I'm guessing main class is a parent class and sum and diff are two child classes descended from it.
If that is right, the best approach is to write a separate function that returns whichever class the user wants, e.g.,
function myClass = ClassSelector(a,b,type)
switch type
case 'sum'
myClass = Sum(a,b);
case 'diff'
myClass = Diff(a,b);
end
end
In good OO design, the parent class should not know anything about the different child classes. For example, you don't want to have to change the parent class if you add a new type of child (e.g, Product(a,b)).
