How to make objects of the same class have independent containers.Map member?

Question

Guanfeng 2013-10-3

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/88989-how-to-make-objects-of-the-same-class-have-independent-containers-map-member

评论： Guanfeng 2013-10-3

I create my own class as below:

classdef testClass < handle
    properties
        value;
        map = containers.Map('KeyType','double','ValueType','any');
    end
end

My goal is for each object of testClass to maintain its own map. However, it turns out that there is only one map object that for the whole class: all objects of testClass access to the same containers.Map. For example, if I create two objects as follows

a = testClass;
b = testClass;
a.value = 'a';
b.value = 'b';
a.map(1) = 123;
b.map(2) = 321;

It ends up both a and b's map contains two key-value pairs:

>> a
a = 
  testClass handle
    Properties:
      value: 'a'
        map: [2x1 containers.Map]
>> b
b = 
  testClass handle
    Properties:
      value: 'b'
        map: [2x1 containers.Map]
    Methods, Events, Superclasses

Both (key,value) pairs (1,123) and (2,321) appears in both a.map and b.map

>> a.map.keys
ans = 
    [1]    [2]
>> a.map.values
ans = 
    [123]    [321]
>> b.map.keys
ans = 
    [1]    [2]
>> b.map.values
ans = 
    [123]    [321]

Is this a bug? How can I keep independent containers.Map for each class object??

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Answer 1

Peter Perkins 2013-10-3

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/88989-how-to-make-objects-of-the-same-class-have-independent-containers-map-member#answer_98539

在 MATLAB Online 中打开

Guanfeng, the problem is that you're initializing the map property in the properties block, and the value you're initializing with is a handle. Your class creates the value on the right hand side of

map = containers.Map('KeyType','double','ValueType','any');

once, and assigns it to that property in each new instance. But because that value is a handle, the assignment just copies the handle, and so all the properties point to the same map. This would not happen if you initialized with a value that wasn't a handle.

The solution is to put the initialization of that property into the testClass constructor.

Hope this helps.