Member variable u of cv::Mat is still NULL

OpenCV version 4.5.4

When a cv::Mat variable is created in a certain constructor, u, a member of cv::Mat, becomes NULL.
The following code is a simple example.

#include <opencv2/opencv.hpp>
#include <iostream>

int main() {
    cv::Mat a = cv::Mat::zeros(3, 3, CV_8UC3);
    cv::Mat b = cv::Mat(a.rows, a.cols, CV_8UC3, a.data);

    std::cout << a << std::endl;
    std::cout << b << std::endl;
    std::cout << a.u << std::endl;
    std::cout << b.u << std::endl;
    return 0;
}

The results of this program are as follows.

[  0,   0,   0,   0,   0,   0,   0,   0,   0;
   0,   0,   0,   0,   0,   0,   0,   0,   0;
   0,   0,   0,   0,   0,   0,   0,   0,   0]
[  0,   0,   0,   0,   0,   0,   0,   0,   0;
   0,   0,   0,   0,   0,   0,   0,   0,   0;
   0,   0,   0,   0,   0,   0,   0,   0,   0]
00000203AC74C3A0
0000000000000000

Is this a constructor bug?
Also, is there any problem with u being NULL?

Thanks.

what is your interest in this member variable?
you have expectations of this member variable. what do you base these expectations on?

these are important questions, not rhetorical.

https://docs.opencv.org/4.x/d3/d63/classcv_1_1Mat.html#a2742469fe595e1b9036f60d752d08461

you must reproduce any issues with the current release or git master. we will not debug anything that could have been fixed in the meantime. 4.5.4 is ancient.