Building Arrays in Superclass Constructors

When a subclass in a heterogeneous class hierarchy calls its superclass to construct an array of objects, you must ensure that the superclass constructor does not return a heterogeneous array to the subclass. The following programming patterns show how to avoid the errors caused by returning the wrong class to the subclass constructor.

When Errors Can Occur

Constructors must return objects that are the same class as the defining class. When working with objects from a heterogeneous class hierarchy, the class of an object array can change as you add array elements of different classes. As a result, heterogeneous superclass constructors can change the class of object arrays when the class design requires all of these techniques:

In addition, either of the following is true:

When assigning to object arrays, MATLAB^® uses the default object to fill in unassigned array elements. In a heterogeneous hierarchy, the default object can be the superclass that is called by the subclass constructor. Therefore, building an array in the superclass constructor can create a heterogeneous array.

If a superclass constructor returns a heterogeneous array to the subclass constructor, MATLAB generates an error (see Potential Error).

Initialize Array in Superclass Constructor

To avoid errors, initialize the object array explicitly in the superclass constructor. For example, use repelem in the superclass constructor to initialize the array before initializing the superclass part of the objects. Initializing the array ensures that all elements assigned into the array are of the same class as the obj argument.

In this code, the superclass constructor creates one object for each element in the input argument, arg:

methods
   function obj = SuperClass(arg)
      ...
      n = numel(arg);
      obj = repelem(obj,1,n);
      for k = 1:n
         obj(k).SuperProp = arg(k);
      end
      ...
   end
end

The subclass constructor calls the superclass constructor to pass the required argument array, a:

methods
   function obj = SubClass(a)
      obj = obj@SuperClass(a);
      for k = 1:numel(a)
         obj(k).SubProp = a(k);
      end
   end
end

Sample Implementation

The following class hierarchy defines a subclass that builds object arrays in its constructor. The root superclass of the hierarchy initializes the superclass part of the objects in the array.

This class hierarchy represents members of an engineering team. The classes in the hierarchy include:

The TeamMembers class is the root of the heterogeneous hierarchy and is a concrete class. Before assigning values to the Name and PhoneX properties, the constructor initializes an array of subclass (ProjectEngineer) objects.

The ProjectEngineer constructor provides the obj argument for the call to repelem with this statement:

obj = obj@TeamMembers(varargin{1:2});

Here is the TeamMembers class:

classdef TeamMembers < matlab.mixin.Heterogeneous
   properties
      Name
      PhoneX
   end
   methods
      function obj = TeamMembers(nme,ext)
         if nargin > 0
            n = numel(nme);
            obj = repelem(obj,1,n);
            for k = 1:n
               obj(k).Name = nme{k};
               obj(k).PhoneX = ext(k);
            end
         else
            obj.Name = '';
         end
      end
   end
end

The ProjectEngineer class represents one type of team member. This class supports array inputs and returns an array of objects.

classdef ProjectEngineer < TeamMembers
   % Inputs: {Name}, [PhoneX], {Rate}
   properties
      Rate
   end
   methods
      function obj = ProjectEngineer(varargin)
         obj = obj@TeamMembers(varargin{1:2});
         for k = 1:numel(varargin{1})
            obj(k).Rate = varargin{3}{k};
         end
      end
   end
end

The ProjectEngineer class requires a cell array of names, a numeric array of phone extensions, and a cell array of billing rates for each engineer in the team.

nm = {'Fred','Nancy','Claudette'};
px = [8112,8113,8114];
rt = {'C2','B1','A2'};
tm = ProjectEngineer(nm,px,rt)

tm = 

  1x3 ProjectEngineer array with properties:

    Rate
    Name
    PhoneX

Potential Error

The TeamMembers constructor initializes the object array with this statement:

obj = repelem(obj,1,n);

Because the obj argument to repelem is a ProjectEngineer object, the array returned is of the same class.

Without this statement, the TeamMembers constructor would create default objects to fill in array elements in the for loop. The resulting heterogeneous array would be of the class of the common superclass (TeamMembers in this case). If the superclass returns this heterogeneous array to the subclass constructor, it is a violation of the rule that class constructors must preserve the class of the returned object.

MATLAB issues this error:

When constructing an instance of class 'ProjectEngineer', the constructor must
preserve the class of the returned object.

Error in ProjectEngineer (line 8)
         obj = obj@TeamMembers(varargin{1:2});