The rand function creates a uniformly distributed random number.

A = rand(1,5) -0.5;

Validate that array elements are positive.

mustBePositive(A)

Error using mustBePositive (line 14)
Value must be positive.

The result of subtracting 0.5 from the array return by rand can contain negative numbers. When a value is negative, mustBePositive issues an error.

This class restricts the value of Prop1 to positive values.

classdef MyClass
   properties
      Prop1 {mustBePositive}
   end
end

Create an object and assign a value to its property.

obj = MyClass;
obj.Prop1 = 0;

Error setting property 'Prop1' of class 'MyClass':
Value must be positive.

When you assign a value to the property, MATLAB calls mustBePositive with the value being assigned to the property. mustBePositive issues an error because the value 0 is not positive.

function r = mbPositive(A,ix)
    arguments
        A (1,:) {mustBeNumeric}
        ix {mustBePositive, mustBeInteger}
    end
    r = A(ix);
end

Calling the function with a value for ix that does not meet the requirement of mustBePositive results in an error.

A = 1:10;
ix = 0;
r = mbPositive(A,ix);

Error using mbPositive
Invalid input argument at position 2. Value must be positive.