Class property validator reports an error if no default value is set

I have the following class defintion which is using a validator function and a custom constructor with an input parser.

classdef MyClass

properties

hWriteFunc(1,1) function_handle

end

methods

function obj = MyClass(varargin)

% Parse input vars

p = inputParser;

addParameter(p,"hWriteFunc",@defaultWriteFunc);

parse(p,varargin{:});

% Set properties

obj.hWriteFunc = p.Results.hWriteFunc;

end

end

end

function defaultWriteFunc(~)

end

When creating an instance of that class by executing

bar = MyClass();

I get the following error:

>> Error defining property 'hWriteFunc' of class 'MyClass'. Unable to construct default object of class function_handle.

Obviously, I can set a default property to avoid the error:

classdef MyClass

properties

hWriteFunc(1,1) function_handle = @defaultWriteFunc;

end

methods

function obj = MyClass(varargin)

% Parse input vars

p = inputParser;

addParameter(p,"hWriteFunc",@defaultWriteFunc);

parse(p,varargin{:});

% Set properties

obj.hWriteFunc = p.Results.hWriteFunc;

end

end

end

function defaultWriteFunc(~)

end

The problem is I now have two points in the class defintion where I need to set the default value (in the properties section and in the custom constructor). I would consider this a bad practise because I am defining the same value on two separate locations and would like to avoid this.

How am I able to define the default value at only one location while still using a validator and the input parser?

Answer 1

[Michael]

With all your help I finally find a satisfying solution!

In the end it is pretty easy.

1. Set default values in the properties section of the class definition
2. Define the NameValue arguments in the arguments section of the constructor
3. Only set the property if the NameValue pair is present in the NameValue struct

By doing so you get rid of the need to redundantly define the default value by simultaneously not cluttering the code to much.

classdef ErrorReportHandler < handle

properties

emailAddress string {mustBeTextScalar} = ""

hWriteFunc {mustBeA(hWriteFunc,"function_handle")} = @(x) []

nDebugLogHistory (1,1) double {mustBeFinite} = 10

end

methods

function obj = ErrorReportHandler(NameValueArgs)

% Use Name-Value arguments to set properties

arguments

NameValueArgs.emailAddress

NameValueArgs.hWriteFunc

NameValueArgs.nDebugLogHistory

end

% Set name value pair properties

assignNameValuePair(obj,NameValueArgs,"emailAddress"); % Use this syntax if you have handle class

obj = assignNameValuePair(obj,NameValueArgs,"hWriteFunc"); % Use this syntax if you have a handle class or a value class

obj = assignNameValuePair(obj,NameValueArgs,"nDebugLogHistory");

end

end

end

function obj = assignNameValuePair(obj,NameValueArgs,Name)

if isfield(NameValueArgs,Name)

obj.(Name) = NameValueArgs.(Name);

end

end

PS: If the class is a handle class (like it is here) you can even get rid of the obj = ... part of the property assignment. I just left it there for the others using this on value classes.

Thanks to all of you for your great support on the way to the solution!

Answer 2

[Aquatris]

You can write your own validation function

classdef MyClass
   properties
      hWriteFunc(1,1) {mustBeFunctionHandle} = @defaultWriteFunc
   end
   
   methods
      function obj = MyClass(varargin)
          if nargin>0
             % if user calls the constructor with a function handle
            obj.hWriteFunc = varargin{1};
         end
      end
   end
end
function mustBeFunctionHandle(x)
if ~isa(x,'function_handle')
   error("Value of Data property must be fHandle")
end
end

[Michael]

Thank you for your answer.

I am aware of the possibility to write custom validation functions.

The problem is though, I need to rewrite dozens of valildation functions because the same issue occurs also for other data types and validation functions. Even worse I would just rewrite the built-in validation functions to cope with this initialization behavior. In that case I would rather have "double" definition than dozens of explicit validatiion functions that clutter my code.

So if you have another suggestion that does not rely on bypassing the problem but rather on using the Matlab class initialization in a more elegant way I would really appreciate it.

[Aquatris]

Then I think here is your solution, for more validation function take a look here:

classdef MyClass
   properties 
       hWriteFunc(1,1) {mustBeA(hWriteFunc,'function_handle')} = @defaultWriteFunc
   end
   
   methods
      function obj = MyClass(varargin)
          if nargin>0
             % if user calls the constructor with a function handle
            obj.hWriteFunc = varargin{1};
         end
      end
   end
end

[Michael]

This comes quite close together with the answer from Matt J. Thank you!

Answer 3

[Catalytic]

classdef MyClass
   properties
        WriteFunc
   end
    
    methods
        function obj = MyClass(varargin)
            % Parse input vars
            p = inputParser;
            addParameter(p,"hWriteFunc",@defaultWriteFunc);
            parse(p,varargin{:});
    
            % Set properties
            obj.hWriteFunc = p.Results.hWriteFunc;
        end
        
        function obj=set.WriteFunc(obj,val)
           arguments
                obj
                val {mustBeA(val,["double","function_handle"])}
           end
           if isnumeric(val), assert( isempty(val) ,'Must be function handle or []') ;end
           obj.WriteFunc=val;
        end
    end
end

[Michael]

This is exactly the solution I was looking for. Based on your hint with the set method I also dug a bit deeper into the documentation of Property Get and Set Methods and found this:

• MATLAB does not call set methods when it assigns default values to the properties during initialization of an object. However, setting property values in the constructor does call set methods.

This is exaclty the behavior I intended.

[Michael]

Below you can find how I implemented your suggestions in case of an error handler (I removed the actual handling function for the sake of shortness here).

I have to admit it is a bit lengthy but at least unambiguous.

Using the set-method prevented the error that occured during the creation of the object.

Is there any shorter solution that allows to achieve the same behavior without having to define all set methods?

classdef ErrorReportHandler < handle

properties

emailAddress % Email address that should be used to send the debug log to in case of an error

emailSubject % Subject of the email template

hWriteFunc % Handle to function that is executed with the exception identifier as a string input argument

nDebugLogHistory % Number of debug log files to keep: -1: unlimited; 0: do not save any debug log files; (int > 0): number of debug log files to keep

end

methods

function obj = ErrorReportHandler(NameValueArgs)

% Use Name-Value arguments to set properties

arguments

NameValueArgs.emailAddress = ""

NameValueArgs.emailSubject = "Debuglog"

NameValueArgs.hWriteFunc = @(x) []

NameValueArgs.nDebugLogHistory = 10

end

% Set properties

obj.emailAddress = NameValueArgs.emailAddress;

obj.emailSubject = NameValueArgs.emailSubject;

obj.hWriteFunc = NameValueArgs.hWriteFunc;

obj.nDebugLogHistory = NameValueArgs.nDebugLogHistory;

end

function set.emailAddress(obj,emailAddress)

arguments

obj

emailAddress string {mustBeValidEmail}

end

obj.emailAddress = emailAddress;

end

function set.emailSubject(obj,emailSubject)

arguments

obj

emailSubject string {mustBeTextScalar}

end

obj.emailSubject = emailSubject;

end

function set.hWriteFunc(obj,hWriteFunc)

arguments

obj

hWriteFunc {mustBeA(hWriteFunc,"function_handle")}

end

obj.hWriteFunc = hWriteFunc;

end

function set.nDebugLogHistory(obj,nDebugLogHistory)

arguments

obj

nDebugLogHistory (1,1) double {mustBeFinite}

end

obj.nDebugLogHistory = nDebugLogHistory;

end

% Removed in this example for the sake of shortness

% reportError(obj,hFigure,ME,data)

end

end

function mustBeValidEmail(emailaddress)

assert(validateemail(emailAddress) || string(emailAddress) == "","Validation:invalidEmailAddress",string(emailAddress) + " is not a valid email address.");

end

% Simplified in this example for the sake of shortness

function valid = validateemail(~)

valid = true;

end

dummyERH = ErrorReportHandler();

Answer 4

[Matt J]

I have the following class defintion which is using a validator function

You're not using a property validation function anywhere that I can see. Property validation functions start with "mustBe", e.g., mustBeReal.

You do have a converter to type "function_handle", which I don't think you really need, but if you really want it there, just define any old function handle for the property default, e.g.,

classdef  MyClass

    properties
        hWriteFunc function_handle = @() []
    end

end