How to define a slicing like x(i).property in a custom class?

One minor but useful thing about image feature classes in MATLAB is that they have varieties of ways to slice the feature points and get a subset. For example, let's say we extract SIFT features from an image:

I = imread('cameraman.tif'); 
points = detectSIFTFeatures(I);
fprintf('Number of detected points: %d\n', points.Count); 

We have the following ways to slice them, which give us the same sets of results:

% Subset of indices
subs = 1:10; 

% Method 1 
a = points(subs); 
aa = a.Location; 

% Method 2
b = points.Location; 
bb = b(subs,:);  

% Method 3
c = points.Location(subs,:); 

% Method 4
d = points(subs).Location; 

I'm wondering how to define a custom class to achieve all these slicing methods. I tried to use subsref function to define a custom class like this:

% File name: MyCustomPoints.m
classdef MyCustomPoints
    properties
        Scale % (n x 1)
        Location % (n x 2)
    end
    
    methods (Access='public')
        % Constructor
        function this = MyCustomPoints(scale, location)
            this.Scale = scale; 
            this.Location = location; 
        end
        % Slicing 
        function varargout = subsref(this, s)
            switch s(1).type
                case '()'
                    subs = s(1).subs{1}; 
                    varargout{1} = MyCustomPoints(this.Scale(subs,:), ...
                                                this.Location(subs,:)); 
                case '.'
                    % Handle property access
                    if isscalar(s)
                        varargout{1} = this.(s(1).subs); 
                    else
                        [varargout{1:nargout}] = builtin('subsref', this, s); 
                    end
            end
        end
    end
end

Below is an example usage of this class:

% Example input
Scale = rand(100,1); 
Location = rand(100,2); 
myPoints = MyCustomPoints(Scale, Location); 
subs = 1:3; % subset indices

% Method 1 
a = myPoints(subs); 
aa = a.Location; % --> Successful. 

% Method 2
b = myPoints.Location; 
bb = b(subs,:);  % --> Successful.

% Method 3
c = myPoints.Location(subs,:); % --> Successful. 

% Method 4
d = myPoints(subs).Location; % --> Failed. 

Only the last slicing method gives me an error message:

Output argument "varargout{2}" (and possibly others) not assigned a value 
in the execution with "MyCustomPoints/subsref" function.

Could you please advise how to modify my class to enable all the slicing methods above?

Answer 1

[Matt J]

        function varargout = subsref(this, s)
        
            switch s(1).type
                case '()'
                    subs = s(1).subs{1}; 
                    this = MyCustomPoints(this.Scale(subs,:), ...
                                          this.Location(subs,:)); 
                case '.'
                    % Handle property access
                    this=this.(s(1).subs);
            end

            s(1)=[];

            if ~isempty(s)
                [varargout{1:nargout}] = builtin('subsref', this, s);
            else
                varargout={this};
            end

        end

[Daigo]

@Matt J Thank you for your answer! Your code works perftctly for getting a single entry (e.g., myPoints(100).Location), but it causes an error when trying to access multiple entries (e.g., myPoints(1:100).Location). Would it be possible to modify the code to enable the slicing?

[Matt J]

but it causes an error when trying to access multiple entries (e.g., myPoints(1:100).Location)

Not on my computer, it doesn't. And not when we run it here in the online environment either , see below:

% Example input
Scale = rand(100,1); 
Location = rand(100,2); 
myPoints = MyCustomPoints(Scale, Location); 
subs = 1:100; % subset indices

% Method 1 
a = myPoints(subs); 
aa = a.Location % --> Successful. 

aa = 100x2

0.2526 0.8973 0.9851 0.0965 0.7341 0.8743 0.8739 0.1660 0.2481 0.2168 0.9970 0.5341 0.8526 0.3431 0.8110 0.6583 0.0565 0.9863 0.8510 0.2150

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% Method 2
b = myPoints.Location; 
bb = b(subs,:)  % --> Successful.

bb = 100x2

0.2526 0.8973 0.9851 0.0965 0.7341 0.8743 0.8739 0.1660 0.2481 0.2168 0.9970 0.5341 0.8526 0.3431 0.8110 0.6583 0.0565 0.9863 0.8510 0.2150

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% Method 3
c = myPoints.Location(subs,:) % --> Successful. 

c = 100x2

0.2526 0.8973 0.9851 0.0965 0.7341 0.8743 0.8739 0.1660 0.2481 0.2168 0.9970 0.5341 0.8526 0.3431 0.8110 0.6583 0.0565 0.9863 0.8510 0.2150

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% Method 4
d = myPoints(subs).Location % --> Failed. 

d = 100x2

0.2526 0.8973 0.9851 0.0965 0.7341 0.8743 0.8739 0.1660 0.2481 0.2168 0.9970 0.5341 0.8526 0.3431 0.8110 0.6583 0.0565 0.9863 0.8510 0.2150

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[Matt J]

Perhaps the real problem you are experiencing is when you try to index without assigning the result to something. This can be addressed by overloading numArgumentsFromSubscript, as in the attached version

% Example input
Scale = rand(100,1); 
Location = rand(100,2); 
myPoints = MyCustomPoints(Scale, Location); 
subs = 1:3; % subset indices

% Method 4
d=myPoints(subs).Location % --> 

d = 3x2

0.3032 0.3080 0.1679 0.6912 0.8951 0.1266

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% Additional Checks: No output arguments
myPoints(subs) % --> 

ans = 
  MyCustomPoints with properties:

       Scale: [3x1 double]
    Location: [3x2 double]

myPoints(subs).Location % -->

ans = 3x2

0.3032 0.3080 0.1679 0.6912 0.8951 0.1266

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myPoints(subs).Location(1)

ans = 0.3032

[Daigo]

@Matt J I haven't heard of numArgumentsFromSubscript before. Your code works like a charm. Thank you so much for your help!