Why does the memory allocation for handle class objects in a cell array cause an error in a loop?

I am unable to generate C++ code for an M-file that allocates memory for a handle class cell array during a loop using MATLAB R2021b.

The class that I am using is defined below. It has a property named "Value" that is assigned by an argument, or by the class function "setValue":

classdef BasicClass < handle  %#codegen
    properties
        Value;
    end

    methods
        function obj = BasicClass(val)
            if nargin == 1
                obj.Value = val;
            end
        end
        function obj = setValue(obj, input)
            obj.Value = input;
            fprintf( 'set.Prop1 method called. Prop1 = %f\n', obj.Value );
        end
    end

end

Here is the relevant code that I am trying to generate code for:

Nodes = cell(1, Nobs);
for nobs = 1:Nobs
    Nodes{nobs} = BasicClass(nobs);
end
tmp = Nodes{5};

"BasicClass" is a class that inherits handle, "Nobs" is a variable that I'm setting at my entry point (I want to set "Nobs" at the cli interface for my C++). I'm receiving this error when running my script to generate C++ code:

### Compiling function(s) my_entry_point ...
### Generating compilation report ...
??? Cannot allocate this handle object. For code
generation, a handle object allocated inside a loop
cannot be referenced outside of the loop.

How can I resolve this error?

Answer 1

[MathWorks Support Team]

The MATLAB coder must statically allocate memory for handle class objects, and allocating in a loop prevents static memory allocation. The 'Nodes' variable should be pre-allocated not only in size, but also in data type.

A workaround is to use "repmat" on a single cell entry to ensure the variable-sized cell array passes the define-it-before-using-it analysis. Then, use a loop to adjust the values of each handle-class object in the array:

% Allocate for a cell array of handle classes
Nodes = repmat({BasicClass(1)}, 1, Nobs);
% Variable-sized for loop
for nobs = 2:Nobs
    % Set the object as you'd like.
    setValue(Nodes{nobs}, nobs);
end
tmp = Nodes{5};