Clarification about modifying properties in handle class

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?

Answer 1

[Steven Lord]

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.