Work Around Language Limitation: Code Generation Does Not Support Object Arrays

Issue

In certain situations, your MATLAB® algorithm uses an array of objects that are instances of the same class. But code generation does not support object arrays. When attempting to generate code for such MATLAB code, you get this or a similar error message:

Code generation does not support object arrays.

Possible Solutions

Use Cell Array of Objects

Code generation supports cell arrays of objects. In your MATLAB code, represent the collection of objects by using a cell array instead of an array.

For example, suppose that your MATLAB algorithm uses the class Square:

classdef Square
    properties(Access = private)
        side
    end

    methods(Access = public)
        function obj = Square(side)
            obj.side = side;
        end

        function area = calculateArea(obj)
            area = obj.side^2;
        end
    end
end

The function addAreas constructs and uses a 1-by-3 array of Square objects:

function y = addAreas(n)
obj = Square(0);
collection = [obj obj obj]; % collection is an array

for i = 1:numel(collection)
    collection(i) = Square(n + i);
end

y = 0;
for i = 1:numel(collection)
    y = y + collection(i).calculateArea;
end
end

Attempt to generate a MEX function for addAreas. Code generation fails because the local variable collection is an object array.

codegen addAreas -args 0 -report

Code generation does not support object arrays.

Redefine collection to be a cell array instead. Modify the code to use cell array indexing to index into collection. Name the modified function addAreas_new.

function y = addAreas_new(n)
obj = Square(0);
collection = {obj obj obj}; % collection is a cell array

for i = 1:numel(collection)
    collection{i} = Square(n + i);
end

y = 0;
for i = 1:numel(collection)
    y = y + collection{i}.calculateArea;
end
end

Attempt to generate a MEX function for addAreas_new. Code generation succeeds and produces addAreas_new_mex.

codegen addAreas_new -args 0 -report
Code generation successful: View report

Verify that addAreas_new and addAreas_new_mex have the same runtime behavior.

disp([addAreas_new(0) addAreas_new_mex(0)])
14    14

For Assignment with Nonscalar Indexing, Use Curly Braces and deal

Suppose that your original MATLAB code performs assignment to the array of objects by using nonscalar indexing. For example, you might add this line after the first for loop in the addAreas function:

collection(1:2) = [Square(10) Square(20)];

In the modified function addAreas_new, index into the corresponding cell array by using curly braces {} and perform assignment by using the deal function. Replace the above line by:

[collection{1:2}] = deal(Square(10),Square(20));

See Also

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