entry.value(), found = true) : false) || ...);
        },
        table);

    return found;
}

template <typename TLookup, typename TValueToSearch, typename TSetting, auto VValueToSearch>
class Lookup {
public:
    static constexpr TSetting setting = [] {
        TSetting tempSetting{};
        constexpr bool found = lookupNeededSetting2<TLookup, TValueToSearch, TSetting, VValueToSearch>(tempSetting);
        static_assert(found, "The given VValueToSearch was not found in the lookup table.");
        return tempSetting;
    }();

    static constexpr auto value = setting;
};

不幸的是编译器抱怨 tempSetting 它不是常量

Answer 1

我现在的解决方案是这样的：

template <typename TSetting>
class Setting
{
public:
  bool m_found = false;
  TSetting m_value;
};

template <typename TLookup, typename TValueToSearch, typename TSetting, auto VValueToSearch>
constexpr Setting<TSetting> lookupNeededSetting2()
{
  Setting<TSetting> setting{};
  typename TLookup::Type const table = typename TLookup::Type{};

  std::apply(
      [&setting](const auto &...entry) {
        // Iterate over each entry in the table
        ((entry.fits(VValueToSearch) ? (setting.m_value = entry.value(), setting.m_found = true) : false) || ...);
      },
      table);

  return setting;
}

template <typename TLookup, typename TValueToSearch, typename TSetting, auto VValueToSearch>
class Lookup
{
public:
  static constexpr auto getSetting = [] {
    return lookupNeededSetting2<TLookup, TValueToSearch, TSetting, VValueToSearch>();
  }();

  static constexpr auto setting = getSetting;

  static_assert(setting.m_found, "Your searched setting was not found.");
};