How to call sub or superclass constructor when recursively creating objects

Question

Tom Clark 2016-8-28

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评论： Guillaume 2016-8-30

Hi,

I'm writing a tree class. I'd like a superclass to contain generic construction / maintenance methods, then be able to subclass it for a range of different, more specific purposes.

Something like:

classdef SuperTree < handle
    properties
      BranchLocation
      Parent
      Children
    end
    methods
      function obj = SuperTree(varargin)
        % construct the the object
      end
      function subdivide(obj, limit_criterion)
        % Lots of code that's difficult to maintain and 
        % shouldn't be overloaded in a subclass
        if limit_criterion
          % Create child branches
          chld1 = SuperTree(args);
          chld2 = SuperTree(args);
          obj.Children = [chld1; chld2]
      end
    end
  end

The problem is that if I create a subclass, then the subdivision method still makes a call to SuperTree() to construct its children.

Is there a way of getting (say) a handle to the constructor function of the current object? I don't want to be overloading the subdivide method for each of my subclasses. Something like this would be ideal:

      function subdivide(obj, limit_criterion)
        % Lots of code that's difficult to maintain and 
        % shouldn't be overloaded in a subclass
        if limit_criterion
          % Create child branches
          chld1 = obj.Constructor(args);
          chld2 = obj.Constructor(args);
          obj.Children = [chld1; chld2]
      end

At the moment I'm working around this with a small utility method, that I overload in each subclass. Working? Yes. Elegant? No. So I'd be interested in any 'proper' solutions.

Thanks!

Tom

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per isakson 2016-8-30

@Guillaume, which problems do you see in using class(obj) in this context?

This "small utility method" does it contain expressions like chld=Tree(args);?

Guillaume 2016-8-30

在 MATLAB Online 中打开

I don't see any problem in using class(obj). It works fine. It's probably because I come from C++ where reflection does not exist that I find it inelegant. It's probably irrational

If I understood correctly (and the way I'd do it), the small utility method is a one-liner method that is overloaded in each class and just call the class constructor:

classdef SuperTree < handle
    %... props definition  
    methods
        function obj = SuperTree(varargin)
          % construct the the object
        end
        function subdivide(obj, limit_criterion)
          % Lots of code that's difficult to maintain and 
          % shouldn't be overloaded in a subclass
          if limit_criterion
            % Create child branches
            chld1 = NewMe(args);
            chld2 = NewMe(args);
            obj.Children = [chld1; chld2]
        end
       %small utility function to be overloaded in each subclass
       function this = NewMe(args)
           error('Must be overloaded by children');
           this = SuperTree(args);
      end
    end
  end

and child

classdef Child < SuperTree
    %... props definition  
    methods
        function obj = Child(varargin)
          % construct the the object
        end
       function this = NewMe(args)
           this = Child(args);
      end
    end
  end

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Answer 1

per isakson 2016-8-28

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/301143-how-to-call-sub-or-superclass-constructor-when-recursively-creating-objects#answer_232969

编辑：per isakson 2016-8-30

在 MATLAB Online 中打开

I think, I understand and that it's doable. Replace

chld1 = obj.Constructor(args);

by

str   = class( obj );
chld1 = feval( str, args );

And in the constructor of the subclass

obj@SuperTree( arguments )

Whether this is the proper way, I don't know.

&nbsp

ADDENDUM 2016-08-29 in response to the comment: "2. this will eliminate any possibility for code generation."

Would this allow code generation? Replace

chld1 = feval( str, args );

by

chld1 = TreeFactory( str, args );

where the simplest form of TreeFactory is an ordinary function

function  tree = TreeFactory( str, args )
    switch str
        case 'oak', tree = Oak( args );
        otherwise
    end
end

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per isakson 2016-8-28

编辑：per isakson 2016-8-28

There has been complains regarding the performance of MATLAB OOP. However, its improving. I've learned to avoid solutions with thousands of simple objects.

"feval() is likely to be quite slow" &nbsp Based on some simple tests (R2016a) I don't think so. (That is, it's not slow compared to other ways to create objects in Matlab.)

"will eliminate any possibility for code generation" &nbsp Yes.

Given the superclass, handle, I cannot think of any other approach. Or maybe, see Clone Handle Object - using MATLAB OOP by Bobby Nedelkovski

per isakson 2016-8-29

I added an alternative to feval to the answer.

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How to call sub or superclass constructor when recursively creating objects

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How to call sub or superclass constructor when recursively creating objects

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