OOP- instantiate a class within a method of a different class

Hi, I have 2 class files, both stored under the same folder. I want to instantiate class A in one of the method of class B but I get an error message: Undefined function or variable 'A'. how can "connect" between the 2 class files so that they know each other? Thanks!

[Adam]

It would help if you show some code.

What does

which classA

show on command line?

There is no reason why a class should not be visible within another class any more than it is from command line or anywhere else.

[Hadar]

Hi Adam, thanks for your reply. I'm new in matlab but familiar with c++. Im trying to practice matlab by implementing algorithms Ive already implemented in c++.

Im have the following class:

classdef MyGraph

      properties
          nodes_num
          vector_of_adj
      end

      methods
          function obj = MyGraph(inputArg1)
              obj.nodes_num = inputArg1;
              obj.vector_of_adj = cell(1,inputArg1);
          end

          function obj = AddEdgeInGraph(obj, src, dest)
               obj.vector_of_adj{src} = [obj.vector_of_adj{src}, dest]
          end

          function DFSutil(obj, start, visited)
              disp(start)
              visited(start) = 1;
              for i = drange(1:numel(obj.vector_of_adj{start}))
                  if visited(obj.vector_of_adj{start}(i)) == 0
                      DFSutil(obj, obj.vector_of_adj{start}(i), visited);
                  end
              end
          end
          function DFS(obj)
              visited_size = obj.nodes_num + 1;
              visited = zeros(1, visited_size);
              for i = 1:obj.nodes_num
                   if visited(i) == 0
                       DFSutil(obj, i, visited);
                  end
              end
          end
      end
  end

I have a problem with the array: visited which is a local variable in function DFS. when I pass it to function DFSutil, it is passed by value and not by reference so its updated values are not saved.

I read and found that I can declare a handle class, like this:

classdef Visited < handle

and to store the array: visited as a property of that class so any change done on its instantiated objects will be saved. So I saved another file for the class above and tried to instantiate an object in the DFS function but I got an error: Undefined function or variable 'Visited'.

so my questions are as follows:

1. is there any way I can pass array by reference to a function in matlab?

if not and I have to save the array as a property of a handle class, then:

2. how can I instantiate a class in a method or as a property of a different class (assuming both classes are saved in their own files but under the same folder)?

Thank you so much for your help!

[Adam]

So what does

which Visited

show on the command line?

What you said you tried to do should work (though depending if DFSutil, which is a public function, is ever called externally with a different 'visited' array than the one in DFS you might as well make it a property on the class).

Alternatively you can just have it passed by value and be an output argument of DFSutil.

handle classes are the only way to pass by reference in Matlab though.

[Adam]

I have the following class defined (and on my path, the location of the class file does not matter so long as it is on your path, and assuming it is not in a package you don't need to reference it in any special way):

classdef ValueByRef < handle
   properties( SetObservable, Access = public )
    value = [];
  end
  
  methods
    function obj = ValueByRef( value )
      obj.value = value;
    end
    
    function empty = isempty( obj )
      empty = isempty( obj.value );
    end
  end
  
end

though I very rarely use it, to be honest, even though at the time I wrote it I thought it would be useful for exactly this purpose.

Pass-by-reference being defined at class level rather than object level and the inability to pass anything around as a 'const' reference are two of the biggest differences from C++ to Matlab that can be very annoying sometimes.

[Hadar]

I dont understand the following question:

So what does

which Visited

show on the command line?

----------------------------

anyway, I changed my code so now visited is a property of MyGraph class and I initialize it in the constructor of MyGraph. therefore, I removed it from argument list of DFSutil so now it can not(!) be called externally with a different 'visited' array. its working now!

here are the 2 modified classes:

classdef MyGraph

    properties
        nodes_num
        vector_of_adj
        visited
    end

    methods
        function obj = MyGraph(inputArg1)

            obj.nodes_num = inputArg1;
            obj.vector_of_adj = cell(1,inputArg1);
            obj.visited = Visited(inputArg1);
        end

        function obj = AddEdgeInGraph(obj, src, dest)

             disp('hi')
             obj.vector_of_adj{src} = [obj.vector_of_adj{src}, dest]
        end

        function DFSutil(obj, start)
            disp(start)
            obj.visited.visited(start) = 1;
            for i = drange(1:numel(obj.vector_of_adj{start}))
                if obj.visited.visited(obj.vector_of_adj{start}(i)) == 0
                    DFSutil(obj, obj.vector_of_adj{start}(i));
                end
            end

        end

        function DFS(obj)
%             
            for i = 1:obj.nodes_num
                 if obj.visited.visited(i) == 0
                     DFSutil(obj, i);

                end
            end
        end

    end
end

classdef Visited < handle here

    properties
        visited;
    end

    methods
        function obj = Visited(input)

            obj.visited = zeros(1, input);
%                 
        end

    end
end

for practice, I intend to also try your other suggestion: "Alternatively you can just have it passed by value and be an output argument of DFSutil".

Thanks alot, you've been very helpful!

[Adam]

which Visited

literally meant what I said - type it on the command line and see what the result is.

[Hadar]

ohh cool, like in bash terminal.

I get the path:

/home/hadar/MATLAB/R2017b/bin/Visited.m % Visited constructor

[Adam]

So it should be able to see it whether inside or outside of your other class, in that case.

Answer 1

[Guillaume]

For somebody coming from C++, I'm surprised you're not using private/public qualifiers for your class methods, rather than having everything public.

The problem with the visited class not being visible is puzzling since which can find it. It's surprising that it is in a bin directory but that doesn't matter.

Irrespective of that issue however, the whole concept of creating a handle class just to emulate a pass-by-reference array is extremely misguided. Matlab is not C++. If you need a mutable object in and out of a function, then you pass it as input and output. Therefore, the proper way to write your DFS would be:

      methods
          %... constructor, etc.

          %DFS method. Note: I prefer calling the object 'this' rather than 'obj'
          function DFS(this)
              visited_size = this.nodes_num + 1;
              visited = zeros(1, visited_size);
              for i = 1:this.nodes_num
                   if visited(i) == 0
                       visited = DFSutil(this, i, visited);
                   end
              end
          end
      end

      methods (Access = private)
          function visited  = DFSutil(this, start, visited)
              disp(start)
              visited(start) = 1;
              for i = drange(1:numel(this.vector_of_adj{start}))
                  if visited(this.vector_of_adj{start}(i)) == 0
                      visited = DFSutil(this, this.vector_of_adj{start}(i), visited);
                  end
              end
          end
     end

Note that in the above case, matlab's JIT compiler should be able to detect that input and output are the same and pass the visited array by reference anyway.

[Hadar]

Hi, I'm familiar with public/private qualifiers in c++ and know this option exists in matlab too. I'm new in matlab, therefore less familiar with its syntax but during learning I intend to add these qualifiers to my classes in addition to pass visited as input and output just like adam previously suggested.

Now Visited class is visible and I have no problem to instantiate it. Regarding bin directory, Ive erroneously saved it there.

Thank you for your help!