Specify Allowed Subclasses

Why Control Allowed Subclasses

A class definition can specify a list of classes that it allows as subclasses. Classes not in the list cannot be defined as subclass of the class. To specify the allowed subclasses, use the AllowedSubclasses class attribute.

The AllowedSubclasses attribute provides a design point between Sealed classes, which do not allow subclassing, and the default behavior, which does not restrict subclassing.

By controlling the allowed subclasses, you can create a sealed hierarchy of classes. That is, a system of classes that enables a specific set of classes to derive from specific base classes, but that does not allow unrestricted subclassing.

See Define Sealed Hierarchy of Classes for more about this technique.

Specify Allowed Subclasses

Specify a list of one or more allowed subclasses in the classdef statement by assigning matlab.metadata.Class objects to the AllowedSubclasses attribute. Create the matlab.metadata.Class object referencing a specific class using the ? operator and the class name:

classdef (AllowedSubclasses = ?ClassName) MySuperClass
   ...
end

Use a cell array of matlab.metadata.Class objects to define more than one allowed subclass:

classdef (AllowedSubclasses = {?ClassName1,?ClassName2,...?ClassNameN}) MySuperClass
   ...
end

Always use the fully qualified class name when referencing the class name:

classdef (AllowedSubclasses = ?namespace.inneramespace.ClassName1) MySuperClass
   ...
end

Assigning an empty cell array to the AllowedSubclasses attribute is effectively the same as defining a Sealed class.

classdef (AllowedSubclasses = {}) MySuperClass
   ...
end

Note

Use only the ? operator and the class name to generate matlab.metadata.Class objects. Values assigned to the AllowedSubclasses attribute cannot contain any other MATLAB^® expressions, including functions that return either matlab.metadata.Class objects or cell arrays of matlab.metadata.Class objects.

Result of Declaring Allowed Subclasses

Including a class in the list of AllowedSubclasses does not define that class as a subclass or require you to define the class as a subclass. It just allows the referenced class to be defined as a subclass. Declaring a class as an allowed subclass also does not affect whether this class can itself be subclassed.

A class definition can contain assignments to the AllowedSubclasses attribute that reference classes that are not currently defined or available on the MATLAB path. Any referenced subclass that MATLAB cannot find when loading the class is effectively removed from the list without causing an error or warning. MATLAB remembers the referenced class in case it becomes available at a later point in time.

Note

If MATLAB does not find any of the classes in the allowed subclasses list, the class is effectively Sealed. A sealed class is equivalent to AllowedSubclasses = {}.

Use the matlab.metadata.Class property RestrictsSubclassing to determine if a class is Sealed or specifies AllowedSubclasses.

Define Sealed Hierarchy of Classes

The AllowedSubclasses attribute enables you to define a sealed class hierarchy by sealing the allowed subclasses:

classdef (AllowedSubclasses = {?SubClass1,?SubClass2}) SuperClass
   ...
end

Define the allowed subclasses as Sealed:

classdef (Sealed) SubClass1
   ...
end

classdef (Sealed) SubClass2
   ...
end

Sealed class hierarchies enable you to use the level of abstraction that your design requires while maintaining a closed system of classes.