I recently consulted to a very large industrial client. Following a merger/acquisition, the relevant R&D department had two development groups using different technologies: one group uses Matlab, the other does not. My client wanted both groups to use Matlab, something that would naturally please MathWorks.
Unfortunately, it turns out that a technical challenge was preventing this move: the other technology enabled data field names (identifiers) longer than Matlab’s namelengthmax=63 characters, and these names also sometimes contained illegal identifier characters, such as spaces or symbols. This prevented an easy code transition, indefinitely delaying the migration to Matlab.

Accessing improper fieldnames

I suggested to this client to use Matlab class objects that overloaded the subsref ^[1]() and subsasgn ^[2]() methods: the long original identifiers would be stored in some internal container (cell array or containers.Map etc.), and they would be accessed not directly (which would be impossible since they are not valid Matlab identifiers) but via the overloaded methods. Something along the following lines:

classdef ExtendedFieldsClass
    % The internal data implementation is not publicly exposed
    properties (Access = 'protected')
        props = containers.Map;
    end
    methods (Access = 'public', Hidden=true)
        % Overload property assignment
        function obj = subsasgn(obj, subStruct, value)
            if strcmp(subStruct.type,'.')
                try
                    obj.props(subStruct.subs) = value;
                catch
                    error('Could not assign "%s" property value', subStruct.subs);
                end
            else  % '()' or '{}'
                error('not supported');
            end
        end
        % Overload property retrieval (referencing)
        function value = subsref(obj, subStruct)
            if strcmp(subStruct.type,'.')
                try
                    value = obj.props(subStruct.subs);
                catch
                    error('"%s" is not defined as a property', subStruct.subs);
                end
            else  % '()' or '{}'
                error('not supported');
            end
        end
    end
end

This works splendidly, as the following snippet shows:

>> c = ExtendedFieldsClass
c =
  ExtendedFieldsClass with no properties.
 
>> c.(' asd f @#$^$%&') = -13.5;  % no error
 
>> c.(' asd f @#$^$%&')
ans =
                     -13.5
 
>> c.(' asd f @#$^$%& xyz')  % note the extra "xyz"
Error using ExtendedFieldsClass/subsref (line 27)
" asd f @#$^$%& xyz" is not defined as a property

Note how we need to use the () parentheses in order to access the “properties” as dynamic fieldnames. We would naturally get an error if we tried to directly access the field:

>> c. asd f @#$^$%&
 c. asd f @#$^$%&
        |
Error: Unexpected MATLAB expression.

Tab completion

So far so good.
The problem is that we would also like to see the defined “properties” when in the desktop’s tab completion. i.e., when I type “c.” and then click <tab> in the Matlab command prompt, I’d like to see the list of defined “properties” in a tooltip (in the example above: ” asd f @#$^$%&”). Instead, I get the message “No Completions Found.”:

classdef ExtendedFieldsClass
    % The internal data implementation is not publicly exposed
    properties (Access = 'protected')
        props = containers.Map;
    end
    methods (Access = 'public', Hidden=true)
        % Overload property assignment
        function obj = subsasgn(obj, subStruct, value)
            ...  (as above)
        end
        % Overload property retrieval (referencing)
        function value = subsref(obj, subStruct)
            ...  (as above)
        end
        % Overload fieldnames retrieval
        function names = fieldnames(obj)
            names = sort(obj.props.keys);  % return in sorted order
        end
    end
end

When we now run this in the command prompt, we get the expected behavior:

R2014a

Unfortunately, this works only up to and including Matlab release R2013b. In R2014a, MathWorks made some internal change that prevents overloading the fieldnames function. To be more precise, we can still overload it as above, and it will indeed work if we directly call fieldnames(c), but it no longer has any effect on the tab completion. On R2014a, the tab-completion remains broken and returns “No Completions Found.” When this was reported to MathWorks some time ago, the official response was that the previous behavior was considered a “bug”, and this was “fixed” in R2014a (don’t bother searching for it in the official bugs parade). Go figure…
So what do you think I should now do? Remember: this is a large client, who knows how many licenses are at stake. Should I suggest to my client not to switch to Matlab? Or should I suggest that they keep using R2013b across the entire organization and cancel their annual maintenance? Or maybe I should simply tell them to accept the fact that some important functionality should be expected to get broken whenever Matlab is upgraded?
These sort of things just blow me away. Sometimes I feel as if I am swimming against the current, and that’s frustrating. I admit it doesn’t happen very often. Then again, I guess if things were straight-forward, nobody would need me to consult them…
Don’t mind me – just blowing off some steam. I’m allowed to, every now and then, aren’t I? 🙂
Addendum July 21, 2014: I found out today that on R2014a+ we can simply overload the properties method. This is a function that returns the properties of a class, and so it makes perfect sense for a class object’s tab-completion to use properties rather than fieldnames. So I can now indeed see why the past behavior was considered by MathWorks to be a bug that should be fixed. Still, it would have been nice if for backward-compatibility considerations, Matlab (or at least mlint) would have detected the fact that fieldnames is being overloaded in a user class and warned/alerted regarding the fact that we should now overload properties. In any case, to be fully backward compatible, simply overload both methods, and make one of them call the other. For example:

        % Overload property names retrieval
        function names = properties(obj)
            names = fieldnames(obj);
        end

My client would be quite happy to hear of this new development 🙂

Related odds and ends

Michal Kutil described a mechanism for overloading the methods function, which is also part of the tab-completion tooltip. The problem here is that we cannot simply overload methods in our class, since Matlab calls methods with the class name (not the class object reference) when it wants to determine the relevant methods to display in the tooltip. Michal’s solution ^[4] was to create a wrapper function that calls the overloaded variant. This wrapper function can then be placed within a @char folder somewhere in the Matlab path. I used a similar trick for my profile_history utility ^[5] last month.
Related newsgroup posts by Eric Salemi here ^[6] and here ^[7].
Similarly, in order to overload the data-value tooltip (when hovering over the object in the editor), or when displaying the object in the Matlab command prompt, simply overload the disp() function (see related ^[8]):

        % Overload class object display
        function disp(obj)
            disp([obj.props.keys', obj.props.values']);  % display as a cell-array
        end

In a related matter, we can limit the values that a property can accept using the matlab.system.StringSet ^[9] class of the matlab.System ^[10] package, as recently discovered ^[11] by Oleg Komarov (additional details ^[12]; a different way to constrict property data type ^[13]):

classdef foo < matlab.System
    properties
        Coordinates
    end
    properties(Hidden,Transient)
        CoordinatesSet = matlab.system.StringSet({'north','south','east','west'});
    end
end

15 Comments To "Class object tab completion & improper field names"

#1 Comment By seb On July 18, 2014 @ 00:48

#2 Comment By Yair Altman On July 18, 2014 @ 01:08

#3 Comment By David On July 18, 2014 @ 12:42

#4 Comment By TheBlackCat On July 20, 2014 @ 09:15

#5 Comment By Yair Altman On July 20, 2014 @ 09:49

#6 Comment By TheBlackCat On July 21, 2014 @ 04:04

#7 Comment By TheBlackCat On July 21, 2014 @ 05:42

#8 Comment By Yair Altman On July 21, 2014 @ 21:25

#9 Comment By Amro On July 20, 2014 @ 09:44

% create struct
s = struct();
s = my_setfield(s, 'a a', 1);
s = my_setfield(s, 'b.b', 2);
s = my_setfield(s, 'c-c', 3);
s = my_setfield(s, '1dd', 4);
s = my_setfield(s, repmat('e',1,70), 5);
s = my_setfield(s, ' asd f @#$^$%&', 6);

% retrieve fields using "dynamic field names" syntax
disp(s)
a = s.('a a')
b = s.('b.b')
c = s.('c-c')
d = s.('1dd')
%e = s.(repmat('e',1,70))   % this one didn't work 🙂
f = s.(' asd f @#$^$%&')

#10 Comment By Daniel On July 22, 2014 @ 03:31

#11 Pingback By Setting class property types | Undocumented Matlab On August 12, 2014 @ 10:21

#12 Comment By Lorin On November 12, 2014 @ 13:31

#13 Comment By Nick Gaul On February 9, 2015 @ 09:54

obj = myclass(x);
obj. % this shows the property names that my overloaded properties method returns
i = [1,3];
obj(i). % this shows a popup message saying no completions found. Do you know if that is considered correct/normal Matlab behavior?

#14 Comment By Jim Hokanson On March 20, 2016 @ 20:20

#15 Comment By Sky On January 18, 2020 @ 17:43

classdef foo < matlab.mixin.SetGet
    properties
        Coordinates {mustBeMember(Coordinates,{'north','south','east','west'})} = 'north'
    end
end