Function at

3rd/nlohmann_json/single_include/nlohmann/json.hpp:20987–21001

! @brief access specified array element with bounds checking Returns a reference to the element at specified location @a idx, with bounds checking. @param[in] idx index of the element to access @return reference to the element at index @a idx @throw type_error.304 if the JSON value is not an array; in this case, calling `at` with an index makes no sense. See exampl

    can be thrown.,at__size_type}
    */
    reference at(size_type idx)
    {
        // at only works for arrays
        if (JSON_HEDLEY_LIKELY(is_array()))
        {
            JSON_TRY
            {
                return set_parent(m_value.array->at(idx));
            }
            JSON_CATCH (std::out_of_range&)
            {
                // create better exception explanation
                JSON_THROW(out_of_range::create(401, "array index " + std::to_string(idx) + " is out of range", *this));
            }
        }
        else
        {
            JSON_THROW(type_error::create(304, "cannot use at() with " + std::string(type_name()), *this));