Why "saveobj" is invoked two times when saving a object?

I defined a class that customs the save process.

% file ClassCustomSave.m
classdef ClassCustomSave < handle
    %ClassCustomSave
    
    properties
        val
    end
    
    methods
        function obj = ClassCustomSave()
            %ClassCustomSave Constructor
        end
        function obj = saveobj(obj)
            warning("val changed.");
            obj.val = obj.val*2;
        end
    end
end

I run this main script.

clear;
o = ClassCustomSave();
o.val = 111;
save sv.mat

clear;
load sv.mat

o.val

I get 2 warnings and "o.val" is 444. How this happens?

Answer 1

[Matt J]

I don't know why saveobj is called twice. Perhaps the first call is so save() can obtain a template of the object to be stored in the .mat file? In any case, even if saveobj were called only once, it would still be hazardous practice for you to use saveobj to edit and save the original object. Because of handle copy semantics, such changes would also be felt by the other copies of the object that remain in your workspace.

What you need to do, when saving handle objects is to move the property data to an unshared copy of the object. If you have R2024b+, you might find it helpful to use the newer framework with matlab.mixin.CustomElementSerialization. This automatically populates a serialization sObj with all the property data from your object. Since sObj is not handle-linked to the original instance(s) of obj, you can make modifications freely, without side effects:

classdef ClassCustomSave < handle & matlab.mixin.CustomElementSerialization

%ClassCustomSave

properties

val

end

methods (Static)

function modifyOutgoingSerializationContent(sObj,~)

warning('Val changing')

sObj.val = 2*sObj.val;

end

end

end

If we now run your test, you can see that the saving routine is still called twice, but the property modifications are not disturbed by the handle copy semantics.

o = ClassCustomSave();
o.val = 111;
save sv.mat

Warning: Val changing

Warning: Val changing

clear;
load sv.mat

o.val

ans = 222

[Matt J]

If you have R2024b+...

But if not, you will have to do something like below,

% file ClassCustomSave.m
classdef ClassCustomSave < handle
    methods
        function s = saveobj(obj)
            disp 'Called'
            s.val = obj.val*2;
        end
    end
    methods (Static)
        function obj=loadobj(s)
            obj = ClassCustomSave();
            obj.val = s.val;
        end
    end
end

[Walter Roberson]

@Matt J

I do not understand how you were able to get that output in MATLAB Answers? Your classdef is in style "code example" and there is no attached .m file ClassCustomSave.m ? To run the code in Answers did you used to have an attached ClassCustomSave.m that you removed later after getting the output ?

[Matt J]

To run the code in Answers did you used to have an attached ClassCustomSave.m that you removed later after getting the output ?

Yes, that is what I did.