Dear all,
I overloaded subsref() and subsasgn() in a class that I built and I seem unable to reproduce the behavior that built-in types/classes, typically cell/struct arrays, have when linearly indexed with {} (this generalizes to any case of linear/mixed indexing with n subs on cell arrays with more than n dimensions). Here are a few tests to illustrate:
>> a = {3, 4} ; >> a{:} % Comma separated list of elements
ans = % (cells content).
3 ans = 4 >> S = substruct( '{}', {':'} ) ; % Build S struct array.
>> subsref( a, S ) % Call class cell subsref().
ans = 3 >> builtin( 'subsref', a, S ) % Same, just in case.
ans = 3 >> b = a{:} % Single LHS => only first RHS saved.
b = 3 >> ans = a{:} % Special behavior with ans? No.
ans = 3 >> c = cell( numel(a), 1 ) ; % Matching LHS, RHS when we know
>> [c{:}] = a{:} % a priori the size of the block
c = % targeted by the sub.
[3] [4] >> ans, ans = a{1}, a{2} % Hand-made comma separated list
ans = % when we know a priori the size
3 % of the block targeted by the sub.
ans = 4>> ans, ans = subsref( a, S ) % Even for built-ins, subsref()
ans = % outputs at most a cell array.
4 % <- this is the previous ans
ans = % <- this is the output of subsref()
3 % So a{:} is not managed by subsref(),
% but transformed into
% ans, ans = a{1}, a{2}
% by the interpreter that is able to
% manage linear indexing with {} for
% built-in classes only??
It seems to me that managing a{:} is not or cannot be done at the subsref() level as the method must output a cell array (varargout) and cannot output a comma separated list. Then, the way I understand the behavior above when using the syntax a{:}, is that the interpreter checks whether "a" is an instance of a built-in type/class (typ. cell and struct array), and if so is able to compute a priori the number of elements of a{:} and evaluate the following
ans, ans = a{1}, a{2}If I am not too wrong, it is therefore not possible to build user-defined classes that are treated the same way by the interpreter (that cannot know a priori the size of the block being indexed)?, .. unless there is a way to tell MATLAB that instances of this user-defined class behave like built-ins, or some numel()-like method call including subs, that we could manage with an overload? What did I understand/misunderstand?
Thank you and best regards,
Cedric
(R2011b, 64-bit)
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