Cannot define a cell array of custom classes in function argument validation

I've a class like the following one:

classdef Session < handle
  %SESSION Summary of this class goes here
  %   Detailed explanation goes here

  properties (Access = private)
    configurations (1, :) cell {mustBeA(configurations, "App.Core.Configuration")}
  end
  
  methods (Access = public)
      function this = Session()
          this.configurations = {}
      end
  end

What I want to do is to declare che configuration property as a unidimensional cell array of a custom class, that can have 0 or more elements.

The problem is that I cannot instantiate it. When I try to do something like:

  s = App.Core.Session()

I obtain the following error

  Error using implicit default value of property 'configurations' of class 'App.Core.Session'. Value must be one of the following types: 'App.Core.Configuration'.

even if the App.Core.Configuration() class exists and it works (I've already unit tested it).

How can I declare correctly my configurations property?

Answer 1

[Sandeep Mishra]

Hi Daniele,

I see that you are trying to implement a cell array of a custom class with function argument validation. You can accomplish this by using either of the following two workarounds:

1. You can define a custom function argument validation by creating a separate validation function, such as the ‘validator’ function in the following example:

function validator(configurations) 
    for i = 1: numel(configurations) 
        config = configurations(i); 
        mustBeA(config{1}, 'Configuration'); 
    end 
end 

Then, you can initialize the ‘configurations’ parameter using this validation function as shown:

properties (Access = private) 
    configurations (1,:) cell {validator} 
end 

2. You can create a validation check when adding a new configuration. Each configuration object will be validated during the addition process in the ‘Session’ class. Below is the implementation of an ‘addConfiguration’ function:

function addConfiguration(this, config) 
    mustBeA(config, 'Configuration');  
    this.configurations{end+1} = config; 
end 

With this approach, no validation is required in the properties section, configuration parameter can be defined as follows:

properties (Access = private) 
    configurations % No direct validation here 
end 

You can now create a ‘Session’ object and add multiple ‘Configuration’ class objects as required:

% Session object creation 
session = Session(); 
% Add configuration objects (config1, config2) to the session 
session.addConfiguration(config1); 
session.addConfiguration(config2);

For more information, refer to the following MathWorks Documentation:

1. Argument validation for cell arrays: https://www.mathworks.com/matlabcentral/answers/2018441-argument-validation-for-cell-arrays#answer_1305481
2. Define Validation Functions: https://in.mathworks.com/help/releases/R2022b/matlab/matlab_prog/argument-validation-functions.html#mw_3e8f7e5b-fbb5-4725-be02-4751c675a92a
3. ‘mustBeA’ function: https://www.mathworks.com/help/releases/R2022b/matlab/ref/mustbea.html

I hope this helps!

[Daniele Lupo]

Thanks. I'll go with the first option, but usually I also check the arguments of function/methods, even if I prefer the arguments method since from the semamtic point of view the intent of the code is clearer:

function addConfiguration(this, config) 
    arguments
        this (1, 1) Session
        config (1, 1) App.Core.Configuration
    end
    this.configurations{end+1} = config; 
end 

Answer 2

[Matt J]

classdef Session < handle
  %SESSION Summary of this class goes here
  %   Detailed explanation goes here

  properties (Access = private)
    configurations (1, :) 
  end
  
  methods (Access = public)
      function this = Session()
          this.configurations = {};
      end
      
      function set.configurations(obj,val)
          
          inputValid=iscell(val);
          if inputValid && ~isempty(val)
            inpuValid = all(  cellfun(  @(z)isa(z,'App.Core.Configuration') , val)   );
          end
          assert(inputValid,"property value must be a cell array of 0 or more Configuration objects")
      end
      
  end

Answer 3

[Steven Lord]

What's the default value for your property?

  properties (Access = private)
    configurations (1, :) cell {mustBeA(configurations, "App.Core.Configuration")}
  end

The code you've written looks like the definition of Prop1 in the first code block on this documentation page. "The property definition does not specify a default value, so MATLAB initializes the property value to an empty double ([])." [Though actually that's not quite correct since you're using class validation as well, so as per the "Specify Valid Default" section on this documentation page, "If you do not specify a default value, MATLAB creates a default value by assigning an empty object of the specified class or by calling the default constructor if size restriction does not allow the use of an empty default value."]

Is that a valid value for your property? Since it is not (it fails the property validation function check), MATLAB correctly throws an error.

Error using implicit default value of property 'configurations' of class 'App.Core.Session'. Value must be one of the following types: 'App.Core.Configuration'.

Depending on the details of what the App.Core.Configurations class actually is, using App.Core.Configuration.empty() along the lines of the Prop4 definition might be an option (and defining configurations to be an array of that class rather than a cell array.) Though if it is a handle object as I suspect it is, see the "Handle Objects as Default Property Values" section on the first documentation page to which I linked.

[Daniele Lupo]

the default value is an empty cell array, I define it in the constructor. this logic works with other object types like the string, but not for my custom class.

[Steven Lord]

configurations = cell(1, 0);
try
    mustBeA(configurations, "App.Core.Configuration")
    fprintf("mustBeA succeeded")
catch ME
    fprintf("mustBeA threw error:\n%s", ME.message)
end

mustBeA threw error:
Value must be one of these types: 'App.Core.Configuration'.

A cell array is not an App.Core.Configuration.

Even if you had a cell array containing an instance of a class, that doesn't make the cell array be an instance of that class. mustBeA checks for an is-a relationship not a contains-a relationship.

g = graph;
try
    mustBeA(g, "graph") % Succeeds, g isa graph
    fprintf("mustBeA succeeded")
catch ME
    fprintf("mustBeA threw error:\n%s", ME.message)
end

mustBeA succeeded

gc = {g};
try
    mustBeA(gc, "graph") % fails, gc isnota graph. gc containsa graph.
    fprintf("mustBeA succeeded")
catch ME
    fprintf("mustBeA threw error:\n%s", ME.message)
end

mustBeA threw error:
Value must be one of these types: 'graph'.

this logic works with other object types like the string, but not for my custom class.

You don't wrap a string in a cell array in your object. If you did you'd have the same contains-a versus is-a problem.

s = "abracadabra"; % is-a
try
    mustBeA(s, "string")
    fprintf("mustBeA succeeded")
catch ME
    fprintf("mustBeA threw error:\n%s", ME.message)
end

mustBeA succeeded

sc = {s}; % contains-a
try
    mustBeA(sc, "string")
    fprintf("mustBeA succeeded")
catch ME
    fprintf("mustBeA threw error:\n%s", ME.message)
end

mustBeA threw error:
Value must be one of these types: 'string'.