constructor called twice when creating array of object. why?

Question

Christof 2012-6-20

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/41635-constructor-called-twice-when-creating-array-of-object-why

This is sort of a double post, as it is an issue I ran into when following a solution to a previous problem. So, sorry for that, but this really confuses me. As an example, I have a class

 classdef someClass
  properties
      name
      value
  end
  methods
      function obj = someClass(name)
          if ~nargin
              % the default constructor. Needed for array creation
              obj.name = '';
          else
              % only take care of the array part for simplicity:
              if iscell(name)
                  [obj(1:length(name)).name] = deal(name{:});
              end
          end          
          display('object created')
      end
  end
end

When I run c=someClass({'a' 'b' 'c' 'd' 'e'}) I get the output object created object created

c =

1x5 someClass
Properties:
  name
  value
Methods

So, the constructor seems to be called twice. The second time, it is called w/o input arguments. So if I comment the blocks that checks for number of input arguments to be >0, I get the result Error using someClass (line 13) Not enough input arguments.

Error in someClass (line 14) [obj(1:length(name)).name] = deal(name{:});

when I run c=someClass({'a' 'b' 'c' 'd' 'e'}). Why does it run the constructor twice? How can I avoid that.

Thanks for any help, Chris

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Answer 1

Daniel Shub 2012-6-20

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/41635-constructor-called-twice-when-creating-array-of-object-why#answer_51270

The documentation says that "During the creation of object arrays, MATLAB might need to call the class constructor with no arguments ..." While you might be able to avoid it, I don't think it is a good idea.