Error Handling

Error handling in arg_router is exclusively done through exceptions. The only exception the user will encounter is parse_exception.

parse_exception is not used internally though, internally multi_lang_exception is used instead and this carries an error code rather than a string. As the name suggests, this so it can be mapped (upon exit from the library) to a translated error message which is then re-thrown as a parse_exception.

The translation mapping is done via a translation::error_code_translations subtype, for example:

template <>
class translation<str<"ja">>
{
    public:
      ...
 
    using error_code_translations = std::tuple<
        std::pair<traits::integral_constant<error_code::unknown_argument>, str<"不明な引数">>,
        std::pair<traits::integral_constant<error_code::unhandled_arguments>, str<"未処理の引数">>,
        std::pair<traits::integral_constant<error_code::argument_has_already_been_set>,
                  str<"引数はすでに設定されています">>,
        std::pair<traits::integral_constant<error_code::failed_to_parse>,
                  str<"解析に失敗しました">>,
        std::pair<traits::integral_constant<error_code::no_arguments_passed>,
                  str<"引数が渡されませんでした">>,
        std::pair<traits::integral_constant<error_code::minimum_value_not_reached>,
                  str<"最小値に達していません">>,
        std::pair<traits::integral_constant<error_code::maximum_value_exceeded>,
                  str<"最大値を超えました">>,
        std::pair<traits::integral_constant<error_code::minimum_count_not_reached>,
                  str<"最小数に達していません">>,
        std::pair<traits::integral_constant<error_code::mode_requires_arguments>,
                  str<"モードには引数が必要です">>,
        std::pair<traits::integral_constant<error_code::missing_required_argument>,
                  str<"必要な引数がありません">>,
        std::pair<traits::integral_constant<error_code::too_few_values_for_alias>,
                  str<"エイリアス値が少なすぎる">>,
        std::pair<
            traits::integral_constant<error_code::dependent_argument_missing>,
            str<"従属引数がありません (コマンドラインで必要なトークンの前に置く必要があります)">>>;
};

If translation::error_code_translations is not provided for a language specialisation, then the library will fall back to the internal en_GB version.

Node or policy developers will need to define their own error codes for errors specifc to their type. Taking the is_even policy from the custom_node_and_policy example to demonstrate:

constexpr auto not_even_ec = std::size_t{1000};
 
namespace arg_router::multi_lang
{
template <>
class translation<str<"ja">>
{
    public:
    ...
 
    using error_code_translations = std::tuple<
        std::pair<traits::integral_constant<not_even_ec>, str<"非偶数値">>,
        ...
    >;
};
}
 
template <typename ValueType>
class is_even
{
    static_assert(std::is_integral_v<ValueType>, "ValueType must be an integer");
 
public:
    template <typename... Parents>
    void validation_phase(const ValueType& value, [[maybe_unused]] const Parents&... parents) const
    {
        using node_type = boost::mp11::mp_first<std::tuple<Parents...>>;
 
        if (value % 2 != 0) {
            throw ar::multi_lang_exception{not_even_ec, parsing::node_token_type<node_type>()};
        }
    }
};

To avoid clashes upon upgrading the library, developers are encouraged to define their own error codes above 1000. If a custom error code is not available in error_code_translations then the error string is:

Untranslated error code (N): <tokens>