Operator overloading did not work for a second operand if the first operand has no operator overloading

I create two classes, DataSet and Transformation. The class Transformation has the operator & overloaded.

classdef Transformation 
methods
  function obj = and(A,B)
     % do something
  end
end

classdef DataSet
methods
  %...
end

Now i can call

X = Dataset();
Y = Transformation();
Z = Y & X;

but not

Z = X & Y;  % ERROR: no and() operator for X

Matlab uses the function of the most left operand if both classes have the same precedence and both have a function with the same name. But in this example there is just one class with the and() function. Why can't Matlab use the and() function of the second operand if there is no and() function in the first operand?

(I tried to use the classdef (InferiorClasses = {?class1,...} attributes, but with this solution i have to add every DataSet class to every Transformation class because these attributes don't inherit. Too much work and it can lead to errors if someone forget same classes in this list.)

Answer 1

[Matt J]

Why MATLAB couldn't have been designed to look for the first class that defines a method, I can't say. However, I wonder if this has to be a huge inconvenience. Somewhere in your code, the operation

 Z=X&Y

is going to have to determine the class of X and figure out how to process it appropriately. You seem to want to do that work in and@Transformation, but why not do it in and@DataSet and then make a generic call to and() within that?

Or, assuming your AND operations are going to be commutative, you could just add a simple AND method to class DataSet which inverts the order of the operands

 function Z=and(X,Y)  %in classdef DataSet

    Z=and(Y,X); %This calls the AND method of class Y, whatever that is.

 end

[Walter Roberson]

Why MATLAB couldn't have been designed to look for the first class that defines a method, I can't say.

If I call

set(HandleGraphicsHandle, 'userdata', DataSetObject)

then I would be unlikely to want DataSet's set() method to be called.

[Matt J]

Not sure. HandleGraphicsHandle defines a set() method, so it would still take precedence. The OP's proposal was that the precedence rules simply ignore arguments that don't define a version of the method.

[Walter Roberson]

When I wrote HandleGraphicsHandle, I was referring to those double() values, not to handle() of them. There is no set() for double() or char()

[Matt J]

OK, but then I'm not understanding the example. In your example, the set method of DataSet will in fact be called if DataSet defines one. Try the following

>> clear classes; h=figure; set(h,'UserData',myclass)
Error using myclass/set
Too many input arguments.

where

classdef myclass

          methods

             function set(obj)

                  disp 'Wrong'

             end

          end
  end

[Walter Roberson]

When you mention a class name, you are invoking the constructor for it, no matter what methods it has. That is why I wrote DataSetObject, to indicate an object of DataSet class (e.g., the X of the original posting)

[Matt J]

Not sure why that's relevant. My example also applies when an object is passed.

>> x=myclass; h=figure; set(h,'UserData',x)
Error using myclass/set
Too many input arguments.

[Walter Roberson]

How odd. If you change the method to varargin will it proceed to the 'Wrong' ?

[Cedric]

This is really weird .. yes, it proceeds to the 'Wrong' ... (?!)

[Matt J]

Perhaps it's a little inconvenient, but the behavior does obey the documentation. I guess that means that if you want to store user-defined objects as user data, you have to do things like this

set(h,'UserData',{x}), %encapsulate in cell

[Matt J]

@Matt J: If I use a AND() method in my DataSet class which inverts the operands, this can be dangerous. A user of the class could accidentally write X & X and this would be an infinit loop.

If your intention was to allow users to do X&X, it is unclear why you don't have an AND method in DataSet already. You're going to need such a method if for no other reason than to define X&X.

If it is not your intention to enable X&X, but you're worried about users doing it as a typo, only a simple modification is required to prevent the infinite loop,

function Z=and(X,Y)  %in classdef DataSet

    if isa(X,'DataSet') & isa(Y,'DataSet'), error 'Improper operation'; end

    Z=and(Y,X); 

 end

It doesn't even matter that much if you do avoid the infinite loop. MATLAB will time out when its recursion limit is reached. One way or another, the user will get an error message.

If I add a AND() method to my DataSet class I have a stronger connection from DataSet to Transformation.

Not true. My proposal for and@DataSet makes no reference to Transformation at all!

[Walter Roberson]

Matt, could you point to the relevant documentation ?

[Matt J]

http://www.mathworks.com/help/matlab/matlab_oop/ordinary-methods.html#brd2n2o-1

Answer 2

[Michael]

Thx for the help.

@Matt J: If I use a AND() method in my DataSet class which inverts the operands, this can be dangerous. A user of the class could accidentally write X & X and this would be an infinit loop.

I try to avoid adding another AND() method to the DataSet class because I want to keep the Transformation logic as local as possible. If I add a AND() method to my DataSet class I have a stronger connection from DataSet to Transformation. If I just want to use the DataSet class in another project, it would bring the Transformation class with it. But i can't see a better solution at the moment.

Is there a reason for ignoring the second (right) operand if the first (left) operand has no convenient method?