UDTs

User-defined types (UDTs) are data types created by programmers to represent domain concepts that are not covered by the language's built-in primitives. UDTs provide a way to model real objects, units, or values with tailored structure and constraints. They can be scalar, composite, or alias types, depending on the language or system.

In programming languages, UDTs include classes, structures, enums, unions, or type aliases that encapsulate data and, in some languages, behavior. They enable stronger type checking, clearer interfaces, and code reuse. They may be defined with fields and methods, and can be serialized or mapped to storage, interchanged via APIs, or bridged between languages via bindings. Some languages support advanced features for UDTs, such as generics, inheritance, or interface contracts.

In databases, UDTs allow storing domain-specific values consistently across tables and procedures. Examples include domain or alias types in SQL Server, composite types or enums in PostgreSQL, and user-defined types in Oracle or other systems. A composite UDT might define an address with fields for street, city, and postal code, while a domain enforces constraints like a pattern or range (for example, a postal code format). UDTs can improve data integrity, readability, and cross-cutting validation, but can complicate migrations and cross-system portability.

Design considerations include naming, versioning, compatibility, serialization format (JSON, XML), and performance implications. When used across APIs or storage backends, careful mapping between the UDT and primitive types is required.