Entries in array of class instances affecting others in my array

Question

Rosco Hunter 2020-4-5

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https://ww2.mathworks.cn/matlabcentral/answers/515537-entries-in-array-of-class-instances-affecting-others-in-my-array

评论： Rosco Hunter 2020-4-5

采纳的回答： Steven Lord

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I can't understand why changing instances of a class in an array effect the other entires.

An example of how this works is as follows:

I create a class as follows in one file:

classdef Terminal < handle
    properties
        Point_1=[]
        Point_2=[]
    end  
end

And wish to make an array where the entires are instances of this class which I create in another file:

%The line below is to make an array with 3 entires all of which are instances of the class Terminal
Terminal_array(1:3)=Terminal
Terminal_array(1).Point_1=1;
%If I call here 'Terminal_array(1).Point_1' the output is 1
Terminal_array(2).Point_1=2;
Terminal_array(3).Point_1=3;
Terminal_array(1).Point_1

If I call Terminal_array(1).Point_1 at the bottom it is 3 instead of the desried 1. I can't understand why this is happening.

Any help or advice would be appriciated.

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Answer 1

Steven Lord 2020-4-5

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You're assigning the same handle object to all three elements of Terminal_array. That means changing one of the elements of Terminal_array will change all of them. You can see that they refer to the same object using the == operator as described on this documentation page.

Terminal_array(1:3) = Terminal;
Terminal_array(1) == Terminal_array(2) % true
Terminal_array(1) == Terminal_array(3) % true

If you created the elements of the array through separate calls to the constructor, they would not refer to the same object.

Terminal_array2(1) = Terminal;
Terminal_array2(2) = Terminal;
Terminal_array2(3) = Terminal;
Terminal_array2(1) == Terminal_array2(2) % false
Terminal_array2(1) == Terminal_array2(3) % false
Terminal_array2(2) == Terminal_array2(3) % false