Use subsasgn from a value class to update its value

Question

Vincent 2014-3-26

0
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https://ww2.mathworks.cn/matlabcentral/answers/123214-use-subsasgn-from-a-value-class-to-update-its-value

评论： Vincent 2014-3-31

I have a huge value class. Because I'm afraid of further problems, I don't want it to be a HandleClass by now.

Anyway, I want to change a class property from a class method. Is this possible under any circumstances? I made an example and modifyValue is the critical function:

testClass.m:

 classdef testClass
    properties
        a
    end 
    methods
        function obj = testClass()
            obj.a = [1 2 3; 4 5 6];
        end 
        function obj = modifyValue(obj)
            obj(:,2) = [];
            % won't work as well
            obj.a(:,2) = [];
        end 
    end
 end

subsasgn.m:

 function obj = subsasgn(obj, S, value)
    if isempty(value)
        obj.a(:,S.subs{2}) = [];
    else
        obj = builtin('subsasgn',obj, S, value);
    end 
 end

Why isn't it possible to call obj(:,2) = []; from within the object? This confuses me. I'm still using Matlab R2007b, but solutions for newer versions are also very welcome.

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Answer 1

per isakson 2014-3-26

2
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/123214-use-subsasgn-from-a-value-class-to-update-its-value#answer_130602

编辑：per isakson 2014-3-26

在 MATLAB Online 中打开

The overloaded subsasgn doesn't work in the objects own methods. R2014a doc says:

    Within a class's own methods, MATLAB calls the built-in subsasgn, not
    the class defined subsasgn. This behavior enables to use the default
    subsasgn behavior when defining specialized indexing for your class. 
    See subsref and subsasgn Within Class Methods — Built-In Called for 
    more information.

In R2007b the new "OO-syntax" was still beta.

.

Try this (it works according to my expectations)

    >> clear all, clear classes
    >> ca = testClass_a;
    >> ca.a
    ans =
         1     2     3
         4     5     6
    >> ca = ca.modifyValue;
    >> ca.a
    ans =
         1     3
         4     6
    >>

where

    classdef testClass_a
        properties
            a
        end
        methods
            function obj = testClass_a()
                obj.a = [1 2 3; 4 5 6];
            end
            function obj = modifyValue(obj)
                obj.a(:,2) = [];
            end
        end
    end