How to circumvent codegen error "Initial values of class types can only be used for reading properties."

I am writing some logging functionality for my project that involves parallel processing and code generation:

For ease of use, I want to be able to have a logger object 'globally' available throughout my code, so that I do not have to pass logger objects to subfunctions in which I want to create log statements. In order to avoid global variables, I came up with the following structure:

I created a value class "Logger"

classdef Logger

properties(Constant)

FileHandle = PersistentProperty();

end

methods

function obj = Logger()

end

function start(obj, file_name)

file_id = fopen(file_name, 'w');

obj.FileHandle.setValue(file_id);

end

function log(obj, varargin)

fprintf(obj.FileHandle.getValue(), varargin{:});

end

function stop(obj)

fclose(obj.FileHandle.getValue());

end

end

end

which uses handle class "PersistentProperty"

(I do not use setter and getter methods on purpose since this is not compatible with constant properties)

classdef PersistentProperty < handle

properties

Value

end

methods

function setValue(obj, value)

obj.Value = value;

end

function value = getValue(obj)

value = obj.Value;

end

end

end

In my main, I use class Logger as follows:

function main()

logger = Logger(); % create logger object in base workspace

logger.start('main.log');

logger.log('Before parallel loop\n');

sub1();

parfor (idx = 1:2, 2)

par_logger = Logger(); % create logger objects in parallel worker's workspaces

par_logger.start(sprintf('worker%i.log', int8(idx)));

par_logger.log('This is worker %i\n', int8(idx));

sub2();

par_logger.stop();

end

logger.log('After parallel loop\n');

logger.stop();

end

function sub1()

logger = Logger();

logger.log('sub1\n'); % this log statement goes to main.log

end

function sub2()

logger = Logger();

logger.log('sub2\n'); % this log statement goes to worker1.log and worker2.log

end

The code above creates three log files. One in the base workspace (main.log) and one for each parallel workspace (worker1.log, worker2.log). The nice thing is that although functions sub1() and sub2() create their own logger instances, "PersistentProperty" (since it is a handle class) contains the original file handle that it was assigned at the time when logger.start(...) was called. This way the logger objects in sub1() and sub2() 'know' the correct file handle without having to pass it directly via a function argument or without using globals.

The above works perfectly fine when I run it in a Matlab console. The content of the log files is as expected

main.log

Before parallel loop

sub1

After parallel loop

worker1.log

This is worker 1

sub2

worker2.log

This is worker 2

sub2

However, when I try to generate code from it, I get the error message

Initial values of class types can only be used for reading properties.

I understand that, in this case, codegen has a limitation that only allows reading properties of class "PersistentProperty".

1. Is there a way to work around this limitation? Maybe there is some detail that I am missing.
2. Do I need to change my structure completely? Any suggestions for a good alternative way?

[Tony Mohan Varghese]

To reproduce the error can you please specify the verison of MATLAB and the also paste the codegen command that you are using the generate the code?