~2025-42678-33: /* C pointers */

2025-12-26T23:15:50Z

C pointers

imported>Kaltenmeyer: WP:AWB clean up, typo(s) fixed: entirely eliminated → eliminated

2025-06-25T00:07:47Z

WP:AWB clean up, typo(s) fixed: entirely eliminated → eliminated

← Previous revision		Revision as of 00:07, 25 June 2025
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	In [[computer science]], a '''pointer''' is an [[object (computer science)\|object]] in many [[programming language]]s that stores a [[memory address]]. This can be that of another value located in [[computer memory]], or in some cases, that of [[memory-mapped I/O\|memory-mapped]] [[computer hardware]]. A pointer ''references'' a location in memory, and obtaining the value stored at that location is known as ''dereferencing'' the pointer. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on that page. The actual format and content of a pointer variable is dependent on the underlying [[computer architecture]].		In [[computer science]], a '''pointer''' is an [[object (computer science)\|object]] in many [[programming language]]s that stores a [[memory address]]. This can be that of another value located in [[computer memory]], or in some cases, that of [[memory-mapped I/O\|memory-mapped]] [[computer hardware]]. A pointer ''references'' a location in memory, and obtaining the value stored at that location is known as ''dereferencing'' the pointer. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on that page. The actual format and content of a pointer variable is dependent on the underlying [[computer architecture]].

	Using pointers significantly improves [[Computer performance\|performance]] for repetitive operations, like traversing [[Collection (abstract data type)#Linear collections\|iterable]] data [[Data structure\|structures]] (e.g. [[String algorithms\|strings]], [[lookup table]]s, [[control table]]s, [[~~Linked~~ list~~\|linked lists~~]], and [[Tree (data structure)\|tree]] structures). In particular, it is often much cheaper in time and space to copy and dereference pointers than it is to copy and access the data to which the pointers point.		Using pointers significantly improves [[Computer performance\|performance]] for repetitive operations, like traversing [[Collection (abstract data type)#Linear collections\|iterable]] data [[Data structure\|structures]] (e.g. [[String algorithms\|strings]], [[lookup table]]s, [[control table]]s, [[linked list]]s, and [[Tree (data structure)\|tree]] structures). In particular, it is often much cheaper in time and space to copy and dereference pointers than it is to copy and access the data to which the pointers point.

	Pointers are also used to hold the addresses of entry points for [[system call\|call]]ed subroutines in [[procedural programming]] and for run-time linking to [[Dynamic link library#Explicit run-time linking\|dynamic link libraries (DLLs)]]. In [[object-oriented programming]], [[Function pointer\|pointers to functions]] are used for [[Name binding\|binding]] [[method (computer programming)\|methods]], often using [[virtual method table]]s.		Pointers are also used to hold the addresses of entry points for [[system call\|call]]ed subroutines in [[procedural programming]] and for run-time linking to [[Dynamic link library#Explicit run-time linking\|dynamic link libraries (DLLs)]]. In [[object-oriented programming]], [[Function pointer\|pointers to functions]] are used for [[Name binding\|binding]] [[method (computer programming)\|methods]], often using [[virtual method table]]s.

	A pointer is a simple, more concrete implementation of the more abstract ''[[reference (computer science)\|reference]]'' [[data type]]. Several languages, especially [[low-level programming language\|low-level languages]], support some type of pointer, although some have more restrictions on their use than others. While "pointer" has been used to refer to references in general, it more properly applies to [[data structures]] whose [[Application programming interface\|interface]] explicitly allows the pointer to be manipulated (arithmetically via ''{{vanchor\|pointer arithmetic}}'') as a memory address, as opposed to a [[magic cookie]] or [[capability-based security\|capability]] which does not allow such.{{Citation needed\|date=May 2009}} Because pointers allow both protected and unprotected access to [[~~Memory~~ address~~\|memory addresses~~]], there are risks associated with using them, particularly in the latter case. Primitive pointers are often stored in a format similar to an [[integer]]; however, attempting to dereference or "look up" such a pointer whose value is not a valid memory address could cause a program to [[Crash (computing)\|crash]] (or contain invalid data). To alleviate this potential problem, as a matter of [[type safety]], pointers are considered a separate type parameterized by the type of data they point to, even if the underlying representation is an integer. Other measures may also be taken (such as [[Data validation and reconciliation\|validation]] and [[bounds checking]]), to verify that the pointer variable contains a value that is both a valid memory address and within the numerical range that the processor is capable of addressing.		A pointer is a simple, more concrete implementation of the more abstract ''[[reference (computer science)\|reference]]'' [[data type]]. Several languages, especially [[low-level programming language\|low-level languages]], support some type of pointer, although some have more restrictions on their use than others. While "pointer" has been used to refer to references in general, it more properly applies to [[data structures]] whose [[Application programming interface\|interface]] explicitly allows the pointer to be manipulated (arithmetically via ''{{vanchor\|pointer arithmetic}}'') as a memory address, as opposed to a [[magic cookie]] or [[capability-based security\|capability]] which does not allow such.{{Citation needed\|date=May 2009}} Because pointers allow both protected and unprotected access to [[memory address]]es, there are risks associated with using them, particularly in the latter case. Primitive pointers are often stored in a format similar to an [[integer]]; however, attempting to dereference or "look up" such a pointer whose value is not a valid memory address could cause a program to [[Crash (computing)\|crash]] (or contain invalid data). To alleviate this potential problem, as a matter of [[type safety]], pointers are considered a separate type parameterized by the type of data they point to, even if the underlying representation is an integer. Other measures may also be taken (such as [[Data validation and reconciliation\|validation]] and [[bounds checking]]), to verify that the pointer variable contains a value that is both a valid memory address and within the numerical range that the processor is capable of addressing.

	==History==		==History==
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	char (C::* M1CFcvE)() /* pointer-to-member-function which returns a char */		char (C::* M1CFcvE)() /* pointer-to-member-function which returns a char */
	char (C::* A5_M1CFcvE[5])(); /* an array of pointers-to-member-functions which return a char */		char (C::* A5_M1CFcvE[5])(); /* an array of pointers-to-member-functions which return a char */


	</syntaxhighlight>		</syntaxhighlight>
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	===Pascal===		===Pascal===
	Unlike many languages that feature pointers, standard [[International Organization for Standardization\|ISO]] [[Pascal (programming language)\|Pascal]] only allows pointers to reference dynamically created variables that are anonymous and does not allow them to reference standard static or local variables.<ref>ISO 7185 Pascal Standard (unofficial copy), section [http://standardpascal.org/iso7185.html#6.4.4%20Pointer-types 6.4.4 Pointer-types] {{Webarchive\|url=https://web.archive.org/web/20170424183023/http://www.standardpascal.org/iso7185.html#6.4.4%20Pointer-types \|date=2017-04-24}} and subsequent.</ref> It does not have pointer arithmetic. Pointers also must have an associated type and a pointer to one type is not compatible with a pointer to another type (e.g. a pointer to a char is not compatible with a pointer to an integer). This helps eliminate the type security issues inherent with other pointer implementations, particularly those used for [[PL/I]] or [[C (Programming Language)\|C]]. It also removes some risks caused by [[dangling pointers]], but the ability to dynamically let go of referenced space by using the <code>dispose</code> standard procedure (which has the same effect as the <code>free</code> library function found in [[C (Programming Language)\|C]]) means that the risk of dangling pointers has not been ~~entirely~~ eliminated.<ref>J. Welsh, W. J. Sneeringer, and C. A. R. Hoare, "Ambiguities and Insecurities in Pascal," ''Software: Practice and Experience 7'', pp. 685–696 (1977)</ref>		Unlike many languages that feature pointers, standard [[International Organization for Standardization\|ISO]] [[Pascal (programming language)\|Pascal]] only allows pointers to reference dynamically created variables that are anonymous and does not allow them to reference standard static or local variables.<ref>ISO 7185 Pascal Standard (unofficial copy), section [http://standardpascal.org/iso7185.html#6.4.4%20Pointer-types 6.4.4 Pointer-types] {{Webarchive\|url=https://web.archive.org/web/20170424183023/http://www.standardpascal.org/iso7185.html#6.4.4%20Pointer-types \|date=2017-04-24}} and subsequent.</ref> It does not have pointer arithmetic. Pointers also must have an associated type and a pointer to one type is not compatible with a pointer to another type (e.g. a pointer to a char is not compatible with a pointer to an integer). This helps eliminate the type security issues inherent with other pointer implementations, particularly those used for [[PL/I]] or [[C (Programming Language)\|C]]. It also removes some risks caused by [[dangling pointers]], but the ability to dynamically let go of referenced space by using the <code>dispose</code> standard procedure (which has the same effect as the <code>free</code> library function found in [[C (Programming Language)\|C]]) means that the risk of dangling pointers has not been eliminated.<ref>J. Welsh, W. J. Sneeringer, and C. A. R. Hoare, "Ambiguities and Insecurities in Pascal," ''Software: Practice and Experience 7'', pp. 685–696 (1977)</ref>

	However, in some commercial and open source Pascal (or derivatives) compiler implementations —like [[Free Pascal]],<ref>Free Pascal Language Reference guide, section [http://www.freepascal.org/docs-html/ref/refse15.html#x43-490003.4 3.4 Pointers]</ref> [[Turbo Pascal]] or the [[Object Pascal]] in [[Embarcadero Delphi]]— a pointer is allowed to reference standard static or local variables and can be cast from one pointer type to another. Moreover, pointer arithmetic is unrestricted: adding or subtracting from a pointer moves it by that number of bytes in either direction, but using the <code>Inc</code> or <code>Dec</code> standard procedures with it moves the pointer by the size of the [[data type]] it is ''declared'' to point to. An untyped pointer is also provided under the name <code>Pointer</code>, which is compatible with other pointer types.		However, in some commercial and open source Pascal (or derivatives) compiler implementations —like [[Free Pascal]],<ref>Free Pascal Language Reference guide, section [http://www.freepascal.org/docs-html/ref/refse15.html#x43-490003.4 3.4 Pointers]</ref> [[Turbo Pascal]] or the [[Object Pascal]] in [[Embarcadero Delphi]]— a pointer is allowed to reference standard static or local variables and can be cast from one pointer type to another. Moreover, pointer arithmetic is unrestricted: adding or subtracting from a pointer moves it by that number of bytes in either direction, but using the <code>Inc</code> or <code>Dec</code> standard procedures with it moves the pointer by the size of the [[data type]] it is ''declared'' to point to. An untyped pointer is also provided under the name <code>Pointer</code>, which is compatible with other pointer types.
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	==Notes==		==Notes==
	{{~~Noteslist~~}}		{{Notelist}}

	==References==		==References==

24.118.62.152: /* Formal description */ Reverted the garbage changes

2025-03-20T04:54:23Z

Formal description: Reverted the garbage changes

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Pointer (computer programming) - Revision history

~2025-42678-33: /* C pointers */

imported>Kaltenmeyer: WP:AWB clean up, typo(s) fixed: entirely eliminated → eliminated

24.118.62.152: /* Formal description */ Reverted the garbage changes