Union type

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In computer science, a union is a value that may have any of multiple representations or formats within the same area of memory; that consists of a variable that may hold such a data structure. Some programming languages support a union type for such a data type. In other words, a union type specifies the permitted types that may be stored in its instances, e.g., float and integer. In contrast with a record, which could be defined to contain both a float and an integer; a union would hold only one at a time.

A union can be pictured as a chunk of memory that is used to store variables of different data types. Once a new value is assigned to a field, the existing data is overwritten with the new data. The memory area storing the value has no intrinsic type (other than just bytes or words of memory), but the value can be treated as one of several abstract data types, having the type of the value that was last written to the memory area.

In type theory, a union has a sum type; this corresponds to disjoint union in mathematics.

Depending on the language and type, a union value may be used in some operations, such as assignment and comparison for equality, without knowing its specific type. Other operations may require that knowledge, either by some external information, or by the use of a tagged union.