Why do I have to create empty object array before calling constructor in constructor?

If I call constructor in the workspace using MATLAB's cheap copies, I get an object array that is exactly what MATLAB's help documentation says. Ex. #1

>> obj(3) = myClass(1,2);a = [obj.a],b = [obj.b]

a =

     2     2     1

b =

     4     4     2

But if I put the same commands in the constructor, I get an object array that is nearly the same, except that the first element now only has the defaults from my properties block. Ex. #2

>> obj = myClass(1,2,3);a = [obj.a],b = [obj.b]

a =

   999     2     1

b =

   999     4     2

Unless I create an empty object array first. Ex. #3

>> obj = myClass(1,2,3);a = [obj.a],b = [obj.b]

a =

     2     2     1

b =

     4     4     2

The examples above use the following m-file:

classdef myClass < handle
  properties
      a = 999
      b = 999
  end
  methods
      function obj = myClass(a,b,c)
          if nargin<3
              c = 1;
          end
          if c>1
              % uncomment for Ex. #3
              % obj = myClass.empty(0, c);
              obj(c) = myClass(a,b);
          else
              if nargin<1
                  a = 2;
              end
              obj.a = a;
              if nargin<2
                  b = 4;
              end
              obj.b = b;
          end
      end
  end
end

Answer 1

[Mark Mikofski]

When the constructor is called, it instantiates the object with any defaults given in the properties block ([] for any properties without defaults), so in my example above, a scalar object was already instantiated.

If c>2 the call

obj(c) = myClass(a,b);

requires MATLAB to fill in

obj(2:c-1)

with cheap copies (created by calling the constructor without arguments). Calling the empty method replaces the object with an empty array, so the call

obj(c) = myClass(a,b);

now acts as expected.

Another option would be to delete the default object before expanding it.

obj.delete

BTW: MATLAB states that an object is destroyed if its variable is assigned to a new value or when the function ends, so you do not need to free memory. http://www.mathworks.com/help/techdoc/matlab_oop/brfylzt-1.html#bsxw4_c

[Mark Mikofski]

here's another example of this behavior with doubles:

>> clear
>> x= 1.23    # <1x1 double>
x =
  1.23
>> x(10) = 5  # grow x to <1x10 double>
x =
  1.23 0 0 0 0 0 0 0 0 5.00
>> y(10) = 5  # create <1x10 double>
y =
  0 0 0 0 0 0 0 0 0 5