MATLAB Force constructor method that should be called when assigning default array elements

Question

I have a class that assigns an immutable UUID to an object in its constructor. The following is a simplified example:

classdef mytest properties (GetAccess = public, SetAccess = immutable) uuid end properties val end methods function obj = mytest(valIn) obj.uuid = char(java.util.UUID.randomUUID.toString); if nargin < 1 valIn = 0; end obj.val = valIn; end end end 

This works fine overall, but I just discovered a problem with the behavior of MATLAB when assigning default elements to an array. I suggested that if I typed

 >> a(5) = mytest(2); 

the constructor will be called 5 times - 4 times without input arguments to populate the default elements from 1 to 4 from the array and once with an input argument of 2 to assign the final element.

However, this is not what MATLAB does - instead, it calls the constructor only once without inputs, and then copies the result four times to populate elements 1 through 4 (and then, as expected for the final element). This means that UUIDs ef elements 1 through 4 end in the same way:

 >> {a.uuid}' ans = '4424b91b-0977-4b4c-b18b-c4564875b952' '4424b91b-0977-4b4c-b18b-c4564875b952' '4424b91b-0977-4b4c-b18b-c4564875b952' '4424b91b-0977-4b4c-b18b-c4564875b952' 'dbb8d862-8a1c-4bf9-876f-ef786e11a896' 

It turns out this is a documented behavior that I just did not expect, and I understand why MathWorks chose this for this (you may not need the overhead of calling the constructor without input many times separately).

But this is not the behavior that I want for this class. Does anyone know how to make the calling constructor be called for each element? Or maybe you can suggest a different class design that will have the desired behavior?

PS I investigated whether MATLAB can copy these elements by default using the copy method - in this case, inheriting my class from matlab.mixin.Copyable and overriding copyElement behavior can help - but it doesn't seem like copying does this: (.

Answer 1

Disclaimer: I am not too familiar with MATLAB classes.

Looking at the relevant documents , it seems that you are doing everything right, as well as MATLAB:

a(1,7) = SimpleValue(7)

The object assigned to the array element a(1,7) uses the input argument passed to the constructor as the value assigned to the property:

...

MATLAB created objects contained in a elements a(1,1:6) with no input arguments. The default value for the empty [] properties.

...

MATLAB calls the SimpleValue constructor once and copies the returned object to each element of the array.

This suggests that what you are experiencing is a documented behavior: automatic distribution of previously unassigned array elements is performed with a single constructor invocation with zero input arguments.

This is also consistent with another example using a random initializer: the code example uses

 for k = 1:5 a(k) = ObjProp; end 

to set 5 elements of an array so that they contain different random data. At least from the documentation, this seems like a problem for your problem: initializing each empty element manually (which is probably not what you want to do).