In computer science, object composition (not to be confused with function composition) is a way and practice to combine simple objects or data types into more complex ones. Compositions are a critical building block of many basic data structures, including the tagged union, the linked list, and the binary tree, as well as the object used in object-oriented programming. Computer science, or computing science, is the study of the theoretical foundations of information and computation and their implementation and application in computer systems. ... In computer science, function composition is an act or mechanism to combine simple functions to build more complicated ones. ... In strictly mathematical branches of computer science the term object is used in a purely mathematical sense to refer to any thing. While this interpretation is useful in the discussion of abstract theory, it is not concrete enough to serve as a primitive datatype in the discussion of more concrete... A data type is a constraint placed upon the interpretation of data in a type system in computer programming. ... In computer science, a tagged union, also called a variant, variant record, discriminated union, or disjoint union, is a data structure used to hold a value that could take on several different, but fixed types. ... In computer science, a linked list is one of the fundamental data structures. ... In computer science, a binary tree is a tree data structure in which each node has at most two children. ... In strictly mathematical branches of computer science the term object is used in a purely mathematical sense to refer to any thing. While this interpretation is useful in the discussion of abstract theory, it is not concrete enough to serve as a primitive datatype in the discussion of more concrete...

Composition is contrasted with subtyping, which is the process of adding detail to a general data type to create a more specific data type. In composition, the composite type "has an" object of a simpler type, while in subtyping, the subtype "is an" instance of its parent type. Composition does not form a subtype but a new type. In computer science, a subtype states that if given type A is compatible with type B, then A is a subtype of B while not always vice versa. ... In database design and program architecture, Has-a is a relationship where one object belongs to another object, and behaves according to the rules of ownership. ... In computer science, the term inheritance may be applied to a variety of situations in which certain characteristics are passed on from one context to another. ...

A real-world example of composition may be seen in an automobile: the objects wheel, steering wheel, seat, gearbox and engine may have no functionality by themselves, but if you compose them, they may form an automobile object, which has a higher function, greater than the sum of its parts in a trite sense. The force bearing on the axle has an eccentricity e with the point of contact to the rolling surface and exerts a moment about the contact point. ... A modern road cars steering wheel Steering wheels from different periods A steering wheel is a type of steering control used in most modern land vehicles, including all mass-production automobiles. ... A gearbox is an assembly of gears allowing the rotational speed of an input shaft to be changed to a different speed. ... An engine in the broadest sense, is something that produces an output effect from a given input. ... “Car” and “Cars” redirect here. ...

Composited (composed) objects are called fields, items, members or attributes, and the resulting composition a structure, storage record, tuple, user-defined type (UDT), or composite type. The terms usually vary across languages. Fields are given a unique name so that each one can be distinguished from the others. Sometimes an issue of ownership arises: when a composition is destroyed, should objects belonging to it be destroyed as well? If not, the case is sometimes called aggregation. For more, see the aggregation section below. In computer science, a storage record is: a group of related data, words, or fields treated as a meaningful unit; for instance, a Name, Address, and Telephone Number can be a Personal Record; a self-contained collection of information about a single object; a record is made up of a... In computer science, composite types are datatypes which can be constructed in a programming language out of that languages primitive types and other composite types. ...

UML Notation

In UML, composition is depicted as a filled diamond and a solid line. It always implies a multiplicity of 1 or 0..1, as no more than one object at a time can have lifetime responsibility for another object. In the field of software engineering, the Unified Modeling Language (UML) is a standardized specification language for object modeling. ...

The more general form, aggregation, is depicted as an unfilled diamond and a solid line. The image below shows both composition and aggregation. The code below shows what the source code is likely to look like. This article does not cite any references or sources. ...

Image File history File links No higher resolution available. ...

 // Composition class Car { private: Carburetor* itsCarb; public: virtual ~Car() {delete itsCarb;} }; 

 // Aggregation class Pond { private: vector<Duck*> itsDucks; }; 

Composite types in C

This is an example of composition in C. C is a general-purpose, block structured, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ...

 typedef struct { int age; char *name; enum { male, female } gender; } Person; 

In this example, the primitive types int, char *, and enum {male, female} are combined to form the composite type of Person. Each object of type Person then "has an" age, name, and sex.

If a Person type were instead created by subtyping, it might be a subtype of Organism, and it could inherit some attributes from Organism (every organism has an age), while extending the definition of Organism with new attributes (not every organism has a gender, but every person does).

Recursive composition

Objects can be composited recursively with the use of recursive types or references. Consider a tree. Each node in a tree may be a branch or leaf; in other words, each node is a tree at the same time when it belongs to another tree. In computer programming languages, a recursive type is a data type for values that may contain other values of the same type. ... This article discusses a general notion of reference in computing. ...

One implementation for the recursive composition is to let each object have references to others of the same type. In C, for example, a binary tree can be defined like:

 struct bintree { struct bintree *left, *right; // some data }; 

If pointers left and right are valid, the node is thought to be a branch referring to each tree to which left and right point. If not, the node is a leaf. In this way, the recursion can be terminated.

Another is to use a tagged union. See tagged union for an example. In computer science, a tagged union, also called a variant, variant record, discriminated union, or disjoint union, is a data structure used to hold a value that could take on several different, but fixed types. ...

Timeline of composition in various languages

C calls a record a struct or structure; object-oriented languages such as Java, Smalltalk, and C++ often keep their records hidden inside objects (class instances); languages in the ML family simply call them records. COBOL was the first programming language to support records directly; ALGOL 68 got it from COBOL and Pascal got it, more or less indirectly, from ALGOL 68. C is a general-purpose, block structured, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ... A struct is the C programming languages notion of a record, a datatype that aggregates a fixed set of labelled objects, possibly of different types, into a single object. ... Object-oriented programming (OOP) is a computer programming paradigm in which a software system is modeled as a set of objects that interact with each other. ... Java is a programming language originally developed by Sun Microsystems and released in 1995. ... For other uses, see Small talk. ... C++ (pronounced see plus plus, IPA: ) is a general-purpose programming language with high-level and low-level capabilities. ... In strictly mathematical branches of computer science the term object is used in a purely mathematical sense to refer to any thing. While this interpretation is useful in the discussion of abstract theory, it is not concrete enough to serve as a primitive datatype in the discussion of more concrete... In object-oriented programming, a class is a programming language construct that is used to group related instance variables and methods. ... 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. Historically, ML stands for metalanguage as it was conceived to develop proof tactics in the LCF theorem prover (the language of... COBOL (pronounced //) is a third-generation programming language, and one of the oldest programming languages still in active use. ... A programming language is an artificial language that can be used to control the behavior of a machine, particularly a computer. ... ALGOL 68 (short for ALGOrithmic Language 1968) is an imperative computer 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 a more rigorously defined syntax and semantics. ... Pascal is an imperative computer programming language, developed in 1970 by Niklaus Wirth as a language particularly suitable for structured programming. ...

1959 – COBOL:

 01 customer-record. 03 customer-number pic 9(8) comp. 03 customer-name. 05 given-names pic x(15). 05 initial-2 pic x. 05 surname pic x(15). 03 customer-address. 05 street. 07 house-number pic 999 comp. 07 street-name pic x(15). 05 city pic x(10). 05 country-code pic x(3). 05 postcode pic x(8). 03 amount-owing pic 9(8) comp. 

1964 – PL/I:

 dcl 1 newtypet based (P); 2 (a,b,c) fixed bin(31), 2 (i,j,k) float, 2 r ptr; allocate newtypet; 

1960 – ALGOL 60:

Arrays were the only composite data type in Algol 60. ALGOL (short for ALGOrithmic Language) is a programming language originally developed in the mid 1950s which became the de facto standard way to report algorithms in print for almost the next 30 years. ...

1968 – ALGOL 68:

 int max=99; mode newtypet = [0..9][0..max]struct ( long real a, b, c, short int i, j, k, ref real r ); newtypet newarrayt=(1,2,3,4,5,6, heap real:=7) 

For an example of all this, here is the traditional linked list declaration:

 mode node = union (real, int, compl, string), list = struct (node val, ref list next); 

Note that for ALGOL 68 only the newtypet name appears to the left of the equality, and most notably the construction is made – and can be read – from left to right without regard to priorities.

1970 – Pascal:

 type a = array [1..10] of integer; b = record a,b,c: real; i,j,k: integer; end; 

1972 – K&R C: C is a general-purpose, block structured, procedural, imperative computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system. ...

 #define max 99 struct newtypet { double a, b, c; short i, j, k; float &r; } newarrayt[10][max+1]; 

1977: FORTRAN 77

Fortran 77 has arrays, but lacked any formal record/structure definitions. Typically compound structures were built up using EQUIVALENCE and/or COMMON statements:

 CHARACTER NAME*32, ADDR*32, PHONE*16 REAL OWING COMMON /CUST/NAME, ADDR, PHONE, OWING 

1983 – ADA:

 type Cust is record Name : Name_Type; Addr : Addr_Type; Phone : Phone_Type; Owing : Integer range 1..999999; end record; 

1985 – C++:

 const int max=99; typedef class { public: double a, b, c; short i, j, k; float &r; } newtypet[10][max+1]; 

1991 – Python:

 max=99 class newtypet: def __init__(self): self.a=self.b=self.c=0; self.i=self.j=self.k=0.0 class R: def __init__(self): self.r=None; self.r=R() # now initialise an example array of this class # newarrayt=[ [ newtypet() for i in range(max+1) ] for j in range(10) ] 

1992 – FORTRAN 90: Arrays and strings were inherited from FORTRAN 77, and a new reserved word was introduced: type

 type newtypet double precision a,b,c integer*2 i,j,k * No pointer type REF REAL R end type type (newtypet) t(10,100) 

FORTRAN 90 updated and included FORTRAN IV's concept called NAMELIST. Fortran (also FORTRAN) is a statically typed, compiled, programming language originally developed in the 1950s and still heavily used for scientific computing and numerical computation half a century later. ...

 INTEGER :: jan=1, feb=2, mar=3, apr=4 NAMELIST / week / jan, feb, mar, apr 

For more details about composition in C/C++, see Composite type. In computer science, composite types are datatypes which can be constructed in a programming language out of that languages primitive types and other composite types. ...

Aggregation

Aggregation differs from ordinary composition in that it does not imply ownership. In composition, when the owning object is destroyed, so are the contained objects. In aggregation, this is not necessarily true. For example, a university owns various departments (e.g., chemistry), and each department has a number of professors. If the university closes, the departments will no longer exist, but the professors in those departments will continue to exist. Therefore, a University can be seen as a composition of departments, whereas departments have an aggregation of professors. In addition, a Professor could work in more than one department, but a department could not be part of more than one university. Representation of a university class, 1350s. ... For other uses, see Chemistry (disambiguation). ...

Composition is usually implemented such that an object contains another object. For example, in C++: C++ (pronounced see plus plus, IPA: ) is a general-purpose programming language with high-level and low-level capabilities. ...

 class Professor; class Department { ... private: // Aggregation Professor* members[5]; ... }; 

 class University { ... private: // Composition Department faculty[20]; ... }; 

In aggregation, the object may only contain a reference or pointer to the object (and not have lifetime responsibility for it):

Sometimes aggregation is referred to as composition when the distinction between ordinary composition and aggregation is unimportant.

The above code would transform into the following UML Class diagram:

Image File history File links No higher resolution available. ...

Containment

Composition that is used to store several instances of the composited data type is referred to as containment. Examples of such containers are arrays, linked lists, binary trees and associative arrays. In computer science, a linked list is one of the fundamental data structures. ... In computer science, a binary tree is a tree data structure in which each node has at most two children. ... An associative array (also map, hash, dictionary, finite map, lookup table, and in query-processing an index or index file) is an abstract data type composed of a collection of keys and a collection of values, where each key is associated with one value. ...

In UML, containment is depicted with a multiplicity of 1 or 0..n (depending on the issue of ownership), indicating that the data type is composed of an unknown amount of instances of the composited data type. In the field of software engineering, the Unified Modeling Language (UML) is a standardized specification language for object modeling. ...

Aggregation in COM

In Microsoft's Component Object Model aggregation means that an object exports, as if it were their owner, one or several interfaces of another object it owns. Formally, this is more similar to composition or encapsulation than aggregation. However, instead of implementing the exported interfaces by calling the interfaces of the owned object, the interfaces of the owned object themselves are exported. The owned object is responsible for assuring that methods of those interfaces inherited from IUnknown actually invoke the corresponding methods of the owner. This is to guarantee that the reference count of the owner is correct and all interfaces of the owner are accessible throught the exported interface, while no other (private) interfaces of the owned object are accessible.^[1] Image File history File links This is a lossless scalable vector image. ... Component Object Model (COM) is a platform for software componentry introduced by Microsoft in 1993. ... An interface defines the communication boundary between two entities, such as a piece of software, a hardware device, or a user. ... The published COM specification mandates that COM objects must minimally implement the IUnknown interface. ...

See also

• C++ structure
• Law of Demeter
• Implementation inheritance
• Inheritance semantics
• Object association
• Virtual inheritance

A C++ structure is a datatype in the C++ programming language which, in addition to the properties of its C counterpart, can have its own member functions and operator overloads. ... The Law of Demeter (LoD) is a design guideline for developing software, particularly object-oriented programs. ... In object-oriented programming, there are two kinds of object association, aggregation and composition. ... In C++, virtual inheritance is a kind of inheritance that solves some of the problems caused by multiple inheritance (particularly the diamond problem) by clarifying ambiguity over which ancestor class members to use. ...

References

1. ^ Aggregation ms-help://MS.PSDKXPSP2.1033/com/htm/com_9xym.htm Platform SDK for Windows XP SP2. Microsoft. Retrieved on 2007-08-29.