What is the best way to work with variable size class properties?

Hi,

I would like to generate C++ code for a object oriented matlab algorithm that fully exploits dynamic memory allocation.

Right now my code contains data objects (see obj_data_fixedsize10000) that include a property 'data' with fixed size. For the code generation I'm using the following configuration:

cfgexe = coder.config('exe');
cfgexe.DynamicMemoryAllocation = 'Off';

The memory that is allocated after code generation is huge, so this approach won't work for my project.

Instead I'd like to work with variable size arrays. Unfortunately, this feature is currently explicity not supported for class properties ('coder.varsize' is not supported for class properties, see [1]).

I figured out a way to use a persistent variable 'data_pers' (see obj_data_persistent below) that can be declared as variable size to accomplish my needs. However, the object obj_data_persistent can be instanced only once since a second instance would use the same persistent variable.

Besides, I assume that it could also be kind of tricky to pass an instance of obj_data_persistent as an input to an entry point function.

Therefore my question: Are there any more elegant procedures to work with objects that contain variable size variables?

Thanks

Johannes

classdef obj_data_fixedsize10000

    properties (Access = private)
        data = zeros(10000,1);
        n_valid = 1;
    end        
    properties (Constant = true)
        n_max = 10000;
    end
    
    methods
        
        function val = getData(obj)
            val = obj.data(1:obj.n_valid);            
        end
        
        function obj = setData(obj,val)
            obj.n_valid = min([length(val) obj.n_max]);
            obj.data(1:obj.n_valid) = val(1:obj.n_valid);
        end                                           
        
    end     

end

classdef obj_data_persistent   
    
    methods        
        function val = getData(obj)
            val = obj.persData;            
        end        
        function obj = setData(obj,val)
            obj.persData(val);
        end                 
    end
        
    methods (Access = private)        
        % Handling Persistent Data
        function varargout = persData(~,varargin)            
            persistent data_pers
            if isempty(data_pers)
                data_pers = 0;
            end
            coder.varsize('data_pers',[inf 1],[1 0]);            
            if nargin == 2
                data_pers = varargin{1};
            end            
            if nargout == 1
                varargout{1} = data_pers;
            end            
        end        
    end
    
end

[1] https://de.mathworks.com/help/simulink/ug/how-working-with-matlab-classes-is-different-for-code-generation.html

Answer 1

[Denis Gurchenkov]

Hi John,

MATLAB Coder does support variable-sized arrays inside classes. I think you got confused by the fact that coder.varsize does not work for class properties. There are other ways to tell Coder that this property is variable-sized, without using coder.varsize (and that is a good catch, we should make the documentation more clear). The end result should be gneration of C or C++ code that uses dynamic memory allocation.

In your example, try this:

classdef obj_data
  properties
    data
  end
  methods
    function obj = obj_data
      % These two sequential assignments with same base type, but different sizes
      % tell MATLAB Coder that obj.data is a variable-sized array that should
      % be allocated in the heap.
      obj.data = 1;
      obj.data = zeros(1,0);
    end
    function val = getData(obj)
      val = obj.data; 
    end
    function obj = setData(obj,val)
      obj.data = val;
    end                                           
  end
end

And use it like this:

function out = main(n)
  x = obj_data;
  if n < 10
    x = x.setData(1:10);
  else
    x = x.setData([]);
  end
  for i = 1:n
    x = x.setData([x.getData(), i]);
  end
  out = x.getData();
end  

Now code generation should work fine:

codegen main -args {1}

[Johnop]

Thanks Denis! Indeed, it was not clear to me that varsize not working does not imply that variable size variables do not work.