bugGNU Octave - Bugs: bug #55856, subsref called in a subscripted...


bug #55856: subsref called in a subscripted assignment operation

Submitted by:  Guillaume <gyom>
Submitted on:  Thu 07 Mar 2019 05:40:37 PM UTC  
Category:  Interpreter Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Unexpected Error or Warning
Status:  None Assigned to:  None
Originator Name:  Guillaume Open/Closed:  Open
Release:  dev Operating System:  Any

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Tue 15 Sep 2020 11:16:29 AM UTC, comment #2: 

¯\(°_o)/¯ with the foo class defined in comment #1, subsref is indeed called in Matlab for f.a(end)=1;

>> f = foo ();
>> f.a = 1;
>> f.a(1)
ans =
>> f.a(end)
ans =
>> f.a(end)=1
f =
       foo object: 1-by-1

If I define an "end" method like this:

function ind = end(obj, k, n)
  disp ("end");
  ind = 1;

the above doesn't change and the new method is only called for:

>> f(end)
ans =
  struct with fields:
    a: 1

Guillaume <gyom>
Tue 15 Sep 2020 04:51:49 AM UTC, comment #1: 

Are there instances where subsref should be called?  For example, if the LHS of the assignment includes "end", maybe like this:

obj.data (end) = 1;

What happens if the class defines an "end" method?

In a case like above, what is responsible for setting things up so that "end" will be called with the correct object and index info?

Does the behavior change if there is a subsref method but no subsasgn method (I hope not)?

John W. Eaton <jwe>
Project Administrator
Thu 07 Mar 2019 05:40:37 PM UTC, original submission:  

Define the following class with subsref and subsasgn methods

%%% @foo/foo.m
function obj = foo ()
  obj = struct ("store", []);
  obj = class(obj, 'foo');

%%% @foo/subsref.m
function val = subsref (obj, S)
  disp ("subsref");
  val = builtin ("subsref", obj.store, S);

%%% @foo/subsasgn.m
function obj = subsasgn (obj, S, val)
  disp ("subsasgn");
  obj.store = builtin ("subsasgn", obj.store, S, val);

In Matlab:

>> f = foo ();
>> f.a.b = 1;
>> f.a.b
ans =

In Octave 5.1 or 6.0:

octave:1> f = foo ();
octave:2> f.a.b = 1;
error: null_matrix cannot be indexed with .
error: called from
    subsref at line 3 column 7
octave:2> f.a = 1;

I don't think that subsref should be called when evaluating "f.a.b = 1;"

Guillaume <gyom>


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