Creating User-Defined Casts

When the database server does not provide built-in casts to perform conversions between two data types, you can create a user-defined cast to handle the data type conversion. User-defined casts are typically used to provide data type conversions for the following extended data types:

• Opaque data types. Developers of opaque data types must define casts to handle conversions between the internal/external representations of the opaque data type. For information about how to create and register casts for opaque data types, see IBM Informix: User-Defined Routines and Data Types Developer's Guide.
• Distinct data types. You cannot directly compare a distinct data type to its source type. However, Dynamic Server automatically registers explicit casts from the distinct type to the source type and vice versa. A distinct type does not inherit the casts that are defined on its source type. In addition, the user-defined casts that you might define on a distinct type are not available to its source type. For more information and examples that show how to create and use casts on distinct types, see Creating Cast Functions for User-Defined Casts.
• Named row types. In most cases, you can explicitly cast a named row type to another row-type value without having to create the cast. However, to convert between values of a named row type and some other data type, you must first create the cast to handle the conversion.

For an example of how to create and use a user-defined cast, see An Example of Casting Between Distinct Data Types. For the syntax of the CREATE CAST statement, see the IBM Informix: Guide to SQL Syntax.