Difference between revisions of "c/language/declarations"

int a, *b=NULL; // "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 type 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

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 = NULL, 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] = NULL;
// 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];

int foo(double); // declaration
int foo(double x){ return x; } // definition

extern int n; // declaration
int n = 10; // definition

struct X; // declaration
struct X { int n; }; // definition

extern int x;
int x = 10; // OK
extern int x; // OK
 
static int n;
static int n = 10; // OK
static int n; // OK

typedef int int_t; 
typedef int int_t; // OK

struct X;
struct X { int n; };
struct X;