Executing the constructor of a handle object with or without semicolon

Hi,

I'm seeing something that appears to me as very strange. I have handle class (called 'cube') which when I call the constructor like this:

a = cube(x,y,z,color,alpha);

Everything works as expected. However when I call the constructor methods without the semicolon, i.e.:

a = cube(x,y,z,color,alpha)

I find that the execution seems to continue pass the end of the constructor and run other functions in the methods section of my class. This is the class code...

   classdef cube < handle
      properties (GetAccess='public', SetAccess='private')
          xIndex; % location of the cube in 3D space
          yIndex;
          zIndex;
      end
      properties (GetAccess='private', SetAccess='private')
          graphicsHandle;
          color;
          alpha;
      end
      properties (Constant, GetAccess='private')
          vertOffsets = [0 0 0; ... % cube vertices offsets configuration
                         1 0 0; ...
                         1 1 0; ...
                         0 1 0; ...
                         0 0 1; ...
                         1 0 1; ...
                         1 1 1; ...
                         0 1 1];
          faces =       [1 2 6 5; ... % cube faces configuration
                         2 3 7 6; ...
                         3 4 8 7; ...
                         1 4 8 5; ...
                         5 6 7 8; ...
                         1 2 3 4];
      end
      methods
          function obj = cube(x, y, z, color, alpha) % constructor
              obj.xIndex = x;
              obj.yIndex = y;
              obj.zIndex = z;
              obj.graphicsHandle = [];
              obj.color = color;
              obj.alpha = alpha;
          end;

          function delete(obj) % destructor
              if isempty(obj.graphicsHandle) == 0
                  delete(obj.graphicsHandle);
              end
          end;

          function display(obj)
              disp('displazing!');
              dvert = obj.vertOffsets;
              dvert(:,1) = dvert(:,1) + obj.xIndex; % translate vertices for drawing
              dvert(:,2) = dvert(:,2) + obj.yIndex;
              dvert(:,3) = dvert(:,3) + obj.zIndex;

              % move the cube to the new location, creating the object if required
              if isempty(obj.graphicsHandle)
                  obj.graphicsHandle = patch('Faces',obj.faces,'Vertices',dvert, ...
                      'FaceColor', obj.color, 'FaceAlpha', obj.alpha); 
              else
                  set(obj.graphicsHandle, 'Faces',obj.faces,'Vertices',dvert);
              end;
          end;
          function move(obj, x, y, z)
              obj.xIndex = x;
              obj.yIndex = y;
              obj.zIndex = z;
              display(obj);
          end;
      end
  end

Answer 1

[Philip Borghesani]

Your class has a display method which is called to display the variable "a" when the assignment is done without a semicolon.

If you enter a=5 without the semicolon the same is done for a with the value 5.

[Michael Allwright]

You were exactly right. Good spot, I had no idea I was overloading a special method. perhaps in the MATLAB editor there should be some sort of syntax highlighting when you override a method, as this can lead to a lot of confusion!

Answer 2

[Michael Allwright]

For future reference, this is question is related to using 'display' as method name. This use of variable name caused the inbuilt display method to be overridden, and when not using a semicolon, would cause this method to execute as MATLAB echo'd or displayed the result of the assignment to the console.

http://www.mathworks.de/help/techdoc/ref/display.html