MATLAB Class Object Not Updated

问题:

I am writing a simple MATLAB class that has a few properties and a method. The constructor of the class initializes the properties with the default values. The method of the class gets an additional input after the class is constructed in order to update the class properties.

classdef classTest
    properties
        p1
        p2
        p3
        p4
    end

    methods

        function obj = classTest()
            obj.p1 = 0;
            obj.p2 = 0;
            obj.p3 = [];
            obj.p4 = '';
        end

        function obj = updateSomeProperties( obj, p1 )
            obj.p1 = p1;
        end
    end

end

However, when I call the method of the class it does not update the properties.

>> b = classTest

b = 

  classTest with properties:

    p1: 0
    p2: 0
    p3: []
    p4: ''

>> b.updateSomeProperties(10)

ans = 

  classTest with properties:

    p1: 10
    p2: 0
    p3: []
    p4: ''

>> b % still used the default values.

b = 

  classTest with properties:

    p1: 0
    p2: 0
    p3: []
    p4: ''

I wanted to know why after calling the method of the class, b is not updated although updateSomeProperties updates the class object.

回答1:

The issue is that your class is a value class which is passed (even to it's own methods) as a copy. This is the default behavior for MATLAB classes as this is what all fundamental MATLAB datatypes are. We can verify that this is the case by looking at the output of your call to updateSomeProperties(). You will see that the returned result (shown as ans) contains the modification that you expect but these changes are not present in your original object, b. If you wanted to stick with a value class, you would need to return the new object from the method and re-assign the variable when calling the method.

b = classTest();
b = b.updateSomeProperties(10);

What you want is a handle class which is always passed by reference. This allows a method to operate on the same object rather than modifying a copy of the original object.

To do this, you will want to inherit from the built-in handle class.

classdef classTest < handle

There is a detailed comparison of handle and value classes in the documentation.

As a side-note, rather than manually setting all default property values in the constructor, it is possible to simply specify these defaults within the properties block itself.

properties
    p1 = 0;
    p2 = 0;
    p3 = [];
    p4 = '';
end

回答2:

Your class needs to inherit from the handle superclass. Try:

classdef classTest < handle
properties
    p1
    p2
    p3
    p4
end

methods

    function obj = classTest()
        obj.p1 = 0;
        obj.p2 = 0;
        obj.p3 = [];
        obj.p4 = '';
    end

    function obj = updateSomeProperties( obj, p1 )
        obj.p1 = p1;
    end
end
end 

This will give,

>> b = classTest

b = 

classTest with properties:

p1: 0
p2: 0
p3: []
p4: ''

>> b.updateSomeProperties(10)

ans = 

  classTest with properties:

p1: 10
p2: 0
p3: []
p4: ''

>> b

b = 

classTest with properties:

p1: 10
p2: 0
p3: []
p4: ''