Clarification about modifying properties in handle class

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Bandar
Bandar on 20 Jul 2021
Answered: Steven Lord on 21 Jul 2021
Take a look at the following class
classdef Counter < handle
properties
count
end
methods
function this = Counter(x)
if nargin == 1
this.count = x;
end
end
function increment(this)
this.count = this.count + 1;
end
end
end
In the constructor, if the argument is zero, count is initliazed according to its value in properties block. In the preceding code, count is empty. It is interesting to notice that in this particular case, calling increment() has no effect on incrementing count. I have to explicitly initializing it with any value in order to make `increment()` works. Any suggestions why I need to explicitly initialize properties?

Accepted Answer

Steven Lord
Steven Lord on 21 Jul 2021
In the preceding code, count is empty.
That is correct, the default value for that propery is the 0-by-0 empty double array [] as stated in the "Property Default Values" section of this documentation page.
calling increment() has no effect on incrementing count
That is also correct.
x = []
x = []
y = x + 1
y = []
The 1 is scalar "expanded" to be a 0-by-0 and a 0-by-0 plus a 0-by-0 results in a 0-by-0.
Any suggestions why I need to explicitly initialize properties?
Because you don't want the default value for that property to be empty in this case.
By the way, despite you mentioning "handle class" in the subject of this post, this behavior holds for value classes as well.

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