How can I initialize nested handle class with independent values

Hello, my code is like this:

'ClassA.m'

classdef ClassA    <  handle
    properties
        t                    double = 0
    end
end

'ClassB.m'

classdef ClassB < handle
    properties
        a   ClassA  = ClassA;
    end
end

'TestFun.m'

clear all
test_num = 1e5;
tic
% initialization
class_b(test_num,1) = ClassB;
initialization_a = 0;
if initialization_a % loop assignment of  element 'a'
    for ii=1:test_num
        class_b(ii).a = ClassA;
    end
end
toc

% test if each b.a is independent, cause the default value of  b.a.t  is 0
class_b(1).a.t = 1;
disp(class_b(2).a.t)

Here is the problem description:

Array 'b' (type ClassB,length 1e5) has a member 'a' (type ClassA), they're both handle class.

when initialize 'b', each b.a shares the same value because 'a' is a handle class.

for some reason, i want each b.a have independent value, so i have 2 ways:

1st way is change the classA type as value class,

from "classdef ClassA < handle" to "classdef ClassA"

this will take about 0.15s;

2nd way is enabled loop assignment 'initialization_a = 1' in 'testfun.m'

this will take about 0.70s.

My question is, if i want to use handle class 'a'(2nd way), can i have a faster way to initialize 'a' and have independent b.a value,

the loop assignment seems to be too slow, especially when classA has more member variables.

Thanks a lot.

Answer 1

[per isakson]

"My question is, if i want to use handle class 'a'(2nd way), can i have a faster way to initialize 'a' and have independent b.a value," AFAIK: The answer is no!

However, see Class object creation performance at UndocumentedMatlab. That article is a bit old and hopefully the performance has been improved since then.

I think the recommended way is as ClassC below, but I didn't find it in the documentation. (I failed to come up with appropriate search terms.)

>> tic, cc = ClassC( 1e5 ); toc
Elapsed time is 0.564107 seconds.

where

classdef    ClassC < handle
    properties
        a   ClassA 
    end
    methods
        function  this = ClassC( n )
            if nargin >= 1
                this( 1, n ) = ClassC(); 
                for jj = 1 : n
                    this(jj).a = ClassA();
                end
            end
        end
    end
end

[pingping zhang]

Thanks for your advice.

What I don't understand is that, this nested structure of handle class is not unusual for a large project.

Maybe I can only deal with the problem in this way. Thanks ^_^

[per isakson]

I'm sure that The Mathworks are aware of this mediocre performance. They have the resources and they made the design decisions based on research. We can just speculate. There has been critisism from the outside regarding the performance based on example with linked list and trees where each node is represented by an object.

My conclusion is that if I chose to use Matlab I need to be aware of this.

The somewhat artifical example below shows the same thing.

>> tic, dd = ClassD( 1e5 ); toc
Elapsed time is 0.156734 seconds.

where

classdef    ClassD < handle
    properties
        a   double 
    end
    methods
        function  this = ClassD( n )
            if nargin >= 1
                this( 1, n ) = ClassD(); 
                for jj = 1 : n
                    this(jj).a = jj;
                end
            end
        end
    end
end

[pingping zhang]

Yes I totally agree with you, Mathworks is not good enough when dealing with For Loop.

I'm glad to share a case that i met a few days ago, it's about the validator from the property of class.

The code is like this:

classdef ClassA 
    properties
        t       (:,1)     double
    end
end

test_num = 1e5;
class_a = ClassA;
class_a.t = zeros(test_num,1);
tic
for ii=1:test_num
    class_a.t(ii) = ii;
end
toc

This will take about 4.0s.

I'm confused by this phenomenon, so i make contact with technical support, and get advice here

https://ww2.mathworks.cn/support/bugreports/details/2069154

If we remove the validator from the property t in ClassA, it will be only 0.04s. :)

Answer 2

[Steven Lord]

The section titled "Initializing Properties to Handle Objects" on this documentation page states what happens when you initialize a property containing a handle object in the properties block of a class definition. If you want the properties (containing handle objects) of multiple instances of the same class to be independent rather than all being handles to the same object, assign to it in the constructor.

[pingping zhang]

This page is really helpful for me, it shows me more useful information, Thank you very much!