Question

Why does my Instantiate function not create a ‘Blank’ instance of That?

I have the following minimal class :

classdef That < handle
 properties
  This = ''
 end
 methods
  function Self = That(String)
   if exist('String','var') == 1
    Self.This = String;
   end
  end
  function Self = Instantiate(Self)
   if isempty(Self)
    Self(1,1) = That;
   end
  end
 end
end

If I run

This = That;
disp(size(This))     % 1x1
disp(isempty(This))  % False

and it’s all good, I have a ‘Blank’ instance of the class

If I run

TheOther = That.empty;
disp(size(TheOther))     % 0x0
disp(isempty(TheOther))  % True
TheOther.Instantiate;  
disp(size(TheOther))     % 0x0   - Expecting 1x1
disp(isempty(TheOther))  % True  - Expecting False

As you can see running my Instantiate doesn’t work and I can’t see why ? Surely it should replace the empty instance with a non empty, yet blank, one ?

UPDATE :

The Link from SCFrench lead to this http://www.mathworks.com/help/techdoc/matlab_oop/brd4btr.html under the heading Creating Empty Arrays, though this didn’t work either :

function Self = Instantiate(Self)
 if isempty(Self)
  Blank = That;
  Props = properties(Blank)
  for idp = 1:length(Props)
   Self(1,1).(Props{idp}) = Blank.(Props{idp});
  end
 end
end

Answer 1

Maybe you should make it a static function:

methods (Static)
    function Self = Instantiate(Self)
        if isempty(Self)
            Self(1,1) = That;
        end
    end
end

Then:

>> TheOther = That.empty;
>> size(TheOther)
ans =
     0     0
>> isempty(TheOther)
ans =
     1
>> TheOther = That.Instantiate(TheOther);
>> size(TheOther)
ans =
     1     1
>> isempty(TheOther)
ans =
     0