How to use c++ codegen to write to a ROS2 message, erroring at struct type

I have the following code in maltab to be generated using codegen:

%change function to file name
function LidarObstacle()
%#codegen

pointnode_lane = ros2node("/ouster_driver_lane");
pubdistance_lane = ros2publisher(pointnode_lane,"/LaneDist","std_msgs/Float64MultiArray");

msg = ros2message(pubdistance_lane);
msg.data = coder.nullcopy(zeros(1, 20));

The error that is happening is:

Attempt to write a value of type 'std_msgs_Float64MultiArrayStruct_T' into a variable defined as type

'struct_T'. Code generation does not support changing types through assignment. To investigate the cause

of the type mismatch, check preceding assignments or input type specifications.

Is there a fix to this issue? I've tried multiple different ways. I've even tried Simulink at writing the

Answer 1

[Prabeen Sahu]

Hi,

This is likely a limitation with your current version of MATLAB. When you create a ROS 2 message in MATLAB (e.g., std_msgs/Float64MultiArray), the message fields such as data are preallocated with a specific type and size. If you attempt to assign a whole new array (e.g., rmsg.data = zeros(1, 20)) or use coder.nullcopy, MATLAB Coder detects a type or size mismatch and produces an error.

Workaround:

Instead of coder.nullcopy(zeros(1, 20)), please use a loop to initialize msg.data as shown below:

function LidarObstacle()
%#codegen
pointnode_lane = ros2node("/ouster_driver_lane");
pubdistance_lane = ros2publisher(pointnode_lane,"/LaneDist","std_msgs/Float64MultiArray");
msg = ros2message(pubdistance_lane);
for ii = 1:20
    msg.data(ii) = 0;
end

[squared]

Thank you! Worked like a charm!