const (computer programming)

When applied in an object declaration,[lower-alpha 1] it indicates that the object is a constant: its value may not be changed, unlike a variable. This basic use – to declare constants – has parallels in many other languages.

However, unlike in other languages, in the C family of languages the const is part of the type, not part of the object. For example, in C, int const x = 1; declares an object x of int const type – the const is part of the type, as if it were parsed "(int const) x" – while in Ada, X : constant INTEGER := 1_ declares a constant (a kind of object) X of INTEGER type: the constant is part of the object, but not part of the type.

This has two subtle results. Firstly, const can be applied to parts of a more complex type – for example, int const * const x; declares a constant pointer to a constant integer, while int const * x; declares a variable pointer to a constant integer, and int * const x; declares a constant pointer to a variable integer. Secondly, because const is part of the type, it must match as part of type-checking. For example, the following code is invalid:

void f(int& x);
// ...
int const i;
f(i);

because the argument to f must be a variable integer, but i is a constant integer. This matching is a form of program correctness, and is known as const-correctness. This allows a form of programming by contract, where functions specify as part of their type signature whether they modify their arguments or not, and whether their return value is modifiable or not. This type-checking is primarily of interest in pointers and references – not basic value types like integers – but also for composite data types or templated types such as containers. It is concealed by the fact that the const can often be omitted, due to type coercion (implicit type conversion) and C being call-by-value (C++ and D are either call-by-value or call-by-reference).

The idea of const-ness does not imply that the variable as it is stored in computer memory is unwritable. Rather, const-ness is a compile-time construct that indicates what a programmer should do, not necessarily what they can do. Note, however, that in the case of predefined data (such as char const * string literals), C const is often unwritable.

While a constant does not change its value while the program is running, an object declared const may indeed change its value while the program is running. A common example are read only registers within embedded systems like the current state of a digital input. The data registers for digital inputs are often declared as const and volatile. The content of these registers may change without the program doing anything (volatile) but you shall not write to them either (const).

In addition, a (non-static) member-function can be declared as const. In this case, the this pointer inside such a function is of type object_type const * rather than merely of type object_type *.[2] This means that non-const functions for this object cannot be called from inside such a function, nor can member variables be modified. In C++, a member variable can be declared as mutable, indicating that this restriction does not apply to it. In some cases, this can be useful, for example with caching, reference counting, and data synchronization. In these cases, the logical meaning (state) of the object is unchanged, but the object is not physically constant since its bitwise representation may change.

In C, C++, and D, all data types, including those defined by the user, can be declared const, and const-correctness dictates that all variables or objects should be declared as such unless they need to be modified. Such proactive use of const makes values "easier to understand, track, and reason about,"[3] and it thus increases the readability and comprehensibility of code and makes working in teams and maintaining code simpler because it communicates information about a value's intended use. This can help the compiler as well as the developer when reasoning about code. It can also enable an optimizing compiler to generate more efficient code.[4]

void Foo( int * ptr,
          int const * ptrToConst,
          int * const constPtr,
          int const * const constPtrToConst )
{
    *ptr = 0; // OK: modifies the pointed to data
    ptr  = NULL; // OK: modifies the pointer

    *ptrToConst = 0; // Error! Cannot modify the pointed to data
    ptrToConst  = NULL; // OK: modifies the pointer

    *constPtr = 0; // OK: modifies the pointed to data
    constPtr  = NULL; // Error! Cannot modify the pointer

    *constPtrToConst = 0; // Error! Cannot modify the pointed to data
    constPtrToConst  = NULL; // Error! Cannot modify the pointer
}

int *ptr;                          // *ptr is an int value
int const *ptrToConst;             // *ptrToConst is a constant (int: integer value)
int * const constPtr;              // constPtr is a constant (int *: integer pointer)
int const * const constPtrToConst; // constPtrToConst is a constant pointer and points
                                   // to a constant value

double (**const (*fun(int))(double))[10]

                  fun

                     (int))

                (*

                           (double))

        (**const

                                    [10]

double

const int *ptrToConst;            //identical to: int const *ptrToConst,
const int *const constPtrToConst; //identical to: int const *const constPtrToConst

int **ptr;            // a pointer to a pointer to ints
int const **ptr       // a pointer to a pointer to constant int value
                      // (not a pointer to a constant pointer to ints)
int *const *ptr       // a pointer to a const pointer to int values
                      // (not a constant pointer to a pointer to ints)
int **const ptr       // a constant pointer to pointers to ints
                      // (ptr, the identifier, being const makes no sense)
int const **const ptr // a constant pointer to pointers to constant int values

int* a;          /* write: */     int *a;    // a is a pointer to an int
int* a, b;       // CONFUSING 
                 /* write: */     int *a, b; // a is a pointer to an int, 
                 //                             but b is a mere int
int* a, *b;      // UGLY: both a and b are pointers to ints
                 /* write: */     int *a, *b;

int var = 22;
int const &refToConst = var;         // OK
int const& ref2 = var, ref3 = var;   // CONFUSING:
                                     // ref2 is a reference, but ref3 isn't:
                                     // ref3 is a constant int initialized with
                                     // var's value
int &const constRef = var;           // ERROR: as references can't change anyway.

// C++:
int&& rref = int(5), value = 10;     // CONFUSING:
                                     // rref is an rvalue reference, but value is
                                     // a mere int. 
                                     /* write: */ int &&rref = int(5), value = 10;

There are several loopholes to pure const-correctness in C and C++. They exist primarily for compatibility with existing code.

The first, which applies only to C++, is the use of const_cast, which allows the programmer to strip the const qualifier, making any object modifiable. The necessity of stripping the qualifier arises when using existing code and libraries that cannot be modified but which are not const-correct. For instance, consider this code:

// Prototype for a function which we cannot change but which
// we know does not modify the pointee passed in.
void LibraryFunc(int* ptr, int size);

void CallLibraryFunc(int const * ptr, int size)
{
    LibraryFunc(ptr, size); // Error! Drops const qualifier

    int* nonConstPtr = const_cast<int*>(ptr); // Strip qualifier
    LibraryFunc(nonConstPtr, size);  // OK
}

However, any attempt to modify an object that is itself declared const by means of a const cast results in undefined behavior according to the ISO C++ Standard. In the example above, if ptr references a global, local, or member variable declared as const, or an object allocated on the heap via new int const, the code is only correct if LibraryFunc really does not modify the value pointed to by ptr.

The C language has a need of a loophole because a certain situation exists. Variables with static storage duration are allowed to be defined with an initial value. However, the initializer can use only constants like string constants and other literals, and is not allowed to use non-constant elements like variable names, whether the initializer elements are declared const or not, or whether the static duration variable is being declared const or not. There is a non-portable way to initialize a const variable that has static storage duration. By carefully constructing a typecast on the left hand side of a later assignment, a const variable can be written to, effectively stripping away the const attribute and 'initializing' it with non-constant elements like other const variables and such. Writing into a const variable this way may work as intended, but it causes undefined behavior and seriously contradicts const-correctness:

size_t const    bufferSize = 8*1024;
size_t const    userTextBufferSize;  //initial value depends on const bufferSize, can't be initialized here

...

int setupUserTextBox(textBox_t *defaultTextBoxType, rect_t *defaultTextBoxLocation)
{
    *(size_t*)&userTextBufferSize = bufferSize - sizeof(struct textBoxControls);  // warning: might work, but not guaranteed by C
    ...
}

Another loophole[11] applies both to C and C++. Specifically, the languages dictate that member pointers and references are "shallow" with respect to the const-ness of their owners — that is, a containing object that is const has all const members except that member pointees (and referees) are still mutable. To illustrate, consider this C++ code:

struct S
{
    int val;
    int *ptr;
};

void Foo(S const & s)
{
    int i  = 42;
    s.val  = i;  // Error: s is const, so val is a const int
    s.ptr  = &i; // Error: s is const, so ptr is a const pointer to int
    *s.ptr = i;  // OK: the data pointed to by ptr is always mutable,
                 //     even though this is sometimes not desirable
}

Although the object s passed to Foo() is constant, which makes all of its members constant, the pointee accessible through s.ptr is still modifiable, though this may not be desirable from the standpoint of const-correctness because s might solely own the pointee. For this reason, Meyers argues that the default for member pointers and references should be "deep" const-ness, which could be overridden by a mutable qualifier when the pointee is not owned by the container, but this strategy would create compatibility issues with existing code. Thus, for historical reasons, this loophole remains open in C and C++.

The latter loophole can be closed by using a class to hide the pointer behind a const-correct interface, but such classes either do not support the usual copy semantics from a const object (implying that the containing class cannot be copied by the usual semantics either) or allow other loopholes by permitting the stripping of const-ness through inadvertent or intentional copying.

Finally, several functions in the C standard library violate const-correctness before C23, as they accept a const pointer to a character string and return a non-const pointer to a part of the same string. strstr and strchr are among these functions. Some implementations of the C++ standard library, such as Microsoft's[12] try to close this loophole by providing two overloaded versions of some functions: a "const" version and a "non-const" version.

The use of the type system to express constancy leads to various complexities and problems, and has accordingly been criticized and not adopted outside the narrow C family of C, C++, and D. Java and C#, which are heavily influenced by C and C++, both explicitly rejected const-style type qualifiers, instead expressing constancy by keywords that apply to the identifier (final in Java, const and readonly in C#). Even within C and C++, the use of const varies significantly, with some projects and organizations using it consistently, and others avoiding it.

char *strchr(char *s, int c);

if (p = strchr(q, '/'))
    *p = ' ';

char* strchr(char* s, int c);
char const* strchr(char const* s, int c);

template <T>
T* strchr(T* s, int c) { ... }

inout(char)* strchr(inout(char)* s, int c);

char *strchr(char const *s, int c);

In Version 2 of the , two keywords relating to const exist.[16] The immutable keyword denotes data that cannot be modified through any reference. The const keyword denotes a non-mutable view of mutable data. Unlike C++ const, D const and immutable are "deep" or , and anything reachable through a const or immutable object is const or immutable respectively.

Example of const vs. immutable in D

int[] foo = new int[5];  // foo is mutable.
const int[] bar = foo;   // bar is a const view of mutable data.
immutable int[] baz = foo;  // Error:  all views of immutable data must be immutable.

immutable int[] nums = new immutable(int)[5];  // No mutable reference to nums may be created.
const int[] constNums = nums;  // Works.  immutable is implicitly convertible to const.
int[] mutableNums = nums;  // Error:  Cannot create a mutable view of immutable data.

Example of transitive or deep const in D

class Foo {
    Foo next;
    int num;
}

immutable Foo foo = new immutable(Foo);
foo.next.num = 5;  // Won't compile.  foo.next is of type immutable(Foo).
                   // foo.next.num is of type immutable(int).

const was introduced by Bjarne Stroustrup in C with Classes, the predecessor to C++, in 1981, and was originally called readonly.[17][18] As to motivation, Stroustrup writes:[18]

"It served two functions: as a way of defining a symbolic constant that obeys scope and type rules (that is, without using a macro) and as a way of deeming an object in memory immutable."

The first use, as a scoped and typed alternative to macros, was analogously fulfilled for function-like macros via the inline keyword. Constant pointers, and the * const notation, were suggested by Dennis Ritchie and so adopted.[18]

const was then adopted in C as part of standardization, and appears in C89 (and subsequent versions) along with the other type qualifier, volatile.[19] A further qualifier, noalias, was suggested at the December 1987 meeting of the X3J11 committee, but was rejected; its goal was ultimately fulfilled by the restrict keyword in C99. Ritchie was not very supportive of these additions, arguing that they did not "carry their weight", but ultimately did not argue for their removal from the standard.[20]

D subsequently inherited const from C++, where it is known as a type constructor (not type qualifier) and added two further type constructors, immutable and inout, to handle related use cases.[lower-alpha 4]

Other languages do not follow C/C++ in having constancy part of the type, though they often have superficially similar constructs and may use the const keyword. Typically this is only used for constants (constant objects).

C# has a const keyword, but with radically different and simpler semantics: it means a compile-time constant, and is not part of the type.

Nim has a const keyword similar to that of C#: it also declares a compile-time constant rather than forming part of the type. However, in Nim, a constant can be declared from any expression that can be evaluated at compile time.[21] In C#, only C# built-in types can be declared as const; user-defined types, including classes, structs, and arrays, cannot be const.[22]

Java does not have const – it instead has final, which can be applied to local "variable" declarations and applies to the identifier, not the type. It has a different object-oriented use for object members, which is the origin of the name.

The Java language specification regards const as a reserved keyword – i.e., one that cannot be used as variable identifier – but assigns no semantics to it: it is a reserved word (it cannot be used in identifiers) but not a keyword (it has no special meaning). It is thought that the reservation of the keyword occurred to allow for an extension of the Java language to include C++-style const methods and pointer to const type. An enhancement request ticket for implementing const correctness exists in the Java Community Process, but was closed in 2005 on the basis that it was impossible to implement in a backwards-compatible fashion.[23]

The contemporary Ada 83 independently had the notion of a constant object and a constant keyword,[24][lower-alpha 5] with input parameters and loop parameters being implicitly constant. Here the constant is a property of the object, not of the type.

JavaScript has a const declaration that defines a block-scoped variable that cannot be reassigned nor redeclared. It defines a read-only reference to a variable that cannot be redefined, but in some situations the value of the variable itself may potentially change, such as if the variable refers to an object and a property of it is altered.[25]

• Single assignment
• restrict
• Pointer aliasing

• "Const-Correctness" by Herb Sutter
• "Constant Optimization?" by Herb Sutter
• The C++ FAQ Lite: Const correctness by Marshall Cline
• Section "Value substitution" from the free electronic book Thinking in C++ by Bruce Eckel
• "Here A Const, There A Const" by Walter Bright
• "Const and Invariant" from D programming language specification, version 2 (experimental)