MATLAB OOP: Return value from constructor not possible

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Milad Omidi 2018-11-2

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评论： Matt J 2018-11-2

I have a simple class called P that is supposed to Read and write a data structure to a file. It can be constructed like this:

P(filename,Data) % Construct object and write data to the file.

Now for the reading part I want to be able to get the Data structure by doing something like this:

Data = P(filename);

However MATLAB constructor is not allowed to return anything other than the object. So I am forced to change the interface such that the object is first created and then some method is used to return the data structure like this:

P1 = P(); 
Data = P1.load(filename);

Is there a better way to design the class (or any other solution) that reads the data with a single command just like writing.

Thanks, Milad

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Answer 1

Matt J 2018-11-2

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编辑：Matt J 2018-11-2

在 MATLAB Online 中打开

To avoid creating an object, you can use a static class method.

 methods (Static)
   function Data=read(filename)
       ...
   end
 end

and then

>> Data=P.read(filename);

Or, you could just create a non-class function,

Data=readP(filename);

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Answer 2

Caglar 2018-11-2

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编辑：Caglar 2018-11-2

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You can use a static method. They can be used like functions. You could do Data=P.load(filename) with them but they do not automatically create an object.

On the other hand, MATLAB constructor is allowed to return variables other than the object.

 classdef P
    properties
    end
    methods
        function [obj,test] = P(inputArg1)
            test=inputArg1*2;
        end
    end
end

>>[P1,result]=P(3)

 P1 = 
P with no properties.
 result =
     6

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Answer 3

Guillaume 2018-11-2

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However MATLAB constructor is not allowed to return anything other than the object.

I have not encountered any programming language where the object constructor returns anything other than the object. That's the definition of the constructor: special function that creates and return the object.

The whole design of your class doesn't make sense. This would make a lot more sense:

obj = P(filename);  %create reader/writer that access a specific file
obj.write(data);    %write data to file assigned to object
data = obj.read;    %read data from file assigned to object

Otherwise, as you explained it, I see no need for a class. Regular functions would do the job.

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Guillaume 2018-11-2

Yes, well don't do that! I'm not sure if that wouldn't break under some circumstances and that's certainly not good designed and may not even be supported in future versions of matlab.

Matlab's own documentation is clear that a constructor should have only one output argument: "The only output argument from a constructor is the object constructed."

Matt J 2018-11-2

I think one can make a case for additional output arguments from constructors. It can be useful in debugging for example, if you want to return intermediate calculations to the workspace for further examination.

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