Subclasses of Built-In Types Without Properties

Specialized Numeric Types

Subclass built-in numeric types to create customized data types that inherit the functionality of the built-in type. Add functionality to that provided by the superclass by implementing class methods. Subclasses without properties store numeric data as the superclass type. If your subclass design does not require properties to store other data, the implementation is simpler because you do not need to define indexing and concatenation methods.

For more information, see Subclasses of MATLAB Built-In Types.

A Class to Manage uint8 Data

This example shows a class derived from the built-in uint8 class. This class simplifies the process of maintaining a collection of intensity image data defined by uint8 values. The basic operations of the class include:

Capability to convert various classes of image data to uint8 to reduce object data storage.
A method to display the intensity images contained in the subclass objects.
Ability to use all the methods supported by uint8 data (for example, size, indexing, reshape, bitshift, cat, fft, arithmetic operators, and so on).

The class data are matrices of intensity image data stored in the superclass part of the subclass object. This approach requires no properties.

The DocUint8 class stores the image data, which converts the data, if necessary:

classdef DocUint8 < uint8
   methods
      function obj = DocUint8(data)
         if nargin == 0
            data = uint8(0);
         end
         obj = obj@uint8(data); % Store data on superclass
      end
      function h = showImage(obj)
         data = uint8(obj);
         figure; colormap(gray(256))
         h = imagesc(data,[0 255]);
         axis image
         brighten(.2)
      end
   end
end

Using the DocUint8 Class

Create DocUint8 Objects

The DocUint8 class provides a method to display all images stored as DocUint8 objects in a consistent way. For example:

cir = imread('circuit.tif');
img1 = DocUint8(cir);
img1.showImage;

Grayscale image of a circuit

Because DocUint8 subclasses uint8, you can use any uint8 methods. For example,

size(img1)

ans =
   280   272

returns the size of the image data.

Indexing Operations

Inherited methods perform indexing operations, but return objects of the same class as the subclass.

Therefore, you can index into the image data and call a subclass method:

showImage(img1(100:200,1:160));

Subscripted reference operations (controlled by the inherited subsref method) return a DocUint8 object.

Grayscale image of a circuit, cropped

You can assign values to indexed elements:

img1(100:120,140:160) = 255;
img1.showImage;

Subscripted assignment operations (controlled by the inherited subsasgn method) return a DocUint8 object.

Grayscale image of a circuit with a white box replacing a portion of the original

Concatenation Operations

Concatenation operations work on DocUint8 objects because this class inherits the uint8 horzcat and vertcat methods, which return a DocUint8 object:

showImage([img1 img1]);

Two copies of a circuit image concatenated horizontally

Data Operations

Methods that operate on data values, such as arithmetic operators, always return an object of the built-in type (not of the subclass type). For example, multiplying DocUint8 objects returns a uint8 object, so calling showImage throws an error:

a = img1.*1.8;
showImage(a);

Check for missing argument or incorrect argument data type in call to function 'showImage'.

To perform operations of this type, implement a subclass method to override the inherited method. The times method implements array (element-by-element) multiplication.

Add this method to the DocUint8 class:

function o = times(obj,val)
   u8 = uint8(obj).*val; 
   o = DocUint8(u8);
end

When you override a uint8 method, MATLAB^® calls the subclass method, not the base class method. The subclass method must:

Call the uint8 times method on the DocUint8 object data.
Construct a new DocUint8 object using the uint8 data.

After adding the times method to the DocUint8 class, the output of multiplication expressions is an object of the DocUint8 class:

showImage(img1.*1.8);

Image of a circuit after values have been multiplied by 1.8