Add elements to matlab cell array member variable - matlab

Question

i have a class called sampleClass with a cell array member variable called list.
Now i am trying to add elements to the internal list. For this purpose, i created the method addToList. The method simply tries to add elements to the list.
classdef sampleClass
properties
list = {}
end
methods
function addToList(obj)
obj.list{end+1}='test';
obj.list
end
end
end
Now the problem is, that the elements are not stored in the list.
Inside the function addToList the following output signalizes that the element is stored in the list:
>> samp = sampleClass();
>> samp.addToList
ans =
'test'
But after leaving the function, the element is no longer accessible:
>> samp.list
ans =
{}
Has anybody an idea what's going wrong? I am using Matlab 7.11.0 (R2010b).
Thanks for your help.
Julien

Answer 1

That's because you forgot to inherit from the handle class:
classdef sampleClass < handle %// Change here!
properties
list = {}
end
methods
function addToList(obj)
obj.list{end+1}='test';
obj.list
end
end
end
If you want to retain the changes made when modifying an instance of your class, you must inherit from the handle class. This can be done by taking your class definition (first line of your code), and doing < handle. If you don't, then any changes you make will not be saved. To show that this works, this is what I get in MATLAB:
>> samp = sampleClass();
>> samp.addToList
ans =
'test'
>> samp.list
ans =
'test'
To be more verbose, here's what the MATLAB docs say about handle:
The handle class is the superclass for all classes that follow handle semantics. A handle is a reference to an object. If you copy an object's handle, MATLAB® copies only the handle and both the original and copy refer to the same object data. If a function modifies a handle object passed as an input argument, the modification affects the original input object.
As such, you need to inherit from the handle class so that you have a reference back to the object you created. Any changes you make to the instance of your class will be remembered if you inherit from handle. If you don't, no changes you make get registered.