Pointer (computer programming)

Donald Knuth, Structured Programming with go to Statements

In computer science, a pointer is a programming language object, whose value refers to (or "points to") another value stored elsewhere in the computer memory using its memory address. 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 the indexed page.

Pointers to data significantly improve performance for repetitive operations such as traversing strings, lookup tables, control tables and 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 called subroutines in procedural programming and for run-time linking to dynamic link libraries (DLLs). In object-oriented programming, pointers to functions are used for binding methods, often using what are called virtual method tables.

A pointer is a simple, more concrete implementation of the more abstract reference data type. Several 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 interface explicitly allows the pointer to be manipulated (arithmetically via pointer arithmetic) as a memory address, as opposed to a magic cookie or capability where this is not possible. Because pointers allow both protected and unprotected access to 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" a pointer whose value was never a valid memory address would cause a program to crash. 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 validation & bounds checking), to verify the contents of the pointer variable contain a value that is both a valid memory address and within the numerical range that the processor is capable of addressing.

Harold Lawson is credited with the 1964 invention of the pointer. In 2000, Lawson was presented the Computer Pioneer Award by the IEEE “or inventing the pointer variable and introducing this concept into PL/I, thus providing for the first time, the capability to flexibly treat linked lists in a general-purpose high level language”. According to the Oxford English Dictionary, the word pointer first appeared in print as a stack pointer in a technical memorandum by the System Development Corporation.

This page was last edited on 4 May 2018, at 11:30.
Reference: https://en.wikipedia.org/wiki/Raw_pointer under CC BY-SA license.

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