Cannot seem to save data to a file within a class

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I have a class which needs to write data to files. However, when I try to write the data, it gives a "variable not found" error, despite the variable being declared in the lines immediately preceding the call. I can manually run the code from the matlab command line and it does what I expect, so I think that the code is fine, but by being put into a class it somehow changes where the variables live. This is a shortened (working) version of what I have:
classdef SaveFiles < handle
properties
data;
fileLoc;
end
methods
function s = SaveFiles(loc)
s.fileLoc = loc;
end
function s = addData(s, data)
s.data = data;
end
function s = writeData(s)
data = s.data;
evalin('base', sprintf('save(''%s'', ''data'')', s.fileLoc));
end
end
end
From the matlab command line:
>> sf = SaveFiles('<file_loc>');
>> a = 1;
>> sf.addData(a);
>> sf.writeData();
This last line causes the following exception:
Error using save
Variable 'data' not found.
Error in SaveFiles/writeData (line 18)
evalin('base', sprintf('save(''%s'', ''data'')', s.fileLoc));
I'm completely stumped. Is there something obvious I'm just overlooking?

Accepted Answer

per isakson
per isakson on 2 Mar 2018
Edited: per isakson on 2 Mar 2018
Yes, there is. To make it work, replace
evalin('base', sprintf('save(''%s'', ''data'')', s.fileLoc));
by
save( s.fileLoc, 'data' )
Why do you think you need to execute save in the base workspace? The variable data is not available in the base workspace, but in the workspace of the method.
.
Then there is a second issue. With handle objects, there is no need to return the object variable in methods, e.g. replace
function s = addData(s, data)
by
function addData(s, data)
However, the constructor shall return the object variable. See Comparison of Handle and Value Classes
  4 Comments
per isakson
per isakson on 3 Mar 2018
"I "need" to return the object for consistency with the rest of the codebase" Fine, then you do it for a reason, not by mistake - which I suspected.

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