Class and Object Creation, forcing argument types

I've made a user defined class, and I want to ensure that the arguments passed to the class constructor are of a certain variable type, and to not create an instance/object unless they are correct variable types. I have the constructor checking to see if it is a certain variable type, displaying some text saying it needs to be a certain type, and returning if it is not, but it still creates an object with empty properties. Anyone know how to prevent this? See my example below

    classdef example

    properties
        exampleProp
    end

    methods
        function exam = example(argument)
             if ~isa(argument,'string)
                   disp('Argument needs to be a string')
                   return
             end
             exampleProp = argument;
        end
    end

My script would call

 a = example(7)

Display my message and say 'Argument needs to be a string' but then i'd get an object with

a = 
    example

     Properties:
         exampleProp = []

But I don't want 'a' in this case. Any info is appreciated.

[Mark]

I just realized my ultimate goal is to not create an object unless the argument is a specific value, e.g. if argument == 'this string' and not if argument == 'that string'.

Thanks

Answer 1

[Matt J]

From Rules for Constructors,

 "Constructors must always return a valid instance of the class."

You cannot have a class constructor returning nothing. Furthermore, any time you have any kind of function call in MATLAB

   a=someFunction(...)

you must return an output argument that can be held in "a". Only function calls that don't request any outputs are permitted to return no outputs, e.g.,

    plot(t,x)

[Mark]

Word. Thanks.