unexpected value for nargout when returning a class/struct

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Markus Leuthold
Markus Leuthold 2014-6-10
编辑： Philip Borghesani 2014-6-11
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);
  2 个评论
Matt J
Matt J 2014-6-10
编辑：Matt J 2014-6-10
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
Markus Leuthold 2014-6-10
编辑：Markus Leuthold 2014-6-10
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?

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回答（1 个）

Philip Borghesani
Philip Borghesani 2014-6-11
编辑：Philip Borghesani 2014-6-11
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.

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