Anyone know how to create a multidimensional subclass constructor?

Hello!

My problem is, that I'm trying to write a subclass to my multidimensional superclass, but i run into errors trying to call the superclasses constructor in the subclass constructor.

The superclasses constructor looks somewhat like this:

function obj = A( arg1, arg2, arg3 )
  if nargin == 0
    return;
  end
  n = numel( arg1 );
  obj( n , 1 ) = A();
  for i=1:n
    % some code
  end
end

Now I would like to build a subclass to this one, with some additional parameters, but I have no idea about the syntax for the subclass. The syntax written in the Matlab Documentation doesn't help at all for this kinda multidimensional cases.

Thanks in advance!

Answer 1

[Guillaume]

That's a very interesting question and I think it shows another problem with matlab class design.

The problem you'll encounter when constructing a derived object (let's call it B) is that in the superclass constructor, obj is still of class B and thus the line obj(n, 1) = A is going to cause an error, since A can't be converted to B. In A constructor obj should never be of type B.

There is an ugly workaround, use eval to construct the array of the required type. You'll end up with:

classdef A
   properties 
      a;
   end
   methods
      function this = A(arg1, arg2, arg3)
         if nargin == 0   %to support array construction
            return;
         else
            this(numel(arg1), 1) = eval(class(this)); %use eval to invoke the correct constructor
            arg1 = num2cell(arg1); [this.a] = arg1{:};  %distribute arg1 into each object
         end
      end
   end
end
classdef B < A
   properties
      b;
   end
   methods
      function this = B(Barg, arg1, arg2, arg3)
         if nargin == 0  %can't call base constructor inside if, so build base constructor arguments 
            args = {};
         else
           args = {arg1, arg2, arg3};
         end
         this@A(args{:}); %and call base constructor
         if nargin > 0
            Barg = num2cell(Barg);
            [this.b] = Barg{:};
         end
      end
   end
end

I'm going to raise that as an issue with mathworks as this shows that there is no decent way of initialising an object array. Assigning to the last element of array is fragile and breaks under your circumstances. I really hate that constructing an object and an array of object is done in the same constructor.

[András]

Thank you for the fast reply!

I suspected that the line obj(n, 1) = A causes the error, but hoped that there is a proper way of dealing with it.

[Andrew Stamps]

This may only be my personal preference in avoiding eval wherever possible, but it could be replaced by str2fun as follows.

...
    con = str2func(class(this));
    this(numel(arg1), 1) = con(); %use str2func to invoke the correct constructor
...