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{{short description|Type qualifier denoting the data as being read-only}}
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In some [[programming language]]s, '''const''' is a [[type qualifier]] (a [[Keyword (computer programming)|keyword]] applied to a [[data type]]) that indicates that the data is read-only. While this can be used to declare [[Constant (computer programming)|constants]], {{mono|const}} in the [[List of C-family programming languages|C family]] of languages differs from similar constructs in other languages in being part of the ''type,'' and thus has complicated behavior when combined with [[Pointer (computer programming)|pointers]], references, [[composite data type]]s, and [[type-checking]]. In other languages, the data is not in a single [[memory ___location]], but copied at [[compile time]] on each use.<ref>{{Cite web |title=Constant items - The Rust Reference |url=https://doc.rust-lang.org/reference/items/constant-items.html |access-date=2022-06-22 |website=doc.rust-lang.org}}</ref> Languages which utilize it include [[C (programming language)|C]], [[C++]], [[D (programming language)|D]], [[JavaScript]], [[Julia (programming language)|Julia]], and [[Rust (programming language)|Rust]].
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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}}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'').<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>https://docs.oracle.com/cd/E19957-01/805-4941/z40000a54ba7/index.html<!-- https://web.archive.org/web/20210921171349/https://docs.oracle.com/cd/E19957-01/805-4941/z40000a54ba7/index.html --></ref><ref>https://techpubs.jurassic.nl/manuals/0630/developer/Ftn_LRM_V1/sgi_html/ch04.html<!-- https://web.archive.org/web/20221223110605/https://techpubs.jurassic.nl/manuals/0630/developer/Ftn_LRM_V1/sgi_html/ch04.html --></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>
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