Using MATLAB Coder with a singleton class accessing Constant property returns error "Initial values of class types can only be used for reading properties"

I am trying to use a singleton model for a class and would like to generate C code from some MATLAB code that uses this class. However, it seems to be returning the error "??? Initial values of class types can only be used for reading properties.". Below, I provide a small example demonstrating the issue. It is unclear what exactly the issue is given the error is occuring on the line trying to access the instance property (obj = SingletonClass.instance), and this line is reading a constant property of the class.

I have two questions:

• Could you help me understand the meaning of this error and why it is occuring?
• Is there a easy fix to this while still following the singleton model for the class?

Class definition:

classdef SingletonClass < handle
  properties (Access = protected)
    data (1,1) double = 5;
  end
  properties (Constant, Access = private)
    instance (1,1) SingletonClass = SingletonClass();
  end
  methods (Access = private)
    function this = SingletonClass()
      % Constructor
    end
  end
  methods (Static, Access = public)
    function obj = Instance()
      % Get singleton instance
      obj = SingletonClass.instance;
    end
  end
  methods (Access = public)
    function out = test(this, val)
    % Test code
      out = this.data + val;
    end
  end
end

The function I am trying to use coder on:

function out = myfunction(val) %#codegen
  obj = SingletonClass.Instance();
  out = obj.test(val);
end

Codegen script:

val = double(0);
codegen myfunction -args {val}

[Mukund Sankaran]

Hi @Curt Laubscher,

For your second question, you could rewrite your static method to use persistent variables, like below, and that should work

classdef SingletonClass < handle
  properties (Access = protected)
    data (1,1) double = 5;
  end

  methods (Access = private)
    function this = SingletonClass()
      % Constructor
    end
  end

  methods (Static, Access = public)
    function obj = Instance()
      % Get singleton instance
      persistent instance
      if isempty(instance)
          instance = SingletonClass();
      end
      obj = instance;
    end
  end

  methods (Access = public)
    function out = test(this, val)
      % Test code
      out = this.data + val;
    end
  end
end

[Mukund Sankaran]

As regards your question #1, the error message is trying to say that you are only allowed to read the properties of "instance". For example, you are allowed to write an expression like this that reads the "data" property of "instance".

someVar = SingletonClass.instance.data + 1;