Overloading subsasgn without breaking class privacy rules

I'm writing a simple class to handle three-vectors - just an array of three numbers. The actual array is stored in the 'Data' property, which is (Access = 'private'). I want to access the values using parenthesis indexing only, so I'm overloading subsasgn. I'm also trying to use the xUnit Test Framework, so I'm trying to keep fairly careful track of the error messages. Here's the subsasgn function I've written for the Vector3 class:

function V = subsasgn(V,s,b)
    switch s(1).type
        case '()'
            V = Vector3(builtin('subsasgn',V.Data,s,b));
        case '{}'
            error('Vector3:subsasgnCell','cell assignment not supported')
        case '.'
            switch s(1).subs
                case 'Data'
                    error('Vector3:setProhibited','... not allowed')
                otherwise
                    error('Vector3:subsasgnDot','dot assign not allowed')
            end
    end
end

Here's where it gets weird: If I define a Vector3 variable at the Command Line, and I then try to assign to the 'Data' property from the Command Line, I get the expected error message:

>> V = Vector3([1 -45 7.9])
     1  -45  7.9
>> V.Data = [2 4 9]
??? Error using ==> ...Vector3.subsasgn...
... not allowed

But if I define a function and try to do the same thing, like...

function out = weirdo(V)
    V.Data = [2 4 9];
    out = V;
end

and call that function from the command line:

>> V = Vector3([1 -45 7.9])
     1  -45  7.9
>> W = weirdo(V)
     2   4   9
>> class(W)
ans = 
Vector3

I've tested it a couple different ways, and basically it seems that when I try setting the private property from the Command Line it fails as designed, but when I set it within a function it succeeds when it shouldn't.

What am I doing wrong?

Answer 1

[Daniel Shub]

My guess is that you are saving weirdo in a location such that it is becoming a method of the class. For good or for bad, methods of a class call the builtin subasgn and subsref functions and not the overloaded functions.

[Matthew M.]

I thought I had gotten around that by putting my unit-testing functions in a subdirectory of the class folder that was not on the MATLAB path, but I guess you can't do that. When I pulled them out of there they work as expected. Rookie mistake. Thanks for the tip.