Method chaining unexpected behaviour?

I have noticed some unexpected behaviour with method chaining. Specifically I have a class A from which I want to call the static method to create class B:

classdef A
  methods (Static)
      function b = getB(args)
          b = B(args);
      end
  end
end

And then I want to use class B to do something e.g. use the print() method, where class B is:

classdef B
  properties
      Property1
  end

    methods
        function self = B(args)
            self.Property1 = args;
        end

        function print(self)
           fprintf(self.Property1); 
        end
    end
end

Trying

A.getB(args).Property1

succeeds.

I would thus also expect

A.getB(args).print()

to succeed, however, it gives the error:

Not enough input arguments.

Error in A.getB (line 5)
          b = B(args);

Would anyone have any suggestions on why this is so? Thanks very much in advance.

Answer 1

[Philip Borghesani]

In general indexing into the output of a function call, what you call method chaining is not supported in MATLAB. Property access currently works on the output of a static function but function calls in general do not. You may find that your code behaves differently on different versions of MATLAB.

Try

 A.getB(args).print;  % works because it looks like property access, not recommended
 print(A.getB(args));  % will work
 obj=A.getB(args); obj.print();  % or print(obj) will also work

[Benjamin Chen]

Thanks for this explanation Philip. I have noticed that when I make getB a non-static method and then call it from an instance of A, i.e:

 a = A()
 a.getB(args).print()

This works. Could you also give some insights into why this is so?

[Philip Borghesani]

It's not static vs nonstatic method that makes the difference it is static call vs call on an object. Even if getB is static then:

a=A();
a.getB(args).print() % or 
A().getB(args).print() %works

a=A();a.getB is simpler to parse then A.getB because from code analysis it is obvious that a is a variable so getB(args) must be a property access or method call but A.getB(args) could also be a call to the static method getB in the package A.

[Benjamin Chen]

That sounds reasonable, thanks. I wish the error message could have been this informative rather than just "Not enough input arguments".

[Philip Borghesani]

I agree, the error message should have been better.

This appears to be fixed so that a static call works correctly in a future version of MATLAB.

[Benjamin Chen]

That's great news, looking forward to it!

Answer 2

[Image Analyst]

fprintf(self.Property1);

needs two inputs, a format string, and a variable. You only passed it the variable without passing it a format specifier string like '%f\n' or something (depends on the type of variable you want to print).

fprintf('The value = %f\n', self.Property1);

[Benjamin Chen]

Thanks for the answer Image Analyst. It seems the error is not at the fprintf part, but in the static getB method of class A.

Even if I replace the fprintf with disp, it still gives the same error.