Matrix/Array of Objects vs. Class Constructor

Let say I have a class Foo

classdef Foo
   function obj = Foo(x,y)
      % constructor's body
   end
end

If I want to preallocate spaces for a matrix of Foo objects

>> foos = Foo(3,4);

How is it different from creating one object with arguments x = 3, y = 4?

>> foo = Foo(3,4);

Note that if I avoid preallocation, Matlab will give me some warning due to speed optimization.

Thanks,

Answer 1

[Khanh]

Thanks for your response, Sean. However, it turns out every (nonabstract) class has a static method called empty, which can be used to create empty array. It makes things much simpler. See details here .

Answer 2

[Sean de Wolski]

It depends on how your class sizes obj:

classdef FooC

      properties
          x
      end
      methods
          function obj = FooC(sz)
              obj.x = 1;
              obj = repmat(obj,sz);
          end
      end

end

Now

fc = FooC([3,4])

I don't particularly like this approach. I would typically recommend constructing it similar to how you would construct a distributed array or gpuArray using static methods. Consider this:

classdef FooC    
    properties
        x
    end
    methods
        function obj = FooC()
            obj.x = 1;
        end
    end
    methods(Static)
        function obj = preallocate(sz)
            obj = FooC();
            obj = repmat(obj,sz);            
        end        
    end    
end

Now FooC always returns a scalar object but FooC.preallocate will give you an sz object, compare:

fcscalar = FooC
fcmat = FooC.preallocate([3,4])

[Khanh]

What if the constructor of FooC takes some argument? Say,

function obj = FooC(x,y)
   obj.x = x;
   obj.y = y;
end

Then the preallocate has to be written as following

      methods(Static)
          function obj = preallocate(sz)
              obj = FooC(dummy1,dummy2);
              obj = repmat(obj,sz);            
          end        
      end

Is there anyway better than that?

Thanks,

[Sean de Wolski]

This is where varargin is your friend:

function preallocate(sz,varargin)
  obj = FooC(varargin{:})
  obj = repmat(obj,sz);
end