Hi Jim,
In MATLAB, when you define an abstract class, you are essentially creating a template for other classes to follow. This template specifies methods that must be implemented by any non-abstract subclasses. However, because an abstract class is only a template, you cannot create instances of it directly.
The error message you're seeing is due to MATLAB requiring a concrete class for property validation when the property is immutable. Immutable properties in MATLAB must be assigned a value when the object is constructed and cannot be changed afterward. MATLAB's type system expects that any property, including immutable ones, has a valid default value even before the constructor is called, which ensures that all properties have valid types at all times.
When you declare a property of an abstract class type without providing a default value, MATLAB does not have a concrete class to instantiate for the default value. Since abstract classes cannot be instantiated, MATLAB cannot assume a zero-element array as the default value for an abstract class property, unlike with non-abstract classes, where it can create a zero-element array by default.
Your suggestion to allow zero element arrays of abstract classes could be a potential solution, but it might conflict with MATLAB's type safety rules, as it would represent an array of a class that cannot be instantiated.
To resolve the issue, you can provide a default value for the fruit property in the top_t class that is a concrete instance of a non-abstract subclass of fruit_t, or you can modify the design to not use an abstract class for the property if the property needs to be immutable and no suitable default can be provided.
Here's a modified version of your code that provides a default value for the fruit property:
classdef top_t < handle
properties (SetAccess = immutable)
fruit fruit_t = banana_t(); % Providing a default value that is a concrete instance
end
methods
function e = top_t(fruitInstance)
if nargin > 0 % Check if a constructor argument was provided
e.fruit = fruitInstance;
end
% If no argument was provided, the default value banana_t() is used
end
end
end
classdef fruit_t < handle
methods (Abstract)
y = foo(x);
end
end
classdef banana_t < fruit_t
methods
function y = foo(e, x)
y = 2*x;
end
end
end
In this code, the fruit property is given a default value of banana_t(), which is a concrete instance of a subclass of fruit_t. When creating an instance of top_t, you can optionally pass a different fruit_t subclass instance to the constructor to override the default value. If no argument is provided, the default banana_t instance is used. This approach satisfies MATLAB's requirement for a default value for immutable properties.
