Function handle to a method in a constant property

Question

In R2018b, I have the following setup:

classdef SomeClass < handle    
    methods
        function SomeMethod(obj)
            disp in!
        end
    end    
end

classdef SomeOtherClass < handle

    properties (Constant)
        instance = SomeClass()
    end

    methods
        function Test(obj)
            hdl = @obj.instance.SomeMethod;
            hdl();            
        end
    end

end

However, running the Test() method gives an error:

>> SomeOtherClass().Test() 
Undefined function or variable 'obj.instance.SomeMethod'.

Changing the Test() method to:

function Test(obj)
    A   = obj.instance;
    hdl = @A.SomeMethod;
    hdl();
end

gives the desired result:

>> SomeOtherClass().Test
in!

I'm puzzled...why do I need the middle man A?

Answer 1

Following @gnovice's finding:

>> obj = struct('instance', SomeClass());
>> hdl = @obj.instance.SomeMethod
hdl =
  function_handle with value:
    @obj.instance.SomeMethod

>> hdl(obj.instance)
Undefined function or variable 'obj.instance.SomeMethod'.

>> hdl()
Undefined function or variable 'obj.instance.SomeMethod'.

But:

>> instance=SomeClass();
>> hdl = @instance.SomeMethod
hdl =
  function_handle with value:
    @(varargin)instance.SomeMethod(varargin{:})

>> hdl(instance)
Error using SomeClass/SomeMethod
Too many input arguments.

Error in @(varargin)instance.SomeMethod(varargin{:})

>> hdl()
in!

Note how the function handle created in this second case is actually an anonymous function, which includes the object in it. This is a special case of the @ operator, not the normal usage, which would be:

>> hdl=@SomeMethod
hdl =
  function_handle with value:
    @SomeMethod

>> hdl(instance)
in!

What this case actually does is create an anonymous function that embeds the object you intend to call this method on. You can create such a function this way:

>> hdl=@()obj.instance.SomeMethod()
hdl =
  function_handle with value:
    @()obj.instance.SomeMethod()

>> hdl()
in!