An example:

As can been seen from the example, an advantage to an object hierarchy is that no casting between different object types is needed. Yet, access to invalid object fields raises an exception.

In the example the kind field is called the discriminator: For safety its address cannot be taken and assignments to it are restricted: The new value must not lead to a change of the active object branch. Also, when the fields of a particular branch are specified during object construction, the corresponding discriminator value must be specified as a constant expression.

Instead of changing the active object branch, replace the old object in memory with a new one completely:

As a special rule, the discriminator kind can also be bounded using a case statement. If possible values of the discriminator variable in a case statement branch are a subset of discriminator values for the selected object branch, the initialization is considered valid. This analysis only works for immutable discriminators of an ordinal type and disregards elif branches. For discriminator values with a range type, the compiler checks if the entire range of possible values for the discriminator value is valid for the chosen object branch.

A small example: