problem explicitly calling subsasgn with buitlin method

Hi

I have a problem when I try to overload the subsasgn method in a class. This is simple example class to show the problem:

classdef TestClass < handle

      properties
          FigHandle = figure(1);
      end

      methods

          function obj = TestClass()
          end

          function val = getFigure( obj )
              val = obj.FigHandle;
          end

          function obj = subsasgn( obj, s, b )
              % redirect to builtin
              obj = builtin( 'subsasgn', obj, s, b );
          end

      end%methods

end%classdef

Now I am used to doing stuff like this:

t = TestClass()
t.getFigure().Units = 'normalized'

This will give the following error code:

Error using TestClass/getFigure
Too many input arguments.

    Error in TestClass/subsasgn (line 21)
                obj = builtin( 'subsasgn', obj, s, b );

    Error in myTestScript (line 6)
    t.getFigure().Units = 'normalized'

When I do not overload the subsasgn method, this call is going through.

What am I doing wrong?

Update: This is working properly: (without the empty braces on the method call)

t.getFigure.Units = 'normalized'

Answer 1

[Guillaume]

This is interesting. There's definitively something not working properly there. Experimenting with your test case, I changed the getFigure method to:

          function val = getFigure( obj, varargin )
              val = obj.FigHandle;
          end

Since that's the function throwing the error Too many input arguments. With that simple change, running your test crashes matlab (R2017a and R2016b, WIN64). This is worthy of a bug report to mathworks.

Of course, that doesn't help with your situation, but it looks like you've found a bug in matlab.

[Andreas Klotzek]

Thank you.

I also have a crash on R2015b with this change. I will report this directly to mathworks.

Answer 2

[Philip Borghesani]

You bumped into an area of loosely defined behavior.

In general you can't index into the results of a function call in MATLAB. There are a few places where it is allowed to work for compatibility with Java and Java algorithms and to avoid breaking too much old code.

The correct way to write this code in MATLAB is

fig=t.getFigure();  %call function getFigure;
fig.Units='normalized';

Or to directly access the FigHandle property which is fine: t.FigHandle.Units = 'normalized'. You could even add a get access method for the property or create a transient property with a get method that returns FigHandle.

I cant promise that when the crash is fixed the code you originally wrote won't error in a reasonable way instead of crashing.

[Andreas Klotzek]

Thanks for your answer.

I already had the impression that I might be doing something that matlab does only support by chance. It could still be some sort of bug because it is working when I am not overloading the method. I will see what mathworks will tell me when I report it.

I guess I will have to adapt to the style you pointed out in the future to be save.

[James Lebak]

The crash is definitely a bug in MATLAB and we will fix it in a future release. I am sorry for the inconvenience.

As Phil said indexing into the results of a method is generally not supported. However, custom subsasgn can give you something like the behavior you want (and also work around the bug) by recognizing the call to 'getFigure' and forwarding the subsequent indexing to the figure. For example:

          function obj = subsasgn( obj, s, b )
            if (numel(s) == 1 && isCallToGetFigure(s(1))) || ...
                    (numel(s) == 2 && isCallToGetFigure(s(1)) && ...
                    isEmptyParens(s(2)))
                % obj.getFigure = b or obj.getFigure() = b;
                obj.FigHandle = b;
            elseif numel(s) > 2 && isCallToGetFigure(s(1)) && ...
                    isEmptyParens(s(2))
                % obj.getFigure() with subsequent indexing
                obj.FigHandle = builtin( 'subsasgn', obj.getFigure, ...
                    s(3:end), b );
            elseif numel(s) > 1 && isCallToGetFigure(s(1))
                % obj.getFigure with subsequent indexing
                obj.FigHandle = builtin( 'subsasgn', obj.getFigure, ...
                    s(2:end), b );
            else
                error('invalid subsasgn syntax.');
            end

The definitions of isCallToGetFigure and isEmptyParens would look like

function b = isCallToGetFigure(s)
b= strcmp(s.type,'.') && strncmp(s.subs,'getFigure',8);
end

function b = isEmptyParens(s)
b = strcmp(s.type,'()') && isempty(s.subs);
end

[James Lebak]

I wanted to include a link to the bug report for completeness.