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Saving a struct property within a class object

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Cyndia Cao
Cyndia Cao on 31 Mar 2020
Edited: Cyndia Cao on 2 Apr 2020
I'm trying to save an instance of a class object I've made for later reference. Some of my properties are structs. I can set and manipulate them as expected, but when I save, the saved object reverts to their default values (or empty if there was no default). Do I need to initialize structs in a special way? Or is there an Attribute I need to set?
This is an example of my class:
classdef testclass
properties
S = struct('d', 0, 'e', 1);
number = 1;
end
methods
function obj = set.S(obj, val)
obj.S.d = val + 1;
obj.S.e = val + 3;
end
end
end
My test output - the save function saves the integer value, but doesn't save changes to the struct:
>> t = testclass
t =
testclass with properties:
S: [1×1 struct]
number: 1
>> t.number = 10;
>> t.S = 3;
>> t.S
ans =
struct with fields:
d: 4
e: 6
>> save test.mat t
>> clear
>> load test.mat
>> t.S
ans =
struct with fields:
d: 0
e: 1
>> t.number
ans =
10
The reason my struct set function is written this way is I'd like to give my function a few parameters to calculate trajectory arrays, and then I save those trajectories for reference. This works fine until I try to save my class object, so I've been saving my structs as separate variables outside the class object, i.e.:
S = t.S; save test.mat t S
Is there a cleaner way to do this instead?

Answers (1)

Jeremy
Jeremy on 31 Mar 2020
Edited: Jeremy on 31 Mar 2020
classdef testclass
properties (GetAccess=public, SetAccess=public)
S
number
end
methods
function obj = testclass(num,val)
obj.number = num;
obj.S.d = val + 1;
obj.S.e = val + 3;
end
end
end
>> t = testclass(10,3)
t =
testclass with properties:
S: [1×1 struct]
number: 10
>> t.S
ans =
struct with fields:
d: 4
e: 6
>> save test.mat t
>> clear
>> load test.mat
>> t
t =
testclass with properties:
S: [1×1 struct]
number: 10
>> t.S
ans =
struct with fields:
d: 4
e: 6
Does this help?
  6 Comments
Cyndia Cao
Cyndia Cao on 31 Mar 2020
edit: realized the problem with this syntax is that testclass is a value class so I'd have to say t = t.update_struct(3) in order for the change to propagate, or I could change testclass to a handle class. Regardless, changing testclass to a handle class doesn't help with the save/load issue of the struct.
--------------------------
To be more explicit, this was my updated class code:
classdef testclass
properties (GetAccess=public, SetAccess=public)
S = struct('d', 0, 'e', 1);
number = 1;
end
methods
function obj = update_struct(obj, val)
disp('in update function')
obj.S.d = val + 1;
disp([obj.S.d, val + 1])
obj.S.e = val + 3;
disp([obj.S.e, val + 3])
end
end
end
And my command line test:
>> t = testclass;
>> t.update_struct(3)
in update function
4 4
6 6
ans =
testclass with properties:
S: [1×1 struct]
number: 1
>> t.S
ans =
struct with fields:
d: 0
e: 1
I've used classes before (though more extensively outside MATLAB). The other functions in the classes I've implemented work as I expect, except for these cases of managing structs.

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