Class Support for Array-Creation Functions

Extend Array-Creation Functions for Your Class

There are several MATLAB^® functions that create arrays of a specific size and type, such as ones and zeros. User-defined classes can add support for array-creation functions without requiring the use of overloaded method syntax.

Class support for any of the array-creation functions enables you to develop code that you can share with built-in and user-defined data types. For example, the class of the variable x in the following code can be a built-in type during initial development, and then be replaced by a user-defined class that transparently overloads zeros:

cls = class(x);
zArray = zeros(m,n,cls);

Array-creation functions create arrays of a specific type in two ways:

Class name syntax — Specify class name that determines the type of array elements.
Prototype object syntax — Provide a prototype object that the function uses to determine the type and other characteristics of the array elements.

For example:

zArray = zeros(2,3,'uint8');

p = uint8([1 3 5 ; 2 4 6]);
zArray = zeros(2,3,'like',p);

After adding support for these functions to a class named MyClass, you can use similar syntax with that class:

zArray = zeros(2,3,'MyClass');

Or pass an object of your class:

p = MyClass(...);
zArray = zeros(size(p),'like',p);

MATLAB uses these arguments to dispatch to the appropriate method in your class.

Array-Creation Functions That Support Overloading

The following functions support this kind of overloading.

Array-Creation Functions
`ones`
`zeros`
`eye`
`nan` (lowercase)
`inf`
`true`
`false`
`cast`
`rand`
`randn`
`randi`

Which Syntax to Use

To create an array of default objects, which require no input arguments for the constructor, then use the class name syntax.

To create an array of objects with specific property values or if the constructor needs other inputs, use the prototype object to provide this information.

Classes can support both the class name and the prototype object syntax.

You can implement a class name syntax with the true and false functions even though these functions do not support that syntax by default.

Class Name Method Called If Prototype Method Does Not Exist

If your class implements a class name syntax, but does not implement a prototype object syntax for a particular function, you can still call both syntaxes. For example, if you implement a static zeros method only, you can call:

zeros(...,'like',MyClass(...))

In the case in which you call the prototype object syntax, MATLAB first searches for a method named zerosLike. If MATLAB cannot find this method, it calls for the zeros static method.

This feature is useful if you only need the class name to create the array. You do not need to implement both methods to support the complete array-creation function syntax. When you implement only the class name syntax, a call to a prototype object syntax is the same as the call to the class name syntax.

Implement Support for Array-Creation Functions

Use two separate methods to support an array-creation function. One method implements the class name syntax and the other implements the prototype object syntax.

For example, to support the zeros function:

Implement the class name syntax:

zeros(...,'ClassName')

As a Static method:

methods (Static)
   function z = zeros(varargin)
      ...
   end
end

Implement the prototype object syntax:

zeros(...,'like',obj)

As a Hidden method with the char vector 'Like' appended to the name.

methods (Hidden)
   function z = zerosLike(obj,varargin)
      ...
   end
end

How MATLAB Interprets the Function Call

The special support for array-creation functions results from the interpretation of the syntax.

A call to the zeros function of this form:
```
zeros(...,'ClassName')
```
Calls the class static method with this syntax:
```
ClassName.zeros(varargin{1:end-1})
```
A call to the zeros function of this form:
```
zeros(...,'like',obj)
```
Calls the class method with this syntax:
```
zerosLike(obj,varargin{1:end-2})
```

Support All Function Inputs

The input arguments to an array-creation function can include the dimensions of the array the function returns and possibly other arguments. In general, there are three cases that your methods must support:

No dimension input arguments resulting in the return of a scalar. For example:
```
z = zeros('MyClass');
```
One or more dimensions equal to or less than zero, resulting in an empty array. For example:
```
z = zeros(2,0,'MyClass');
```
Any number of valid array dimensions specifying the size of the array. For example:
```
z = zeros(2,3,5,'MyClass');
```

When the array-creation function calls your class method, it passes the input arguments, excluding the class name or the literal 'like' and the object variable to your method. You can implement your methods with these signatures:

zeros(varargin) for “class name” methods
zeros(obj,varargin) for “like prototype object” methods

Sample Class

The Color class represents a color in a specific color space, such as, RGB, HSV, and so on. The discussions in Class Name Method Implementations and Prototype Object Method Implementation use this class as a basis for the overloaded method implementations.

classdef Color
   properties
      ColorValues = [0,0,0]
      ColorSpace = 'RGB'
   end
   methods
      function obj = Color(cSpace,values)
         if nargin > 0
            obj.ColorSpace = cSpace;
            obj.ColorValues = values;
         end
      end
   end
end

Class Name Method Implementations

The zeros function strips the final ClassName char vector and uses it to form the call to the static method in the Color class. The arguments passed to the static method are the array dimension arguments.

Here is an implementation of a zeros method for the Color class. This implementation:

Defines the zeros method as Static (required)
Returns a scalar Color object if the call to zeros has no dimension arguments
Returns an empty array if the call to zeros has any dimensions arguments equal to 0.
Returns an array of default Color objects. Use repmat to create an array of the dimensions specified by the call to zeros.

classdef Color
   ...
   methods (Static)
      function z = zeros(varargin)
         if (nargin == 0)
         % For zeros('Color')
            z = Color;
         elseif any([varargin{:}] <= 0)
         % For zeros with any dimension <= 0   
            z = Color.empty(varargin{:});
         else
         % For zeros(m,n,...,'Color')
         % Use property default values
            z = repmat(Color,varargin{:});
         end
      end
   end
end

The zeros method uses default values for the ColorValues property because these values are appropriate for this application. An implementation of a ones method can set the ColorValues property to [1,1,1], for example.

Suppose that you want to overload the randi function to achieve the following objectives:

Define each ColorValue property as a 1-by-3 array in the range of 1 to a specified maximum value (for example, 1–255).
Accommodate scalar, empty, and multidimensional array sizes.
Return an array of Color objects of the specified dimensions, each with random ColorValues.

classdef Color
   ...
   methods (Static)
      function r = randi(varargin)
         if (nargin == 0)
            % For randi('ClassName')
            r = Color('RGB',randi(255,[1,3]));
         elseif any([varargin{2:end}] <= 0)
            % For randi with any dimension <= 0
            r = Color.empty(varargin{2:end});
         else
            % For randi(max,m,n,...,'ClassName')
            if numel([varargin{:}]) < 2
               error('Not enough input arguments')
            end
            dims = [varargin{2:end}];
            r = zeros(dims,'Color');
            for k = 1:prod(dims)
               r(k) = Color('RGB',randi(varargin{1},[1,3]));
            end
         end
      end
   end
end

Prototype Object Method Implementation

The objective of a method that returns an array of objects that are “like a prototype object” depends on the requirements of the class. For the Color class, the zeroLike method creates objects that have the ColorSpace property value of the prototype object, but the ColorValues are all zero.

Here is an implementation of a zerosLike method for the Color class. This implementation:

Defines the zerosLike method as Hidden
Returns a scalar Color object if the call to the zeros function has no dimension arguments
Returns an empty array if the call to the zeros function has any dimension arguments that are negative or equal to 0.
Returns an array of Color objects of the dimensions specified by the call to the zeros function.

classdef Color
   ...
   methods (Hidden)
      function z = zerosLike(obj,varargin)
         if nargin == 1
            % For zeros('like',obj)
            cSpace = obj.ColorSpace;
            z = Color;
            z.ColorSpace = cSpace;
         elseif  any([varargin{:}] <= 0)
            % For zeros with any dimension <= 0
            z = Color.empty(varargin{:});
         else
            % For zeros(m,n,...,'like',obj)
            if ~isscalar(obj)
               error('Prototype object must be scalar')
            end
            obj = Color(obj.ColorSpace,zeros(1,3,'like',obj.ColorValues));
            z = repmat(obj,varargin{:});
         end
      end
   end
end

Full Class Listing

Here is the Color class definition with the overloaded methods.

Note

In actual practice, the Color class requires error checking, color space conversions, and so on. This overly simplified version illustrates the implementation of the overloaded methods.

classdef Color
   properties
      ColorValues = [0,0,0]
      ColorSpace = 'RGB'
   end
   methods
      function obj = Color(cSpace,values)
         if nargin > 0
            obj.ColorSpace = cSpace;
            obj.ColorValues = values;
         end
      end
   end
   methods (Static)
      function z = zeros(varargin)
         if (nargin == 0)
            % For zeros('ClassName')
            z = Color;
         elseif any([varargin{:}] <= 0)
            % For zeros with any dimension <= 0
            z = Color.empty(varargin{:});
         else
            % For zeros(m,n,...,'ClassName')
            % Use property default values
            z = repmat(Color,varargin{:});
         end
      end
      function r = randi(varargin)
         if (nargin == 0)
            % For randi('ClassName')
            r = Color('RGB',randi(255,[1,3]));
         elseif any([varargin{2:end}] <= 0)
            % For randi with any dimension <= 0
            r = Color.empty(varargin{2:end});
         else
            % For randi(max,m,n,...,'ClassName')
            if numel([varargin{:}]) < 2
               error('Not enough input arguments')
            end
            dims = [varargin{2:end}];
            r = zeros(dims,'Color');
            for k = 1:prod(dims)
               r(k) = Color('RGB',randi(varargin{1},[1,3]));
            end
         end
      end
   end
   methods (Hidden)
      function z = zerosLike(obj,varargin)
         if nargin == 1
            % For zeros('like',obj)
            cSpace = obj.ColorSpace;
            z = Color;
            z.ColorSpace = cSpace;
         elseif  any([varargin{:}] <= 0)
            % For zeros with any dimension <= 0
            z = Color.empty(varargin{:});
         else
            % For zeros(m,n,...,'like',obj)
            if ~isscalar(obj)
               error('Prototype object must be scalar')
            end
            obj = Color(obj.ColorSpace,zeros(1,3,'like',obj.ColorValues));
            z = repmat(obj,varargin{:});
         end
      end
   end
end

Documentation