Scott - I think that you are encountering the different behaviours of using a handle class versus a value class. From comparing handle and value classes,
A value class constructor returns an instance that is associated with the variable to which it is assigned. If you reassign this variable, MATLAB® creates a copy of the original object. If you pass this variable to a function, the function must return the modified object.
A handle class constructor returns a handle object that is a reference to the object created. You can assign the handle object to multiple variables or pass it to functions without causing MATLAB to make a copy of the original object. A function that modifies a handle object passed as an input argument does not need to return the object.
Since matlab.mixin.Heterogeneous is a value class (see handle compatibility section of matlab.mixin.heterogeneous class) then you would have to modify your changeParentVariable to
function [obj] = changeParentVariable(obj,a)
obj.parentVariable = a;
end
and change your call to
test = test.changeParentVariable(3); % or test = changeParentVariable(test,3)
so that the parent variable is updated as
test =
child with properties:
childVariable: 2
parentVariable: 3
