const type qualifier
Each individual type in the C MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 35 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE has several qualified versions of that type, corresponding to one, two, or all three of the const, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 34 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, and, for pointers to object types, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 32 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE qualifiers. This page describes the effects of the const qualifier.
Objects MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 31 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE with const-qualified types may be placed in read-only memory by the compiler, and if the address of a const object is never taken in a program, it may not be stored at all.
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const int n = 1; // object of const-qualified type int* p = (int*)&n; *p = 2; // undefined behavior
const semantics apply to MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 28 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE expressions only; whenever a const lvalue expression is used in context that does not require an lvalue, its const qualifier is lost (note that volatile qualifier, if present, isn't lost).
The lvalue expressions that designate objects of const-qualified type and the lvalue expressions that designate objects of struct or union type with at least one member of const-qualified type (including members of recursively contained aggregates or unions), are not modifiable lvalues. In particular, they are not assignable:
const int n = 1; // object of const type n = 2; // error: the type of n is const-qualified int x = 2; // object of unqualified type const int* p = &x; *p = 3; // error: the type of the lvalue *p is const-qualified struct {int a; const int b; } s1 = {.b=1}, s2 = {.b=2}; s1 = s2; // error: the type of s1 is unqualified, but it has a const member
A member of a const-qualified structure or union type acquires the qualification of the type it belongs to (both when accessed using the . operator or the -> operator).
struct s { int i; const int ci; } s; // the type of s.i is int, the type of s.ci is const int const struct s cs; // the types of cs.i and cs.ci are both const int
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If an array type is declared with the const type qualifier (through the use of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 27 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE), the array type is not const-qualified, but its element type is. |
(until 哋它亢23) |
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(since 哋它亢23) |
typedef int A[2][3]; const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of const int int* pi = a[0]; // Error: a[0] has type const int* void *unqual_ptr = a; // OK until 哋它亢23; error since 哋它亢23 // Notes: clang applies the rule in 哋它亢++/哋它亢23 even in 哋它亢89-哋它亢17 modes
If a function type is declared with the const type qualifier (through the use of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 25 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE), the behavior is undefined.
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In a function declaration, the keyword MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 24 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE void f(double x[const], const double y[const]); void f(double * const x, const double * const y); |
(since 哋它亢99) |
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MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 22 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE const int* p1 = (const int[]){1, 2, 3}; const int* p2 = (const int[]){2, 3, 4}; // the value of p2 may equal p1+1 _Bool b = "foobar" + 3 == (const char[]){"bar"}; // the value of b may be 1 |
(since 哋它亢99) |
A pointer to a non-const type can be implicitly converted to a pointer to const-qualified version of the same or MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 21 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE type. The reverse conversion requires a cast expression.
int* p = 0; const int* cp = p; // OK: adds qualifiers (int to const int) p = cp; // Error: discards qualifiers (const int to int) p = (int*)cp; // OK: cast
Note that pointer to pointer to T is not convertible to pointer to pointer to const T; for two types to be compatible, their qualifications must be identical.
char *p = 0; const char **cpp = &p; // Error: char* and const char* are not compatible types char * const *pcp = &p; // OK, adds qualifiers (char* to char*const)
Keywords
Notes
C adopted the const qualifier from 哋它亢++, but unlike in 哋它亢++, expressions of const-qualified type in C are not MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 19 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE; they may not be used as MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 18 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE labels or to initialize MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 17 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE storage duration objects, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 14 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, or MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 13 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE sizes. When they are used as MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 07 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE sizes, the resulting arrays are VLAs.
References
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.7.3 Type qualifiers (p: 87-90)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.7.3 Type qualifiers (p: 121-123)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.7.3 Type qualifiers (p: 108-110)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 6.5.3 Type qualifiers