47:
1031:, which is still accepted as a synonym for union by several compilers. The union declaration is similar to the structure definition, where elements at the same level within the union declaration occupy the same storage. Elements of the union can be any data type, including structures and array. Here vers_num and vers_bytes occupy the same storage locations.
770:, there are two ways to create unions. One is the standard way through a variant record. The second is a nonstandard means of declaring a variable as absolute, meaning it is placed at the same memory location as another variable or at an absolute address. While all Pascal compilers support variant records, only some support absolute variables.
428:, and as a non-standard extension in many compilers, unions can also be anonymous. Their data members do not need to be referenced, are instead accessed directly. They have some restrictions as opposed to traditional unions: in C11, they must be a member of another structure or union, and in C++, they can not have
2170:
The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol's adherents would approve of. The central notion I captured from Algol was a type structure based on atomic types (including structures), composed into arrays,
264:
However, one useful programming function of unions is to map smaller data elements to larger ones for easier manipulation. A data structure consisting, for example, of 4 bytes and a 32-bit integer, can form a union with an unsigned 64-bit integer, and thus be more readily accessed for purposes of
578:
attribute is available for union types. Types contained in the union can be converted transparently to the union type itself in a function call, provided that all types have the same size. It is mainly intended for function with multiple parameter interfaces, a use necessitated by early Unix
281:
has tagged unions, and uses a case clause to distinguish and extract the constituent type at runtime. A union containing another union is treated as the set of all its constituent possibilities, and if the context requires it a union is automatically coerced into the wider union. A union can
260:
of its constituting types, since it can take on any value any of its fields can. Also, because a mathematical union discards duplicates, if more than one field of the union can take on a single common value, it is impossible to tell from the value alone which field was last written.
388:. However, there is no checking of types, so it is up to the programmer to be sure that the proper fields are accessed in different contexts. The relevant field of a union variable is typically determined by the state of other variables, possibly in an enclosing struct.
198:
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
714:
over the first item, the two items share the same address in memory, and therefore share the same underlying data bytes. The first item interprets the two data bytes as a binary value, while the second item interprets the bytes as character values.
1715:
comparison for complex data types. Types with overlapping usage (e.g. a slice method exists on both strings and arrays, the plus operator works on both strings and numbers) don't need additional narrowing to use these features.
373:) also allows for a data member to be any type that has a full-fledged constructor/destructor and/or copy constructor, or a non-trivial copy assignment operator. For example, it is possible to have the standard C++
369:), except that each data member is located at the same memory address. The data members, as in structures, need not be primitive values, and in fact may be structures or even other unions. C++ (since
435:
Simply omitting the class-name portion of the syntax does not make a union an anonymous union. For a union to qualify as an anonymous union, the declaration must not declare an object. Example:
852:
In the first example, each of the elements of the array B maps to one of the specific bytes of the variable A. In the second example, the variable C is assigned to the exact machine address 0.
405:
Structure and union specifiers have the same form. The size of a union is sufficient to contain the largest of its members. The value of at most one of the members can be stored in a union
1074:
An alternative to a union declaration is the DEFINED attribute, which allows alternative declarations of storage, however the data types of the base and defined variables must match.
409:
at any time. A pointer to a union object, suitably converted, points to each of its members (or if a member is a bit-field, then to the unit in which it resides), and vice versa.
595:(66 level) keyword, which effectively maps a second alphanumeric data item on top of the same memory location as a preceding data item. In the example code below, data item
1095:
in most other languages, enum variants in Rust can contain additional data in the form of a tuple or struct, making them tagged unions rather than simple enumerated types.
397:, by assigning to one field of a union and reading from another, as is done in code which depends on the raw representation of the values. A practical example is the
233:
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
380:
The primary use of a union is allowing access to a common location by different data types, for example hardware input/output access, bitfield and word sharing, or
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function or block, as the compiler cannot guarantee that the data in the union will be valid for the type of the field; if this is not the case, it will result in
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Union types were introduced in PHP 8.0. The values are implicitly "tagged" with a type by the language, and may be retrieved by "gettype()".
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Because of the limitations of their use, untagged unions are generally only provided in untyped languages or in a type-unsafe way (as in
2510:
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Union types are supported in TypeScript. The values are implicitly "tagged" with a type by the language, and may be retrieved using a
1110:
attribute will be laid out in memory exactly like the equivalent union in C. Reading the fields of a union can only be done within an
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pointers (references), and functions (procedures). Algol 68's concept of unions and casts also had an influence that appeared later.
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The syntax of the C/C++ union type and the notion of casts was derived from ALGOL 68, though in an untagged form.
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Support for typing was introduced in Python 3.5. The new syntax for union types were introduced in Python 3.10.
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179:. In other words, a union type specifies the permitted types that may be stored in its instances, e.g.,
168:
2241:
79:
249:). They have the advantage over simple tagged unions of not requiring space to store a data type tag.
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Enterprise PL/I for z/OS PL/I for AIX IBM Developer for z
Systems PL/I for windows Language Reference
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In the following example, a record has variants, some of which share the same location as others:
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implements both tagged and untagged unions. In Rust, tagged unions are implemented using the
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keyword. The memory layout of unions in Rust is undefined by default, but a union with the
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Depending on the language and type, a union value may be used in some operations, such as
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The name "union" stems from the type's formal definition. If a type is considered as the
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is defined as a 2-byte binary integer containing a version number. A second data item
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if the data in the union is not valid as the type of the field, and thus requires an
2122:// This results in undefined behavior, as the value stored in x is not a valid bool.
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explicitly contain no value, which can be distinguished at runtime. An example is:
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of all values that that type can take on, a union type is simply the mathematical
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that may have any of multiple representations or formats within the same area of
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is defined as a two-character alphanumeric variable. Since the second item is
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Data type that allows for values that are one of multiple different data types
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A structure can also be a member of a union, as the following example shows:
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For the purposes of this example, the following are all integer types: a
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is defined as a group containing another group and a numeric data item.
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One common C programming idiom uses unions to perform what C++ calls a
1367:(which in turn contains three members), and an integer variable named
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2092:// This will set y to 10, and does not result in undefined behavior.
362:
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1763:// types that support the same operations don't need narrowing
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607:, treating the data bytes continued within it as character data.
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1024:
365:, untagged unions are expressed nearly exactly like structures (
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Unions may occur within structures and arrays, and vice versa:
591:, union data items are defined in two ways. The first uses the
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2421:"typing — Support for type hints — Python 3.9.7 documentation"
399:
method of computing square roots using the IEEE representation
40:
2926:
788:
The following example shows the non-standard absolute form:
207:
of memory), but the value can be treated as one of several
574:
In compilers such as GCC, Clang, and IBM XL C for AIX, a
401:. This is not, however, a safe use of unions in general.
2025:
Reading from the fields of an untagged union results in
414:
ANSI/ISO 9899:1990 (the ANSI C standard) Section 6.5.2.1
1359:), which contains two members, a structure (tagged as
698:
The second way to define a union type is by using the
2299:"How Rust Implements Tagged Unions - Pat Shaughnessy"
603:
is defined as an alphanumeric data item that renames
1923:
keyword, and can contain tuple and struct variants:
1475:
and the first character of string sval by either of
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1981:
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195:an integer; a union would hold only one at a time.
71:. Unsourced material may be challenged and removed.
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191:, which could be defined to contain both a float
2493:Kernighan, Brian W.; Ritchie, Dennis M. (1978).
518:"Hexadecimal representation of 3.14f:"
2446:"PEP 604 -- Allow writing union types as X | Y"
566:definitions to provide a sense of namespacing.
403:
702:keyword. In the example code below, data item
2942:
2572:
1098:Rust also supports untagged unions using the
8:
2548:, differences between union & structure
2243:IBM System/360 PL/I Language Specifications
2223:"Common Type Attributes: transparent_union"
2949:
2935:
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2552:Difference between struct and union in C++
2471:"Handbook - Unions and Intersection Types"
2184:"6.63 Unnamed Structure and Union Fields"
2163:
579:extensions and later re-standardisation.
131:Learn how and when to remove this message
2266:
2264:
2536:, a type-safe alternative to C++ unions
2501:(1st ed.). Prentice Hall. p.
2227:Using the GNU Compiler Collection (GCC)
2135:
269:Unions in various programming languages
2540:MSDN: Classes, Structures & Unions
562:Anonymous unions are also useful in C
384:. Unions can also provide low-level
339:): print(("void:", "EMPTY")),
7:
69:adding citations to reliable sources
2148:"The Development of the C Language"
2347:"Type layout - The Rust Reference"
2323:"Union types - The Rust Reference"
1471:The number ival is referred to as
1027:the original term for a union was
25:
1711:call for primitive values and an
1980:Untagged unions in Rust use the
1826:// distinct types need narrowing
1351:This example defines a variable
488:// Assumes float is 32 bits wide
45:
335:s): print(("string:", s)), (
331:i): print(("int:", i)), (
327:r): print(("real:", r)), (
56:needs additional citations for
2240:IBM Corporation (March 1968).
1917:Tagged unions in Rust use the
1:
3015:Arbitrary-precision or bignum
2371:"Unions - The Rust Reference"
1133:In C and C++, the syntax is:
2271:IBM Corporation (Dec 2017).
2903:Directed acyclic word graph
2669:Double-ended priority queue
3518:
2497:The C Programming Language
29:
3356:Strongly typed identifier
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2542:, for examples and syntax
3502:C (programming language)
2635:Retrieval Data Structure
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377:as a member of a union.
30:Not to be confused with
18:Union (computer science)
3431:Parametric polymorphism
2916:List of data structures
2893:Binary decision diagram
777:consists of 8 bits, a
2898:Directed acyclic graph
2475:www.typescriptlang.org
2396:"PHP 8.0: Union Types"
2205:"CUnionsForNamespaces"
1355:as a union (tagged as
432:or access specifiers.
417:
222:; this corresponds to
2165:10.1145/155360.155580
1901:// has to be a number
187:. In contrast with a
169:programming languages
163:that may hold such a
159:; that consists of a
3497:Composite data types
2764:Unrolled linked list
2394:Karunaratne, Ayesh.
2203:Siebenmann., Chris.
1889:// do something else
65:improve this article
3436:Primitive data type
3341:Recursive data type
3194:Algebraic data type
3070:Quadruple precision
2812:Self-balancing tree
2152:ACM SIGPLAN Notices
1069:pic '(2)A';
781:is 16 bits, and an
209:abstract data types
3399:Abstract data type
3080:Extended precision
3039:Reduced precision
2792:Binary search tree
2657:Double-ended queue
2303:patshaughnessy.net
2144:Ritchie, Dennis M.
2027:undefined behavior
1880:"string"
1784:dependsOnParameter
1124:Syntax and example
1118:undefined behavior
343:print(("?:", n))
315:n := "abc";
32:Union (set theory)
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3211:Associative array
3075:Octuple precision
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2726:Hashed array tree
2625:Associative array
2375:doc.rust-lang.org
2351:doc.rust-lang.org
2327:doc.rust-lang.org
645:PERSON-NAME-FIRST
576:transparent_union
570:Transparent union
141:
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16:(Redirected from
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3336:Opaque data type
3268:Record or Struct
3065:Double precision
3060:Single precision
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265:comparison etc.
226:in mathematics.
218:, a union has a
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145:computer science
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2425:docs.python.org
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1850:// do something
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699:
696:
695:
692:
689:
686:
683:
680:
677:
674:
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665:
662:
659:
657:PERSON-NAME-MID
656:
653:
650:
647:
644:
641:
638:
635:
632:
629:
626:
623:
620:
617:
614:
611:
604:
600:
596:
592:
585:
572:
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541:
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469:
466:
463:
460:
457:
454:
451:
449:<cstdint>
448:
445:
442:
439:
422:
420:Anonymous union
416:
413:
392:
355:
347:
276:
271:
243:
241:Untagged unions
184:
180:
137:
126:
120:
117:
74:
72:
62:
50:
39:
28:
23:
22:
15:
12:
11:
5:
3515:
3513:
3505:
3504:
3499:
3494:
3484:
3483:
3477:
3476:
3474:
3473:
3468:
3463:
3458:
3453:
3448:
3443:
3438:
3433:
3428:
3427:
3426:
3416:
3411:
3409:Data structure
3406:
3401:
3395:
3393:
3387:
3386:
3384:
3383:
3378:
3373:
3368:
3363:
3358:
3353:
3348:
3343:
3338:
3333:
3328:
3323:
3318:
3313:
3308:
3302:
3300:
3296:
3295:
3293:
3292:
3291:
3290:
3280:
3275:
3270:
3265:
3260:
3255:
3254:
3253:
3243:
3238:
3233:
3228:
3223:
3218:
3213:
3208:
3203:
3202:
3201:
3190:
3188:
3182:
3181:
3179:
3178:
3177:
3176:
3166:
3160:
3158:
3152:
3151:
3149:
3148:
3143:
3142:
3141:
3136:
3125:
3123:
3117:
3116:
3114:
3113:
3108:
3103:
3102:
3101:
3091:
3090:
3089:
3088:
3087:
3077:
3072:
3067:
3062:
3057:
3056:
3055:
3050:
3048:Half precision
3045:
3035:Floating point
3032:
3027:
3022:
3017:
3011:
3009:
3005:
3004:
3002:
3001:
2996:
2991:
2986:
2981:
2976:
2970:
2968:
2962:
2961:
2956:
2954:
2953:
2946:
2939:
2931:
2922:
2921:
2919:
2918:
2912:
2909:
2908:
2906:
2905:
2900:
2895:
2889:
2887:
2881:
2880:
2878:
2877:
2876:
2875:
2865:
2864:
2863:
2861:Hilbert R-tree
2858:
2853:
2843:
2842:
2841:
2839:Fibonacci heap
2836:
2831:
2821:
2820:
2819:
2814:
2809:
2807:Red–black tree
2804:
2799:
2789:
2783:
2781:
2775:
2774:
2772:
2771:
2766:
2761:
2756:
2751:
2745:
2743:
2737:
2736:
2734:
2733:
2728:
2723:
2718:
2713:
2708:
2702:
2700:
2694:
2693:
2691:
2690:
2689:
2688:
2683:
2673:
2672:
2671:
2664:Priority queue
2661:
2660:
2659:
2649:
2644:
2639:
2638:
2637:
2632:
2621:
2619:
2613:
2612:
2610:
2609:
2604:
2598:
2596:
2592:
2591:
2586:
2584:
2583:
2576:
2569:
2561:
2555:
2554:
2549:
2543:
2537:
2534:boost::variant
2529:
2528:External links
2526:
2525:
2524:
2512:978-0131101630
2511:
2488:
2487:
2462:
2437:
2412:
2386:
2362:
2338:
2314:
2290:
2260:
2232:
2214:
2195:
2175:
2158:(3): 201–208.
2146:(March 1993).
2134:
2132:
2129:
2038:
1989:
1926:
1914:
1911:
1719:
1704:
1701:
1631:square_and_add
1608:
1602:
1599:
1494:
1488:
1485:
1477:*symtab.u.sval
1377:
1275:
1136:
1130:
1127:
1125:
1122:
1079:
1076:
1034:
1020:
1017:
858:
791:
763:
760:
718:
675:PACKED-DECIMAL
610:
584:
581:
571:
568:
438:
421:
418:
411:
354:
351:
284:
275:
272:
270:
267:
242:
239:
224:disjoint union
165:data structure
139:
138:
53:
51:
44:
26:
24:
14:
13:
10:
9:
6:
4:
3:
2:
3514:
3503:
3500:
3498:
3495:
3493:
3490:
3489:
3487:
3472:
3469:
3467:
3464:
3462:
3459:
3457:
3454:
3452:
3449:
3447:
3444:
3442:
3439:
3437:
3434:
3432:
3429:
3425:
3422:
3421:
3420:
3417:
3415:
3412:
3410:
3407:
3405:
3402:
3400:
3397:
3396:
3394:
3388:
3382:
3379:
3377:
3374:
3372:
3369:
3367:
3364:
3362:
3359:
3357:
3354:
3352:
3349:
3347:
3344:
3342:
3339:
3337:
3334:
3332:
3331:Function type
3329:
3327:
3324:
3322:
3319:
3317:
3314:
3312:
3309:
3307:
3304:
3303:
3301:
3297:
3289:
3286:
3285:
3284:
3281:
3279:
3276:
3274:
3271:
3269:
3266:
3264:
3261:
3259:
3256:
3252:
3249:
3248:
3247:
3244:
3242:
3239:
3237:
3234:
3232:
3229:
3227:
3224:
3222:
3219:
3217:
3214:
3212:
3209:
3207:
3204:
3200:
3197:
3196:
3195:
3192:
3191:
3189:
3187:
3183:
3175:
3172:
3171:
3170:
3167:
3165:
3162:
3161:
3159:
3157:
3153:
3147:
3144:
3140:
3137:
3135:
3132:
3131:
3130:
3127:
3126:
3124:
3122:
3118:
3112:
3109:
3107:
3104:
3100:
3097:
3096:
3095:
3092:
3086:
3083:
3082:
3081:
3078:
3076:
3073:
3071:
3068:
3066:
3063:
3061:
3058:
3054:
3051:
3049:
3046:
3044:
3041:
3040:
3038:
3037:
3036:
3033:
3031:
3028:
3026:
3023:
3021:
3018:
3016:
3013:
3012:
3010:
3006:
3000:
2997:
2995:
2992:
2990:
2987:
2985:
2982:
2980:
2977:
2975:
2972:
2971:
2969:
2967:
2966:Uninterpreted
2963:
2959:
2952:
2947:
2945:
2940:
2938:
2933:
2932:
2929:
2917:
2914:
2913:
2910:
2904:
2901:
2899:
2896:
2894:
2891:
2890:
2888:
2886:
2882:
2874:
2871:
2870:
2869:
2866:
2862:
2859:
2857:
2854:
2852:
2849:
2848:
2847:
2844:
2840:
2837:
2835:
2834:Binomial heap
2832:
2830:
2827:
2826:
2825:
2822:
2818:
2815:
2813:
2810:
2808:
2805:
2803:
2800:
2798:
2795:
2794:
2793:
2790:
2788:
2785:
2784:
2782:
2780:
2776:
2770:
2767:
2765:
2762:
2760:
2757:
2755:
2752:
2750:
2747:
2746:
2744:
2742:
2738:
2732:
2731:Sparse matrix
2729:
2727:
2724:
2722:
2719:
2717:
2716:Dynamic array
2714:
2712:
2709:
2707:
2704:
2703:
2701:
2699:
2695:
2687:
2684:
2682:
2679:
2678:
2677:
2674:
2670:
2667:
2666:
2665:
2662:
2658:
2655:
2654:
2653:
2650:
2648:
2645:
2643:
2640:
2636:
2633:
2631:
2628:
2627:
2626:
2623:
2622:
2620:
2618:
2614:
2608:
2605:
2603:
2600:
2599:
2597:
2593:
2589:
2582:
2577:
2575:
2570:
2568:
2563:
2562:
2559:
2553:
2550:
2547:
2544:
2541:
2538:
2535:
2532:
2531:
2527:
2514:
2508:
2504:
2499:
2498:
2491:
2490:
2476:
2472:
2466:
2463:
2451:
2447:
2441:
2438:
2426:
2422:
2416:
2413:
2401:
2397:
2390:
2387:
2376:
2372:
2366:
2363:
2352:
2348:
2342:
2339:
2328:
2324:
2318:
2315:
2304:
2300:
2294:
2291:
2276:
2275:
2267:
2265:
2261:
2245:
2244:
2236:
2233:
2228:
2224:
2218:
2215:
2210:
2206:
2199:
2196:
2185:
2179:
2176:
2172:
2166:
2161:
2157:
2153:
2149:
2145:
2139:
2136:
2130:
2128:
2036:
2028:
1987:
1924:
1912:
1717:
1702:
1606:
1600:
1492:
1486:
1484:
1481:symtab.u.sval
1473:symtab.u.ival
1375:
1372:
1273:
1134:
1128:
1123:
1121:
1119:
1096:
1094:
1084:
1077:
1075:
1032:
1030:
1026:
1018:
856:
853:
789:
786:
784:
780:
776:
771:
769:
761:
716:
713:
608:
590:
582:
580:
569:
567:
436:
433:
431:
427:
419:
410:
408:
402:
400:
395:
389:
387:
383:
378:
376:
372:
368:
364:
360:
352:
350:
346:
342:
338:
334:
330:
326:
322:
318:
314:
310:
306:
302:
298:
294:
290:
287:
283:
280:
273:
268:
266:
262:
259:
255:
250:
248:
240:
238:
236:
232:
227:
225:
221:
217:
212:
210:
206:
202:
196:
194:
190:
178:
174:
170:
166:
162:
158:
154:
150:
146:
135:
132:
124:
113:
110:
106:
103:
99:
96:
92:
89:
85:
82: –
81:
77:
76:Find sources:
70:
66:
60:
59:
54:This article
52:
48:
43:
42:
37:
33:
19:
3282:
3236:Intersection
2686:Disjoint-set
2516:. Retrieved
2496:
2478:. Retrieved
2474:
2465:
2453:. Retrieved
2449:
2440:
2428:. Retrieved
2424:
2415:
2403:. Retrieved
2399:
2389:
2378:. Retrieved
2374:
2365:
2354:. Retrieved
2350:
2341:
2330:. Retrieved
2326:
2317:
2306:. Retrieved
2302:
2293:
2281:. Retrieved
2273:
2251:. Retrieved
2249:. p. 52
2242:
2235:
2226:
2217:
2208:
2198:
2187:. Retrieved
2178:
2169:
2155:
2151:
2138:
2126:
2024:
1979:
1916:
1706:
1604:
1529:squareAndAdd
1490:
1470:
1373:
1350:
1271:
1132:
1097:
1081:
1073:
1028:
1022:
854:
851:
787:
785:is 32 bits.
782:
778:
774:
772:
765:
711:
697:
586:
573:
561:
542:'\n'
434:
423:
404:
390:
386:polymorphism
382:type punning
379:
356:
348:
344:
340:
336:
332:
328:
324:
320:
316:
312:
308:
304:
300:
296:
292:
288:
285:
277:
263:
251:
244:
235:tagged union
228:
213:
197:
192:
172:
148:
142:
127:
118:
108:
101:
94:
87:
80:"Union type"
75:
63:Please help
58:verification
55:
3466:Type theory
3461:Type system
3311:Bottom type
3258:Option type
3199:generalized
3085:Long double
3030:Fixed point
2829:Binary heap
2754:Linked list
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