MATLAB: A constructor call to superclass appears after a return

classMATLABoopsubclass

I am trying to check if a parameter is of a certain type when instantiating a subclass of a superclass, and I keep getting an error that appears to be associated with my method of checking of the argument type. The debugger won't even let me include a breakpoint without throwing the error "A constructor call to superclass appears after the object is used, or after a return." I think it's pretty obvious what line of code breaks my classes, but why am I not allowed to do this type checking? What other ways are there to confirm that my arguments are of a particular type? Code below.

classdef superClass < handle
  properties
      PropertyOne
      PropertyTwo
  end    
  methods
      function sup = superClass(param1, param2)
          sup.PropertyOne = param1;
          sup.PropertyTwo = param2;
      end
  end
end
classdef subClass < superClass
  properties
      PropertyThree
  end   
  methods
      function sub = subClass(param1, param2, param3)
          if ~isa(param1, 'char')
              disp('param1 must be type char')
              return
          end
          sub@superClass(param1, param2);
          sub.PropertyThree = param3;        
      end
  end
end

Best Answer

Call error instead of disp and return. The constructor must return a valid object in sub when return is called and your code has not initialized it yet.

Related Solutions

MATLAB: Super class get callers name

Found it. Just call this:

 me = class(obj);

this gives the name of the created subclass. I call it in the constructor of the superclass:

 function obj = mySuperClass() %constructor
   me = class(obj);
   obj.name = me;
 end

MATLAB: How are objects converted from one type to another when collected together in an array in MATLAB 7.6 (R2008a)

When MATLAB sees a subscripted assignment to an existing variable, MATLAB checks the class of the right-hand-side against the class of the left-hand-side (LHS) variable. If different, it attempts to convert the right-hand-side (RHS) variable to the type of the LHS class.

To achieve this, MATLAB first looks for a method of the RHS class that has the same name as the LHS class. This is a converter method from one class to another and similar in some ways to a cast operator in C++.

If there is no specific converter, then MATLAB calls the LHS constructor and passes the RHS variable as input. This is irrespective of the class relationship between the LHS and RHS variable.

In the above example, the LHS class is SuperClass and the RHS class is SubClass. When the following line is executed,

vec(2) = subobj

since SubClass does not have a method SuperClass, MATLAB calls the SuperClass constructor with subobj as the single input, resulting in an error.

To facilitate the conversion from SubClass to SuperClass, you can create a method named SuperClass in class SubClass with the following signature

function obj = SuperClass(subobj)
      obj = SuperClass(subobj.prop1, subobj.prop2);
end

Alternatively, you can modify the constructor of SuperClass to accommodate single inputs of type SuperClass. One implementation of the constructor could be,

function obj = SuperClass(prop1, prop2)
      if isa(prop1,'SuperClass')
          obj.prop1 = prop1.prop1;
          obj.prop2 = prop1.prop2;
      else
          obj.prop1 = prop1;
          obj.prop2 = prop2;
      end
end

One drawback of the previous methods is that a copy of the object is created.

As an alternative, you can preserve the original class of the objects and still aggregate the objects by using a cell array as follows:

vec{1} = supobj;
vec{2} = subobj;

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