Construct an object given another object of that class

What I want to do can be accomplished with the following :

classdef X methods function obj = X(x) obj.x = x; end

        function y = f(obj,x)
            y = eval([class(x) '(' num2str(-x.x) ')']);
        end
    end
    properties
        x;
    end
end

a = X(2); b = a.f(a);

but the use of eval is ugly and slow. Is there not a better way to get the class of an object from the object itself?

Matthias