Why do I get an error if do not define a constructor in the inherited class?

27 次查看（过去 30 天）

显示更早的评论

Evgeny Pr 2013-1-24

2
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/59789-why-do-i-get-an-error-if-do-not-define-a-constructor-in-the-inherited-class

评论： Jim Hokanson 2020-5-27

采纳的回答： Daniel Shub

在 MATLAB Online 中打开

Here's a simple example that does not work in the MATLAB language:

classdef Bar < handle
    % Bar class
      properties
          Value
      end
      methods
          function self = Bar(value)
              self.Value = value;
          end
      end
  end
classdef Foo < Bar
    % Foo class
    % No has constructor. We want to use the "Bar" constructor
end
>> f = Foo(10)
Error using Foo
Too many input arguments.

Too many input arguments.???

Ok...

>> f = Foo()
Error using Bar (line 10)
Not enough input arguments.
Error in Foo (line 1)
classdef Foo < Bar

What's going on? This is the ordinary inheritance. I would not want every time to write this:

classdef Foo < Bar
    methods
        function self = Foo(value)
            self = self@Bar(value);
        end
    end
end

3 个评论
显示 1更早的评论隐藏 1更早的评论

Evgeny Pr 2013-1-24

It's not a defect of language design, it's a feature? :)

Jim Hokanson 2020-5-27

This seems to be version specific. I'm hopping beween versions right now and 2016b throws the same error but 2019a doesn't ...

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Daniel Shub 2013-1-25

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/59789-why-do-i-get-an-error-if-do-not-define-a-constructor-in-the-inherited-class#answer_72398

编辑：Daniel Shub 2013-1-25

在 MATLAB Online 中打开

Classes and subclasses do not need a constructor.

classdef mysimpleclass
end

is perfectly valid. If a subclass does not have a constructor it calls the superclass constructor with no arguments. In other words the default constructor looks something like

function obj = mysubclass()
  obj = obj@myclass;
end

It sounds like you would like the default constructor to look like

function obj = mysubclass(varargin)
  obj = obj@myclass(varargin{:});
end

such that it would pass the arguments up the chain. I think both are reasonable defaults. There is a slight difference between MATLAB and Python (and most other languages) which potentially swayed TMW to go with the former approach in which no arguments get passed to the superclass. Because everything is an array in MATLAB, and how TMW chose to implement the OO system, the constructor for every MATLAB class needs to support being called with no input arguments (this is well documented).

EDIT

I think the choice becomes clearer if expand from the no constructor case a little bit. Consider

classdef mysubclass < myclass
  methods
    function obj = mysubclass(varargin)
    end
  end
end

Calling the mysubclass constructor invisibly calls the myclass constructor with no arguments. I believe this is documented. As we are guaranteed that the myclass constructor can be called with no arguments, it makes a little more sense to pass it no arguments (which will not crash) than pass it all the arguments which might result in an error. Either way you will end up with having to explicitly call the superclass constructor sometimes (either to pass all arguments or to pass no/some arguments). Passing all arguments seems less robust than pass no arguments. Passing a subset just seems silly.

5 个评论
显示 3更早的评论隐藏 3更早的评论

Daniel Shub 2013-1-25

@Evgeny, you are correct. I changed "a lot more sense" to "a little more sense". I don't think one is substantially better than another. Just as you don't want to write "self = self@Bar(value);" every time, TMW decided that they/we didn't want to write "self = self@Bar;" every time. You asked "What's going on?" and I tried to explain what is going on and that it essentially was a design decision by the TMW.

Evgeny Pr 2013-1-25

编辑：Evgeny Pr 2013-1-25

@Daniel

Thanks for your reasonable answer. I will choose your answer as an accepted.

@Matt J

Thanks for the discussion!

请先登录，再进行评论。

更多回答（1 个）

Matt J 2013-1-24

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/59789-why-do-i-get-an-error-if-do-not-define-a-constructor-in-the-inherited-class#answer_72316

编辑：Matt J 2013-1-24

在 MATLAB Online 中打开

Write the Bar constructor to handle the case when no input arguments are passed. Otherwise, it needs to get those arguments from somewhere, e.g.,

function self = Bar(varargin)
  if nargin
         value=varargin{1};
  else
         value=somedefault;
  end
              self.Value = value;
   end