unexpected value for nargout when returning a class/struct

What values for nargout would you expect for the last two lines in the run block: 0 or 1?

Class A

classdef A < handle
  methods
      function r=fctA(~)
          disp(['nargout=' num2str(nargout)])
          r=B;
      end
  end    
end

Class B

 classdef B < handle
    methods
        function fctB(~,~)
        end
    end    
 end

run

a=A;
a.fctA.fctB;        
a.fctA.fctB(1);

[Matt J]

I think the bigger surprise is that you can directly dot-index the output of a function call. Since when has that been possible????

[Markus Leuthold]

it only works with methods from new-style classes (classdef). Following code doesn't work, as you've expected:

function r=fcn
  r.value=1;
end

this won't work

fcn.value

Even worse, this code crashes Matlab:

function matlabcrash
  mytest.value
  function a=mytest
        a.value=1;
  end
end

So I'm asking myself if dot-index the output of a method is illegal as well and only works by coincidence?

Answer 1

[Philip Borghesani]

In general you cannot index into the output of a function call in MATLAB.

You have found an under documented feature of dot indexing on objects and a bug in that feature. The ability to index into the output of a dot function call on MATLAB class objects leaked into a previous version. This ability has always existed for Java objects. Because this feature just worked in many places it has been frequently exploited in MATLAB code and removing it would be quite difficult making supporting it necessary. In general we believe that the following code is more conformant MATLAB code for calling a method and will have similar performance:

a=A;
Bobj=fctA(a);
fctB(Bobj);

Nargout in your example code should be 1 and this is fixed in a future version of MATLAB. Note that a.fctA().fctB also returns 1.

I have entered a bug report for the crash with a nested function but I would not expect that code to be legal MATLAB code in the foreseeable future.

Information stated here is not an official guarantee of support or statement of policy from MathWorks.