CODEWITHSUNDEEP
oopclassmatlab
Carel
Carel

Reputation: 3397

MATLAB : Instantiate a class from an empty Instance to a 'Blank' Instance

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

Upvotes: 3

Views: 857

Answers (2)

SCFrench
SCFrench

Reputation: 8374

MATLAB passes arrays of handle objects (including 1-by-1 "scalar" arrays) by value. A handle value is a reference to an object, which can be used to change the state of the object (i.e. its properties), but importantly, it is not the object itself. If you pass an array of handle values to a function or method, a copy of the array is actually passed to the function, and modifying the dimensions of the copy has no effect on the original. In fact, when you call

TheOther.Instantiate;

The instance of That that you assigned to Self(1,1) gets returned as the output of Instantiate and assigned to ans.

This link to a section of MATLAB Object-Oriented Design documentation might also help.

Upvotes: 2

Amro
Amro

Reputation: 124563

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

Upvotes: 0

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