MATLAB: Can’t I use builtin for classes that overload subsref

builtinclassMATLABobscure behavioroopoverloadsubsref

It seems like Matlab's 'builtin' function doesn't work when subsref is overloaded in a class. Consider this class:
    classdef TestBuiltIn
        properties
            testprop = 'This is the built in method';
        end
        methods
            function v = subsref(this, s)
                disp('This is the overloaded method');
            end
        end
    end
To use the overloaded subsref method, I do this:
    t = TestBuiltIn;
    t.testprop
        >> This is the overloaded method
That's as expected. But now I want to call Matlab's built in subsref method. To make sure I'm doing things right, first I try out a similar call on a struct:
    x.testprop = 'Accessed correctly';
    s.type = '.';
    s.subs = 'testprop';
    builtin('subsref', x, s)
        >> Accessed correctly
That's as expected as well. But, when I try the same method on TestBuiltIn:
    builtin('subsref', t, s)
        >> This is the overloaded method
…Matlab calls the overloaded method rather than the built in method. Why does Matlab call the overloaded method when I requested that it call the builtin method?

Best Answer

I'm guessing a bit, but in MATLAB there are certain precedence rules for choosing which versions of functions can apply to a given list of input arguments. I think BUILTIN can only choose between functions that are of equal precedence (other than w.r.t. position on the path, of course).
In your case, you are calling builtin('subsref',...) with an object, t, of a user-defined class testBuiltIn, as an input argument. Under the precedence rules, user-defined class methods dominate everything but subfunctions, so the only versions of subsref that are of equal precedence are those belonging to the TestBuiltIn class. MATLAB's builtin subsref is not a method of TestBuiltIn, so it is ignored by builtin('subsref',t,s).
It might be worth talking to Tech Support to see if this is intended behavior.
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