Return false if there is an alternative object currently managed by this variant object, and true otherwise. A variant object can become valueless by exception if the creation of an alternative object exits via an exception, or if it is copied or assigned from another variant object that is valueless by exception. This method differs from the standard in the following way: * constexpr is not implemented This is because no constructors are currently constexpr and there is no way to test the constexpr property of this function.