Computer science is the study of the theoretical foundations of information and computation, and of practical techniques for their implementation and application in computer systems.... , a tagged union, also called a variant
Variant type
Variant is a data type in certain programming languages, particularly Visual Basic and C++ when using the Component Object Model.In Visual Basic the Variant data type is a tagged union that can be used to represent any other data type except fixed-length string type.... , variant record, discriminated union, or disjoint union
Disjoint union
In set theory, a disjoint union is a modified union operation which indexes the elements according to which set they originated in.Formally, let be a family of sets indexed by I.... , is a data structure
Data structure
A data structure in computer science is a way of storing data in a computer so that it can be used efficiently. It is an organization of mathematical and logical concepts of data.... used to hold a value that could take on several different, but fixed types. Only one of the types can be in use at any one time, and a tag field explicitly indicates which one is in use. It can be thought of as a type which has several "cases," each of which should be handled correctly when that type is manipulated.
Computer science is the study of the theoretical foundations of information and computation, and of practical techniques for their implementation and application in computer systems.... , a tagged union, also called a variant
Variant type
Variant is a data type in certain programming languages, particularly Visual Basic and C++ when using the Component Object Model.In Visual Basic the Variant data type is a tagged union that can be used to represent any other data type except fixed-length string type.... , variant record, discriminated union, or disjoint union
Disjoint union
In set theory, a disjoint union is a modified union operation which indexes the elements according to which set they originated in.Formally, let be a family of sets indexed by I.... , is a data structure
Data structure
A data structure in computer science is a way of storing data in a computer so that it can be used efficiently. It is an organization of mathematical and logical concepts of data.... used to hold a value that could take on several different, but fixed types. Only one of the types can be in use at any one time, and a tag field explicitly indicates which one is in use. It can be thought of as a type which has several "cases," each of which should be handled correctly when that type is manipulated. Like ordinary unions
Union (computer science)
In computer science, a union is a data structure that stores one of several types of data at a single location. There are only two safe ways of accessing a union object.... , tagged unions can save storage by overlapping storage areas for each type, since only one is in use at a time.
Tagged unions are most important in functional languages such as ML
ML programming language
ML is a general-purpose functional programming language developed by Robin Milner and others in the late 1970s at the University of Edinburgh, whose syntax is inspired by ISWIM.... and Haskell
Haskell (programming language)
Haskell is a standardized, purely functional programming language with non-strict programming language, named after logician Haskell Curry. The goals of the language are described as:... , where they are called datatypes (see algebraic data type
Algebraic data type
In computer programming, an algebraic data type is a datatype each of whose value s is data from other datatypes wrapped in one of the constructors of the datatype.... ) and the compiler is able to verify that all cases of a tagged union are always handled, avoiding many types of errors. They can, however, be constructed in nearly any language, and are much safer than untagged unions, often simply called unions, which are similar but do not explicitly keep track of which member of the union is currently in use.
Tagged unions are often accompanied by the concept of a constructor, which is similar but not the same as a constructor
Constructor (computer science)
In object-oriented programming, a constructor in a class is a special block of statements called when an object lifetime#Creating objects, either when it is declared or dynamically constructed on the heap-based memory allocation through the keyword ?new?.... for a class. Constructors produce a tagged union value, given the initial tag value and a value of the corresponding type.
Mathematically, tagged unions correspond to disjoint
Disjoint union
In set theory, a disjoint union is a modified union operation which indexes the elements according to which set they originated in.Formally, let be a family of sets indexed by I.... or discriminated unions, usually written using +. Given an element of a disjoint union A + B, it is possible to determine whether it came from A or B. If an element lies in both, there will be two effectively distinct copies of the value in A + B, one from A and one from B.
In mathematics, logic and computer science, type theory is any of several formal systems that can serve as alternatives to naive set theory, or the study of such formalisms in general.... , a tagged union is called a sum type. Notations vary, but usually the sum type comes with two introduction forms and . The elimination form is case analysis: if has type and and have type under the assumptions and respectively, then the term
has type . The sum type corresponds to logical disjunction
Logical disjunction
File:ORGate2.pngIn logic and mathematics, or, also known as logical disjunction or inclusive disjunction is a logical operator that results in true whenever one or more of its operands are true.... under the Curry-Howard correspondence.
Advantages and disadvantages
The primary advantage of a tagged union over an untagged union is that all accesses are safe, and the compiler can even check that all cases are handled. Untagged unions depend on program logic to correctly identify the currently active field, which may result in strange behavior and hard-to-find bugs if that logic fails.
The primary advantage of a tagged union over a simple record
Record (computer science)
In computer science, a record type or struct is a type whose values are records, i.e. aggregates of several items of possibly different types.... containing a field for each type is that it saves storage by overlapping storage for all the types. Some implementations reserve enough storage for the largest type, while others dynamically adjust the size of a tagged union value as needed. When the value is immutable
Immutable object
In object-oriented computer programming and Functional_programming programming, an immutable object is an object whose state cannot be modified after it is created.... , it is simple to allocate just as much storage as is needed.
The main disadvantage of tagged unions is that the tag occupies space. Since there are usually a small number of alternatives, the tag can often be squeezed into 2 or 3 bits wherever space can be found, but sometimes even these bits are not available. In this case, a helpful alternative may be folded, computed or encoded tags, where the tag value is dynamically computed from the contents of the union field. Common examples of this are the use of reserved values, where, for example, a function returning a positive number may return -1 to indicate failure, and sentinel value
Sentinel value
In computer programming, a sentinel value is a special value that is used to terminate a Control flow that processes data structure data . The value should be selected in such a way that it will not be confused with legal data values.... s, most often used in tagged pointer
Tagged pointer
In computer science, a tagged pointer is a common example of a tagged union, where the primary type of data to be stored in the union is a pointer.... s.
Sometimes, untagged unions are used to perform bit-level conversions between types, called reinterpret casts in C++. Tagged unions are not intended for this purpose; typically a new value is assigned whenever the tag is changed.
The top type in type theory, commonly abbreviated as top or by the down tack symbol is the universal type--that type which contains every possible object in the type system of interest.... , which is a type that includes every value of every other type, and often a way is provided to test the actual type of a value of the universal type. These are sometimes referred to as variants. While universal data types are comparable to tagged unions in their formal definition, typical tagged unions include a relatively small number of cases, and these cases form different ways of expressing a single coherent concept, such as a data structure node or instruction. Also, there is an expectation that every possible case of a tagged union will be dealt with when it is used. The values of a universal data type are not related and there is no feasible way to deal with them all.
In computer science, a binary tree is a Tree in which each node has at most two child node. Typically the child nodes are called left and right.... of integers. In ML, we would do this by creating a datatype like this:
datatype tree = Leaf
| Node of (int * tree * tree)
This is a tagged union with two cases: one, the leaf, is used to terminate a path of the tree, and functions much like a null value would in imperative languages. The other branch holds a node, which contains an integer and a left and right subtree. Leaf and Node are the constructors, which enable us to actually produce a particular tree, such as:
ALGOL 68 is an imperative programming computer programming programming language that was conceived as a successor to the ALGOL 60 programming language, designed with the goal of a much wider scope of application and more rigorously defined syntax and semantics.... , tagged unions are called united modes, the tag is implicit, and the case construct is used to determine which field is tagged:
modenode = union(real, int, compl, string);
Usage example for unioncase of node:
node n := "1234";
case n in
(real r): print(("real:", r)),
(int i): print(("int:", i)),
(compl c): print(("compl:", c)),
(string s): print(("string:", s))
out print(("?:", n))
esac
1970s & 1980s
Although primarily only functional languages such as ML
ML programming language
ML is a general-purpose functional programming language developed by Robin Milner and others in the late 1970s at the University of Edinburgh, whose syntax is inspired by ISWIM.... and Haskell
Haskell (programming language)
Haskell is a standardized, purely functional programming language with non-strict programming language, named after logician Haskell Curry. The goals of the language are described as:... (from 1990s) give a central role to tagged unions and have the power to check that all cases are handled, other languages have support for tagged unions as well. However, in practice they can be less efficient in non-functional languages due to optimizations enabled by functional language compilers that can eliminate explicit tag checks and avoid explicit storage of tags.
Modula-2 is a computer programming language invented by Niklaus Wirth at ETH, around 1978, as a successor to his intermediate language Modula. Modula-2 was implemented in 1980 for the Lilith computer, which was commercialized in 1982 by startup company DISER as MC1 and MC2.... call them variant records, and require the tag field to be manually created and the tag values specified, as in this Pascal example:
type shapeKind = (square, rectangle, circle);
shape = record
centerx : integer;
centery : integer;
case kind : shapeKind of
square : (side : integer);
rectangle : (length, height : integer);
circle : (radius : integer);
end;
C is a general-purpose computer programming language originally developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories to implement the Unix operating system.... and C++
C++
C++ is a general-purpose programming language. It is regarded as a middle-level language, as it comprises a combination of both high-level programming language and low-level programming language language features.... , a tagged union can be created from untagged unions using a strict access discipline where the tag is always checked:
enum ShapeKind ;
struct Shape ;
int getSquareSide(struct Shape* s)
void setSquareSide(struct Shape* s, int side)
/* and so on */
As long as the union fields are only accessed through the functions, the accesses will be safe and correct. The same approach can be used for encoded tags; we simply decode the tag and then check it on each access. If the inefficiency of these tag checks is a concern, they may be automatically removed in the final version.
C and C++ also have language support for one particular tagged union: the possibly-null pointer. This may be compared to the option type in ML or the Maybe type in Haskell, and can be seen as a tagged pointer
Tagged pointer
In computer science, a tagged pointer is a common example of a tagged union, where the primary type of data to be stored in the union is a pointer.... : a tagged union (with an encoded tag) of two types:
Valid pointers,
A type with only one value, null, indicating an exceptional condition.
Unfortunately, C compilers do not verify that the null case is always handled, and this is a particularly prevalent source of errors in C code, since there is a tendency to ignore exceptional cases.
The Cyclone programming language is intended to be a safe dialect of the C . Cyclone is designed to avoid buffer overflows and other vulnerabilities that are endemic in C programs, without losing the power and convenience of C as a tool for systems programming.... has extensive built-in support for tagged unions. See for more information.
The variant library from Boost has demonstrated it was possible to implement a safe tagged union as a library in C++, visitable using functors, a mechanism more or less equivalent to closures.
struct display : booststatic_visitor
;
boostvariant v = 42;
boostapply_visitor(display, v);
boostvariant v = "hello world";
boostapply_visitor(display, v);
As in taxonomy, the classifications of species, a class hierarchy in computer science is a classification of object types, denoting objects as the instantiations of classes inter-relating the various classes by relationships such as "inherits", "extends", "is an abstraction of", "an interface definition".... in object-oriented programming
Object-oriented programming
Object-oriented programming is a programming paradigm that uses "Object_" and their interactions to design applications and computer programs.... , each subclass can encapsulate data unique to that class. The metadata used to perform virtual method lookup (for example, the object's vtable
Virtual method table
A virtual method table, virtual function table, dispatch table, or vtable, is a mechanism used in programming language to support dynamic dispatch .... pointer in most C++ implementations) identifies the subclass and so effectively acts as a tag identifying the particular data stored by the instance.
An object's constructor
Constructor (computer science)
In object-oriented programming, a constructor in a class is a special block of statements called when an object lifetime#Creating objects, either when it is declared or dynamically constructed on the heap-based memory allocation through the keyword ?new?.... sets this tag, and it remains constant throughout the object's lifetime.
The D programming language, also known simply as D, is an Object-oriented programming, Imperative programming, Multi-paradigm programming language system programming language by Walter Bright of Digital Mars.... 2.0
Discriminator is a typed tag field present in Object Management Group Interface description language Tagged union type and value definitions that determines which union member is selected in the current union instance....
