Const (computer programming): Difference between revisions

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=== Pointers and references ===

For pointer and reference types, the meaning of <code>const</code> is more complicated – either the pointer itself, or the value being pointed to, or both, can be <code>const</code>. Further, the syntax can be confusing. A pointer can be declared as a <code>const</code> pointer to writable value, or a writable pointer to a <code>const</code> value, or <code>const</code> pointer to <code>const</code> value. {{anchor|Pointee|Cray pointer|Cray character pointer}}A <code>const</code> pointer cannot be reassigned to point to a different object from the one it is initially assigned, but it can be used to modify the value that it points to (called the ''[[pointee]]''<!-- Might be better described somewhere else, hence circular link with possibilities for now. If you point or move this elsewhere, please also take care of the incoming redirects to "Pointee" and "Cray pointer". -->).<ref>{{cite book |title=The GNU Fortran Compiler |chapter=5.1. Extensions implemented in GNU Fortran: 5.1.16 Cray pointers |date=2006 |url=https://gcc.gnu.org/onlinedocs/gfortran/Cray-pointers.html#Cray-pointers |access-date=2022-12-21 |url-status=live |archive-url=https://web.archive.org/web/20221221175054/https://gcc.gnu.org/onlinedocs/gfortran/Cray-pointers.html#Cray-pointers |archive-date=2022-12-21}}</ref><ref>{{cite web |title=Cray Fortran Pointers vs. Fortran 90 Pointers and Porting from the Cray C90 to the SGI Origin2000 |author-first1=Mark R. |author-last1=Fahey |author-first2=Dan |author-last2=Nagle |publisher=US Army Corps of Engineers Waterways Experiment Station, Major Shared Ressource Center |publication-place=Vicksburg, Massachusetts, USA |date=1999-04-19 |url=https://fs.hlrs.de/projects/par/mooc/cray-pointers.pdf |access-date=2022-12-23 |url-status=live |archive-url=https://web.archive.org/web/20221223105738/https://fs.hlrs.de/projects/par/mooc/cray-pointers.pdf |archive-date=2022-12-23}} (8 pages)</ref><ref>{{cite web |title=Appendix C: Fortran 90 Features and Differences > Features > Cray Pointers |work=Fortran User's Guide |date=2010 |publisher=[[Oracle Corporation]] |url=https://docs.oracle.com/cd/E19957-01/805-4941/z40000a54ba7/index.html<! |access-date=2022-12-23 |url-status=live |archive-url=https://web.archive.org/web/20210921171349/https://docs.oracle.com/cd/E19957-01/805-4941/z40000a54ba7/index.html |archive-date=2021-09-21}}</ref><ref>{{cite web |title=Appendix C: Fortran 90 Features and Differences > Features > Cray Character Pointers |work=Fortran User's Guide |date=2010 |publisher=[[Oracle Corporation]] |url=https://docs.oracle.com/cd/E19957-01/805-4941/z40000a5510b/index.html |access-date=2022-12-23 |url-status=live |archive-url=https://web.archive.org/web/20221223112633/https://docs.oracle.com/cd/E19957-01/805-4941/z40000a5510b/index.html |archive-date=2021-09-21}}</ref><ref>{{cite book |title=Fortran Language Reference Manual, Volume 1 |volume=1 |id=Document Number: 007-3692-004 |chapter=Chapter 4. Data Types |date=1999 |orig-date=1993 |publisher=[[Silicon Graphics, Inc.]] |url=https://techpubs.jurassic.nl/manuals/0630/developer/Ftn_LRM_V1/sgi_html/ch04.html<! |access-date=2022-12-23 |url-status=live |archive-url=https://web.archive.org/web/20221223110605/https://techpubs.jurassic.nl/manuals/0630/developer/Ftn_LRM_V1/sgi_html/ch04.html |archive-date=2022->12-23}} (NB. Derived from "FORTRAN 90 HANDBOOK" (1992, [[McGraw-Hill, Inc.]]) by Walter S. Brainerd, Jeanne C. Adams, Jeanne T. Martin, Brian T. Smith, and Jerrold L. Wagener.)</ref> Reference variables in C++ are an alternate syntax for <code>const</code> pointers. A pointer to a <code>const</code> object, on the other hand, can be reassigned to point to another memory ___location (which should be an object of the same type or of a convertible type), but it cannot be used to modify the memory that it is pointing to. A <code>const</code> pointer to a <code>const</code> object can also be declared and can neither be used to modify the pointee nor be reassigned to point to another object. The following code illustrates these subtleties:

<syntaxhighlight lang=c>