Variadic template class containing array of another template class

Question

I'm creating my own RDBMS to practice C++. I came up with a solution where each column can have a type and is a template. This is what a row looks like, but it doesn't work

template <typename... col_types>
struct row {
  row(std::tuple<std::function<bool(col_types)>, bool, col_types>...col_data){
    int i = 0;
    ([&]{
        columns[i] = std::make_unique<column_impl<decltype(std::get<2>(col_data))>>(std::get<0>(col_data), std::get<1>(col_data), std::get<2>(col_data));
        i++;
    }(), ...);
  }
  std::array<std::unique_ptr<column_impl<col_types>>, sizeof...(col_types)> columns;
};

And here is a column_impl

template <typename data_t>
struct column_impl {
  column_impl<data_t>() {}
  column_impl<data_t>(std::function<bool(data_t)> _constraint) : constraint(_constraint) {}
  column_impl<data_t>(std::function<bool(data_t)> _constraint, bool _is_nullable)
      : constraint(_constraint), is_nullable(_is_nullable) {}

  // This constructor requires to validate whether provided data
  // satisfies provided constraint. If constraint isn't satisfied
  // an exception gets thrown. It needs to be handled
  column_impl<data_t>(std::function<bool(data_t)> _constraint, bool _is_nullable, data_t _value)
      : constraint(_constraint), is_nullable(_is_nullable) {
    if (validate_data()) {
      value = _value;
    } else {
      throw std::invalid_argument("Provived data couldn't satisfy column's constraint");
    }
  }

  std::function<bool(data_t)> constraint = [](data_t) { return true; };
  bool is_nullable = false;
  data_t value;
  auto get_column_type_variant() { return std::variant<data_t>(); }

  bool validate_data() { return constraint(value); }

  template <typename T>
  bool validate_type(T) {
    return std::is_nothrow_convertible<T, data_t>();
  }
};

Any ideas how to create a container for columns inside of a row and create a constructor that unpacks tuples into a row? I can't get it working. I know this array is not supposed to work, but I have no idea how to write it better. And also I don't want a vector. I could write a polymorphic wrapper like
struct column{some_type_containing_column_impls column_obj;}, but I have no idea how to do it. And how to properly unpack those tuples into column_impl obj?

Answer 1

Using only non-type template arguments can do the thing.

template<class T>
bool default_constraint(T) {
    return true;
}

template<class T, bool nullable = true, bool(constraint)(T) = default_constraint>
class column {
    T data;
    bool is_null;
public:
    column() : data(), is_null(true) { ///< T must be DefaultConstructable
        if (!validate())
            throw std::invalid_argument("Provided data couldn't satisfy column's constraint");
    }

    column(T data) : data(data), is_null(false) {
        if (!validate())
            throw std::invalid_argument("Provided data couldn't satisfy column's constraint");
    }

    bool validate() {
        return nullable || is_null || constraint(data);
    }

    T get() {
        return data;
    }
};

template<class... C>
struct row {
    std::tuple<C...> columns;

    row(C&&... c) : columns(std::forward<C>(c)...) {}

    template<int index>
    auto get() {
        return std::get<index>(columns).get();
    }
};

bool require_positive(int i) {
    return i >= 0;
}

int main() {
    typedef row<column<int>, column<float>, column<bool>> MyRow;
    MyRow r{1, 2.3f, true};

    using MyRow2 = row<column<int, false, require_positive>, column<float>>;
    MyRow2 r2{-1, 2.5}; ///< exception thrown
}

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I know maybe you only want the "idea" of how to implement, but I just post the code here for clarity.