Casts

If your DataBlade® module defines types that are similar or comparable, consider defining casts between the types. You can also define casts from DataBlade module types to built-in types, and from data types in one DataBlade module to data types in another DataBlade module.

Casting values allows the query processing engine, implicitly or explicitly, to change the type of a value and use it as an argument to routines that require the destination type.

In an inheritance hierarchy, casting can provide another mechanism for type conversion. In general, subclasses can be implicitly cast to superclass types. However, downward casts (that is, from supertype to subtype) are not automatically supported because subclasses typically add instance variables not present in the supertype.

Similarly, distinct types can often be cast to their source type. For example, a distinct type called LIRA (representing the currency unit of Italy), based on the MONEY data type, might allow casting to MONEY to allow simple math operations on it. However, you probably do not want to cast MONEY to LIRA; if LIRA has only the properties of MONEY, it is not a required type.

Casts can be confusing if overused. Implicit casts hide an important fact from users-that data can be lost during type conversion. Explicit casts, which users must specify in queries, do not have this problem.

Use casts only where necessary. Be sparing in the casts you supply to users, and be sure that you understand the circumstances under which you expect casts to be used.