This article is about a general notion of reference in computing. For the more specific notion of reference used in C++, see Reference (C++). In computer science, a reference is an object containing information which refers to data stored elsewhere, as opposed to containing the data itself. Accessing the value referred to by a reference is called dereferencing it. References are fundamental to constructing many data structures (such as linked lists) and in exchanging information between different parts of a program. In the C++ programming language, a reference is a simple reference datatype that is less powerful but safer than the pointer type inherited from C, which is a reference in the general sense but not in the sense used by C++. // Syntax and terminology The declaration of the form <Type...
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 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 computer science, a value is a sequence of bits that is interpreted according to some data type. ...
A binary tree, a simple type of branching linked data structure. ...
In computer science, a linked list is one of the fundamental data structures, and can be used to implement other data structures. ...
Program or Programme may refer to: Computer program Radio programming Television program(me), Webcast Program (management) 12-step program Program (The Animatrix), a short film in The Animatrix series Event programme British tabloid form of comic book, called prog in short. ...
Address analogy
A reference may be compared to the address of a house. It is a small identifier from which it is possible to find a potentially much larger object. Finding a house based on its address is analogous to dereferencing a reference.
In a more complicated example, suppose you leave a forwarding address in your old house each time you move. A person could visit your first house, then follow the forwarding address to the next house, and so on until they finally find your current house. This is analogous to how references are used in singly linked lists. In computer science, a linked list is one of the fundamental data structures, and can be used to implement other data structures. ...
Another benefit of house addresses is that they're much easier to deal with than actual houses. Say you want to be able to easily locate people on your street based on their last name. One way to do this is to use a large crane to physically pick up and rearrange all the houses based on the last names of the residents. A much easier solution is to make a list of addresses of people on your street and sort it by their last names. References have the same benefit: it is possible to manipulate references to data without actually having to modify the data itself, which in some cases can be much more efficient.
Benefits References increase flexibility in where objects can be stored, how they are allocated, and how they are passed between areas of code. As long as we can access a reference to the data, we can access the data through it, and the data itself need not be moved. They also make sharing of data between different code areas easier; each keeps a reference to it.
The mechanism of references, if varying in implementation, is a fundamental programming language feature common to nearly all modern programming languages. Even some languages that support no direct use of references have some internal or implicit use. For example, the call by reference calling convention can be implemented with either explicit or implicit use of references. In computer science, an evaluation strategy is a set of (usually deterministic) rules for determining the evaluation of expressions in a programming language. ...
Pointers are the most primitive and error-prone but also one of the most powerful and efficient types of references, storing only the address of an object in memory. Smart pointers are opaque data structures that act like pointers but can only be accessed through particular methods. It has been suggested that Software pointer be merged into this article or section. ...
A smart pointer is an abstract data type that simulates a pointer while providing additional features, such as automatic garbage collection or bounds checking. ...
In computer programming, an opaque pointer is a datatype that hides its internal implementation using a pointer. ...
A file handle, or handle is a type of reference used to abstract file content. It usually represents both the file itself, as when requesting a lock on the file, and a specific position within the file's content, as when reading a file. In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. ...
Formal representation More generally, a reference can be considered as a piece of data that allows unique retrieval of another piece of data. This includes primary keys in databases and keys in an associative array. If we have a set of data D, any well-defined (single-valued) function from D onto D ∪ {null} defines a type of reference, where null is the image of a piece of data not referring to anything meaningful. In database design, a primary key is a value that can be used to identify a unique row in a table. ...
This article is about computing. ...
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 computer programming, null is a special value for a pointer (or other kind of reference) used to signify that the pointer intentionally does not have a target. ...
An alternative representation of such a function is a directed graph called a reachability graph. Here, each datum is represented by a vertex and there is an edge from u to v if the datum in u refers to the datum in v. The maximum out-degree is one. These graphs are valuable in garbage collection, where they can be used to separate accessible from inaccessible objects. In the mathematical field of graph theory the degree or valency of a vertex v is the number of edges incident to v (with loops being counted twice). ...
In computer science, garbage collection (GC) is a form of automatic memory management. ...
In computer science, unreachable memory is a block of memory allocated dynamically where the program that allocated the memory no longer has any reachable pointer that refers to it. ...
External and internal storage In many data structures, large, complex objects are composed of smaller objects. These objects are typically stored in one of two ways: - With internal storage, the contents of the smaller object are stored inside the larger object.
- With external storage, the smaller objects are allocated in their own location, and the larger object only stores references to them.
Internal storage is usually more efficient, because there is a space cost for the references and dynamic allocation metadata, and a time cost associated with dereferencing a reference and with allocating the memory for the smaller objects. Internal storage also enhances locality of reference by keeping different parts of the same large object close together in memory. However, there are a variety of situations in which external storage is preferred: In computer science, dynamic memory allocation is the allocation of memory storage for use in a computer program during the runtime of that program. ...
It has been suggested that this article or section be merged with Memory locality. ...
- If the data structure is recursive, meaning it may contain itself. This cannot be represented in the internal way.
- If the larger object is being stored in an area with limited space, such as the stack, then we can prevent running out of storage by storing large component objects in another memory region and referring to them using references.
- If the smaller objects may vary in size, it's often inconvenient or expensive to resize the larger object so that it can still contain them.
- References are often easier to work with and adapt better to new requirements.
Some languages, such as Java and Scheme, do not support internal storage. In these languages, all objects are uniformly accessed through references. “Java language†redirects here. ...
Scheme is a multi-paradigm programming language. ...
Language support In assembly languages, the first languages used, it is typical to express references using either raw memory addresses or indexes into tables. These work, but are somewhat tricky to use, because an address tells you nothing about the value it points to, not even how large it is or how to interpret it; such information is encoded in the program logic. The result is that misinterpretations can occur in incorrect programs, causing bewildering errors. See the terminology section, below, regarding inconsistent use of the terms assembly and assembler. ...
One of the earliest opaque references was that of the Lisp programming language cons cell, which is simply a record containing two references to other Lisp objects, including possibly other cons cells. This simple structure is most commonly used to build singly linked lists, but can also be used to build simple binary trees and so-called "dotted lists", which terminate not with a null reference but a value. Lisp is a family of computer programming languages with a long history and a distinctive fully-parenthesized syntax. ...
CONS, Connection-Oriented Network Service, is one of the two OSI stack network layer protocols, the other being CLNS (Connectionless Network Service). ...
This article does not cite any references or sources. ...
In computer science, a linked list is one of the fundamental data structures, and can be used to implement other data structures. ...
In computer science, a binary tree is a tree data structure in which each node has at most two children. ...
Another early language, Fortran, does not have an explicit representation of references, but does use them implicitly in its call-by-reference calling semantics. Parameters are a way of allowing the same sequence of commands to operate on different data without re-specifying the instructions. ...
The pointer is still one of the most popular types of references today. It is similar to the assembly representation of a raw address, except that it carries a static datatype which can be used at compile-time to ensure that the data it refers to is not misinterpreted. However, because C has a weak type system which can be violated using casts (explicit conversions between various pointer types and between pointer types and integers), misinterpretation is still possible, if more difficult. Its successor C++ tried to increase type safety of pointers with new cast operators and smart pointers in its standard library, but still retained the ability to circumvent these safety mechanisms for compatibility. It has been suggested that Software pointer be merged into this article or section. ...
In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ...
In computing, weak typing, when applied to a programming language, is used to describe how the language handles datatypes. ...
This article needs cleanup. ...
C++ (pronounced see plus plus, IPA: ) is a general-purpose programming language with high-level and low-level capabilities. ...
In computer science, a programming language is type safe when the language does not permit the programmer to treat a value as a type to which it does not belong. ...
Illustration of an application which may use libvorbisfile. ...
A number of popular mainstream languages today such as Java, C#, and Visual Basic have adopted a much more opaque type of reference, usually referred to as simply a reference. These references have types like C pointers indicating how to interpret the data they reference, but they are typesafe in that they cannot be interpreted as a raw address and unsafe conversions are not permitted. In those managed languages, the references are actually pointers of pointers of the referred data. In C/C++, the reference concept of managed languages means two-step pointing. The Garbage Collector is the sole actor that can directly access the mid-step pointers, which cause the opacity. Typically pointer arithmetic is also not supported. “Java language†redirects here. ...
The title given to this article is incorrect due to technical limitations. ...
This article is about the Visual Basic language shipping with Microsoft Visual Studio 6. ...
In computer science, garbage collection (GC) is a form of automatic memory management. ...
Fortran A Fortran reference is best thought of as an alias of another object, such as a scalar variable or a row or column of an array. There is no syntax to dereference the reference or manipulate the contents of the referent directly. Fortran references can be null. As in other languages, these references facilitate the processing of dynamic structures, such as linked lists, queues, and trees. Fortran (previously FORTRAN[1]) is a general-purpose[2], procedural,[3] imperative programming language that is especially suited to numeric computation and scientific computing. ...
Functional languages In all of the above settings, the concept of mutable variables, data that can be modified, often makes implicit use of references. In Standard ML, O'Caml, and many other functional languages, most values are persistent: they cannot be modified by assignment. Assignable "reference cells" serve the unavoidable purposes of mutable references in imperative languages, and make the capability to be modified explicit. Such reference cells can hold any value, and so are given the polymorphic type α ref, where α is to be replaced with the type of value pointed to. These mutable references can be pointed to different objects over their lifetime. For example, this permits building of circular data structures. Standard ML (SML) is a general-purpose, modular, functional programming language with compile-time type checking and type inference. ...
Objective Caml (OCaml) is an advanced programming language which is a member of the ML family. ...
In computer science, polymorphism means allowing a single definition to be used with different types of data (specifically, different classes of objects). ...
To preserve safety and efficient implementations, references cannot be type-cast in ML, nor can pointer arithmetic be performed. It is important to note that in the functional paradigm, many structures that would be represented using pointers in a language like C are represented using other facilities, such as the powerful algebraic datatype mechanism. The programmer is then able to enjoy certain properties (such as the guarantee of immutability) while programming, even though the compiler often uses machine pointers "under the hood". In functional programming, new types can be defined, each of which has one or more constructors. ...
Equality In languages where all objects are accessed through references (like Java and Scheme), there becomes a need to test for two different types of equality: “Java language†redirects here. ...
Scheme is a multi-paradigm programming language. ...
- whether two references reference the same object. In Java, this is done using the equality operator (==). In Scheme, this is done using the procedure
eq?. - whether the objects referenced by two references are equal in some sense (e.g. their contents are the same). In Java, this is done using the
.equals() method, which all reference types possess as it is inherited from Object. In Scheme, this is done with the procedure equal?. The first type of equality implies the second, but the converse is not necessarily true. For example, two objects may be distinct (in the first sense) but contain the same string (in the second sense). See identity for more of this issue. Look up String in Wiktionary, the free dictionary. ...
An identity in object-oriented programming, object-oriented design and object-oriented analysis describes the property of objects that the object can be distinguished from other objects. ...
See also In computer science, abstraction is a mechanism and practice to reduce and factor out details so that one can focus on a few concepts at a time. ...
In computer science, a pointer is a programming language datatype whose value refers directly to (points to) another value stored elsewhere in the computer memory using its address. ...
In computer programming, a magic cookie or cookie is a token or short packet of data passed between communicating programs, where the data is typically not meaningful to the recipient program. ...
In computer programming, a weak reference is a reference that does not protect the referent object from collection by a garbage collector. ...
In computer science a bounded pointer is a pointer that is augmented with additional information that enable the storage bounds within which it may point to be deduced. ...
External links Look up dereference in Wiktionary, the free dictionary. - Pointer Fun With Binky Introduction to pointers in a 3 minute educational video - Stanford Computer Science Education Library
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