Overloading subsref and calling the builtin from within causes the class to only return the first value when called on a nested vector of objects.

To explain this problem, I have made a minimal example. I created two classes, one with an overloaded subsref and one without.

classdef NonSubsrefExample

properties

Property

end

methods

function obj = NonSubsrefExample(prop)

obj.Property = prop;

end

end

end

classdef SubsrefExample

properties

Property

PropertyOverloaded

end

methods

function obj = SubsrefExample(prop)

obj.Property = prop;

obj.PropertyOverloaded = prop;

end

function varargout = subsref(obj, S)

if S(1).subs == "PropertyOverloaded"

varargout = {"Dot"};

return

end

[varargout{1:nargout}] = builtin('subsref', obj, S);

end

end

end

Now if we create a nested vector of these objects:

subsrefExample = SubsrefExample([SubsrefExample(1), SubsrefExample(2)]);

nonSubsrefExample = NonSubsrefExample([NonSubsrefExample(1), NonSubsrefExample(2)]);

We can now operate on these and see what happens

[subsrefExample.Property.Property] % Returns 1

[nonSubsrefExample.Property.Property] % Returns [1, 2] as expected

Answer 1

[James Lebak]

To get the behavior you want, your class needs to overload numArgumentsFromSubscript. One example implementation of this method would look like the following. The rest of your class would remain the same.

function n = numArgumentsFromSubscript(obj, S, ctx)
    n = builtin('numArgumentsFromSubscript', obj, S, ctx);
end

The point of numArgumentsFromSubscript is to tell MATLAB how many outputs the particular instance of your class returns when indexed in a certain way.

When I add this method to your class, the new class works the way you expect:

>> enhancedSubsrefExample = EnhancedSubsrefExample([EnhancedSubsrefExample(1), EnhancedSubsrefExample(2)]);
>> [enhancedSubsrefExample.Property.Property] % Returns [1, 2]

It's also worth nothing that in recent releases, you can use modular indexing mixins such as RedefinesParen and RedefinesDot in place of overloading subsref and subsasgn. If you are able to use modular indexing, it will generally result in better performance when your class overloads indexing.