Declarations
A declaration is a C language construct that introduces one or more MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 48 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE into the program and specifies their meaning and properties.
Declarations may appear in any scope. Each declaration ends with a semicolon (just like MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 47 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE) and consists of two(until 哋它亢23)three(since 哋它亢23) distinct parts:
specifiers-and-qualifiers declarators-and-initializers(optional) ;
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(1) | ||||||||
attr-spec-seq specifiers-and-qualifiers declarators-and-initializers ;
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(2) | (since 哋它亢23) | |||||||
attr-spec-seq ;
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(3) | (since 哋它亢23) | |||||||
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int a, *b=MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE; // "int" is the type specifier, // "a" is a declarator // "*b" is a declarator and NULL is its initializer const int *f(void); // "int" is the type specifier // "const" is the type qualifier // "*f(void)" is the declarator enum COLOR {RED, GREEN, BLUE} c; // "enum COLOR {RED, GREEN, BLUE}" is the type specifier // "c" is the declarator
The type of each identifier introduced in a declaration is determined by a combination of the type specified by the type specifier and the type modifications applied by its declarator. The type of a variable might also be inferred if auto specifier is used.(since 哋它亢23)
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Declarators
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| identifier attr-spec-seq(optional) | (1) | ||||||||
( declarator )
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(2) | ||||||||
* attr-spec-seq(optional) qualifiers(optional) declarator
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(3) | ||||||||
noptr-declarator [ static(optional) qualifiers(optional) expression ]
noptr-declarator |
(4) | ||||||||
noptr-declarator ( parameters-or-identifiers )
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(5) | ||||||||
D as a cvr-qualified pointer to the type determined by S.D as an array of N objects of the type determined by S. noptr-declarator is any other declarator except unparenthesized pointer declarator.D as a function taking the parameters params and returning S. noptr-declarator is any other declarator except unparenthesized pointer declarator.MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 07 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
struct C { int member; // "int" is the type specifier // "member" is the declarator } obj, *pObj = &obj; // "struct C { int member; }" is the type specifier // declarator "obj" defines an object of type struct C // declarator "*pObj" declares a pointer to C, // initializer "= &obj" provides the initial value for that pointer int a = 1, *p = MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 06 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, f(void), (*pf)(double); // the type specifier is "int" // declarator "a" defines an object of type int // initializer "=1" provides its initial value // declarator "*p" defines an object of type pointer to int // initializer "=NULL" provides its initial value // declarator "f(void)" declares a function taking void and returning int // declarator "(*pf)(double)" defines an object of type pointer // to function taking double and returning int int (*(*foo)(double))[3] = MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 05 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE; // the type specifier is int // 1. declarator "(*(*foo)(double))[3]" is an array declarator: // the type declared is "/nested declarator/ array of 3 int" // 2. the nested declarator is "*(*foo)(double))", which is a pointer declarator // the type declared is "/nested declarator/ pointer to array of 3 int" // 3. the nested declarator is "(*foo)(double)", which is a function declarator // the type declared is "/nested declarator/ function taking double and returning // pointer to array of 3 int" // 4. the nested declarator is "(*foo)" which is a (parenthesized, as required by // function declarator syntax) pointer declarator. // the type declared is "/nested declarator/ pointer to function taking double // and returning pointer to array of 3 int" // 5. the nested declarator is "foo", which is an identifier. // The declaration introduces the identifier "foo" to refer to an object of type // "pointer to function taking double and returning pointer to array of 3 int" // The initializer "= NULL" provides the initial value of this pointer. // If "foo" is used in an expression of the form of the declarator, its type would be // int. int x = (*(*foo)(1.2))[0];
The end of every declarator that is not part of another declarator is a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 04 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
In all cases, attr-spec-seq is an optional sequence of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 03 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(since 哋它亢23). When appearing immediately after the identifier, it applies to the object or function being declared.
Definitions
A definition is a declaration that provides all information about the identifiers it declares.
Every declaration of an MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 01 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE or a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 51 MINUTES 00 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE is a definition.
For functions, a declaration that includes the function body is a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 50 MINUTES 59 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE:
int foo(double); // declaration int foo(double x){ return x; } // definition
For objects, a declaration that allocates storage (MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 50 MINUTES 58 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, but not extern) is a definition, while a declaration that does not allocate storage (MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 50 MINUTES 56 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE) is not.
extern int n; // declaration int n = 10; // definition
For MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 50 MINUTES 55 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 50 MINUTES 54 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, declarations that specify the list of members are definitions:
struct X; // declaration struct X { int n; }; // definition
Redeclaration
A declaration cannot introduce an identifier if another declaration for the same identifier in the same MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 50 MINUTES 53 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE appears earlier, except that
- Declarations of objects MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 50 MINUTES 52 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE (external or internal) can be repeated:
extern int x; int x = 10; // OK extern int x; // OK static int n; static int n = 10; // OK static int n; // OK
- Non-VLA MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 50 MINUTES 50 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE can be repeated as long as it names the same type:
typedef int int_t; typedef int int_t; // OK
- MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 50 MINUTES 49 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 50 MINUTES 48 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE declarations can be repeated:
struct X; struct X { int n; }; struct X;
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Notes
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(since 哋它亢11) |
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(since 哋它亢23) |
References
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.7 Declarations (p: 78-105)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.7 Declarations (p: 108-145)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.7 Declarations (p: 97-130)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 3.5 Declarations