Save class object as struct

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Behnam on 20 May 2021
Commented: Matt J on 28 Jul 2021
I want to automatically convert class objects to structs before saving them to MAT files.
This webpage seems to explain the task that I expect. However, when I try the following, it trurn out that it is not as expected:
mySuper = MySuper;
save('m.mat', 'mySuper')
The saved file contains mySuper variable still as a class object, not a struct.
If I delete the class and load the file afterwards, I get:
Warning: Variable 'mySuper' originally saved as a MySuper cannot be instantiated as an object and will be read in as a uint32.
> In uiimport/runImportdata (line 459)
In uiimport/gatherFilePreviewData (line 438)
In uiimport (line 262)
Class MySuper could be extended by changing the saveobj method to the following:
function S = saveobj(obj)
S = struct(obj);
But this doesn't work either.
I'm using MATLAB R2020a.
Michael Jablecki
Michael Jablecki on 23 Jun 2021
This has been a problem for a while. Object serialization tends to have issues, and I have had parallel problems with Python pickled objects. It is a portability problem. When the class changes, the saved object will no longer be able to be loaded in many cases, making debugging challenging if the process involves saved (serialized) instances separated from the code which generated them. I'd stick with saving as structures and reconstitute as described above rather than ever saving a "native class instance". They are trouble.

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Accepted Answer

J. Alex Lee
J. Alex Lee on 11 Jun 2021
I think it means when you implement saveobj/loadobj, it will internally use a struct intermediary, but ultimately when you load the object, the intent is that the output will be an object, not a struct.
if need the struct, I guess you can just do
s = struct(obj)
or just wrap that in a "export" method for your class.
Matt J
Matt J on 28 Jul 2021
Yes, the loadobj/saveobj mechanism is the intended way to handle saved objects that are out of date with their class definitions.

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