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Union type
open-in-new
<p>In <a href="/facts/Computer_science/lN3pEyPz">computer science</a>, a union is a <a href="/facts/Value_(computer_science)/QgQPbewf">value</a> that may have any of multiple representations or formats within the same area of <a href="/facts/Computer_memory/hQKVQ51J">memory</a>; that consists of a <a href="/facts/Variable_(computer_science)/VoCKtwml">variable</a> that may hold such a <a href="/facts/Data_structure/yNsNvL1I">data structure</a>. Some <a href="/facts/Programming_languages/cWTYbgWa">programming languages</a> support a union type for such a <a href="/facts/Data_type/fcx5hgVL">data type</a>. 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 <a href="/facts/Record_(computer_science)/1fvSgenB">record</a>, which could be defined to contain both a float <i>and</i> an integer; a union would hold only one at a time. </p><p>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 <a href="/facts/Byte/BNHUp9Qq">bytes</a> or <a href="/facts/Word_(computer_architecture)/2e5umRaj">words</a> of memory), but the value can be treated as one of several <a href="/facts/Abstract_data_type/BaVGSHNP">abstract data types</a>, having the type of the value that was last written to the memory area. </p><p>In <a href="/facts/Type_theory/xj0xzVqI">type theory</a>, a union has a <a href="/facts/Sum_type/uMFenvCQ">sum type</a>; this corresponds to <a href="/facts/Disjoint_union/x9QSfkr5">disjoint union</a> in mathematics. </p><p>Depending on the language and type, a union value may be used in some operations, such as <a href="/facts/Assignment_statement/kLevAoyJ">assignment</a> 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 <a href="/facts/Tagged_union/uMFenvCQ">tagged union</a>. </p>