overloaded subsref and display vs. command line

Hi,

when I overload subsref in a class, I can explicitly call the builtin subsref to get to the methods and properties of the class. Since I don't know the number of output arguments beforehand I use varargout, for instancce in this simple example

classdef myClass
    properties
        name = 'test'
    end
    methods 
        function varargout = subsref(obj, s)
                varargout = cell(1,nargout);
                [varargout{:}] = builtin('subsref',obj,s);
        end
    end
end

This works fine with assignments, eg. calling

c = MyClass; 
a = c.name;

or when calling a method with no output arguments.

What doesn't seem to work as expected is calling a property in an unterminated statement on the commandline:

>> c.name
??? The left hand side is initialized and has an empty range of indices.
However, the right hand side returned one or more results.

Error in ==> myClass>myClass.subsref at 8
                [varargout{:}] = builtin('subsref',obj,s);

Hoever, calling display explicitly causes no error:

>> display(c.name)
test

Explicitly checking the number of output arguments, ie.

function varargout = subsref(obj, s)
   if nargout > 0
      varargout = cell(1,nargout);
      [varargout{:}] = builtin('subsref',obj,s);
   else
       builtin('subsref',obj,s);
   end
end

instead produces no output on the commandline call,

>> c.name
>>

whereas display(c.name) works as before.

I know this is perhaps a minor annoyance but I'd like to understand it anyway, if anyone has an explanation.

cheers, Pieter

Answer 1

[Jacob Halbrooks]

When you call "disp(c.name)", you are explicitly requesting an output argument from c.name and as a result NARGOUT returns 1 in your SUBSREF method. This is equivalent to doing this:

myName = c.name;
disp(myName);

However, when you execute "c.name", there is no output argument requested, and as a result NARGOUT is 0 in your method, causing the error. The display from leaving off the semi-colon is actually on the ANS variable. This case is then equivalent to doing this:

c.name;
disp(ans);

I think your solution might be to always return a minimum of 1 argument to ensure that ANS gets populated:

function varargout = subsref(obj, s)
   varargout = cell(1,max(1,nargout));
   [varargout{:}] = builtin('subsref',obj,s);
end

[Pieter van den Berg]

Jacob, thanks for your answer.

This would indeed solve the problem of displaying properties but then calling a method that returns zero arguments on the command line would now give a 'Too many output arguments' error.

[Jacob Halbrooks]

You are right. It seems the root problem is that NARGOUT reports 0 for both the case where ANS can be assigned and for the method call (which as you point out throws an error if you try to assign an output). I cannot think of a test to differentiate the two cases. I suppose a TRY/CATCH might be helpful if it's important to handle both cases.