8 Declarators [dcl.decl]

1 A declarator declares a single object, function, or type, within a declaration. The init-declarator-list appearing in a declaration is a comma-separated sequence of declarators, each of which can have an initializer.

init-declarator-list:

init-declarator
init-declarator-list , init-declarator

init-declarator:

declarator initializeropt

2 The two components of a declaration are the specifiers (decl-specifier-seq; 7.1) and the declarators (init-declarator-list). The specifiers indicate the type, storage class or other properties of the objects, functions or typedefs being declared. The declarators specify the names of these objects, functions or typedefs, and (optionally) modify the type of the specifiers with operators such as * (pointer to) and () (function returning). Initial values can also be specified in a declarator; initializers are discussed in 8.5 and 12.6.

3 Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself.⁸⁵⁾

4 Declarators have the syntax

declarator:

direct-declarator
ptr-operator declarator

direct-declarator:

declarator-id
direct-declarator ( parameter-declaration-clause ) cv-qualifier-seqopt exception-specificationopt
direct-declarator [ constant-expressionopt ]
( declarator )

ptr-operator:

* cv-qualifier-seqopt
&
::opt nested-name-specifier * cv-qualifier-seqopt

cv-qualifier-seq:

cv-qualifier cv-qualifier-seqopt

cv-qualifier:

const
volatile

declarator-id:

id-expression
::opt nested-name-specifieropt type-name

85) A declaration with several declarators is usually equivalent to the corresponding sequence of declarations each with a single declarator. That is

T   D1, D2, ... Dn;

is usually equvalent to

T   D1; T D2; ... T Dn;

where T is a decl-specifier-seq and each Di is a init-declarator. The exception occurs when a name introduced by one of the declarators hides a type name used by the dcl-specifiers, so that when the same dcl-specifiers are used in a subsequent declaration, they do not have the same meaning, as in

struct S { ... };
S    S, T;                          // declare two instances of struct S

which is not equivalent to

struct S { ... };
S    S;
S    T;                             // error

[back to text]

8.1 Type names [dcl.name]

1 To specify type conversions explicitly, and as an argument of sizeof, new, or typeid, the name of a type shall be specified. This can be done with a type-id, which is syntactically a declaration for an object or function of that type that omits the name of the object or function.

type-id:

type-specifier-seq abstract-declaratoropt

type-specifier-seq:

type-specifier type-specifier-seqopt

abstract-declarator:

ptr-operator abstract-declaratoropt
direct-abstract-declarator

direct-abstract-declarator:

direct-abstract-declaratoropt
( parameter-declaration-clause ) cv-qualifier-seqopt exception-specificationopt
direct-abstract-declaratoropt [ constant-expressionopt ]
( abstract-declarator )

int                                        // int i
int *                                      // int *pi
int *[3]                                   // int *p[3]
int (*)[3]                                 // int (*p3i)[3]
int *()                                    // int *f()
int (*)(double)                            // int (*pf)(double)

2 A type can also be named (often more easily) by using a typedef (7.1.3).

8.2 Ambiguity resolution [dcl.ambig.res]

1 The ambiguity arising from the similarity between a function-style cast and a declaration mentioned in 6.8 can also occur in the context of a declaration. In that context, the choice is between a function declaration with a redundant set of parentheses around a parameter name and an object declaration with a function-style cast as the initializer. Just as for the ambiguities mentioned in 6.8, the resolution is to consider any construct that could possibly be a declaration a declaration. [Note: a declaration can be explicitly disambiguated by a nonfunction-style cast, by a = to indicate initialization or by removing the redundant parentheses around the parameter name. ] [Example:

struct S {
     S(int);
};

void foo(double a)
{
     S w(int(a));                       // function declaration
     S x(int());                        // function declaration
     S y((int)a);                       // object declaration
     S z = int(a);                      // object declaration
}

2 The ambiguity arising from the similarity between a function-style cast and a type-id can occur in different contexts. The ambiguity appears as a choice between a function-style cast expression and a declaration of a type. The resolution is that any construct that could possibly be a type-id in its syntactic context shall be considered a type-id.

3 [Example:

#include <cstddef>
char *p;
void *operator new(size_t, int);
void foo()     {
          const int x = 63;
          new (int(*p)) int;            // new-placement expression
          new (int(*[x]));              // new type-id
}

4 For another example,

template <class T>
struct S {
          T *p;
};
S<int()> x;                             // type-id
S<int(1)> y;                            // expression (ill-formed)

5 For another example,

void foo()
{
          sizeof(int(1));               // expression
          sizeof(int());                // type-id (ill-formed)
}

6 For another example,

void foo()
{
          (int(1));                     // expression
          (int())1;                     // type-id (ill-formed)
}

7 Another ambiguity arises in a parameter-declaration-clause of a function declaration, or in a type-id that is the operand of a sizeof or typeid operator, when a type-name is nested in parentheses. In this case, the choice is between the declaration of a parameter of type pointer to function and the declaration of a parameter with redundant parentheses around the declarator-id. The resolution is to consider the type-name as a simple-type-specifier rather than a declarator-id. [Example:

class C { };
void f(int(C)) { }                       // void f(int (*fp)(C c)) { }
                                         // not: void f(int C);

int g(C);

void foo() {
          f(1);                          // error: cannot convert 1 to function pointer
          f(g);                          // OK
}

class C { };
void h(int *(C[10]));                    // void h(int *(*_fp)(C _parm[10]));
                                         // not: void h(int *C[10]);

8.3 Meaning of declarators [dcl.meaning]

1 A list of declarators appears after an optional (clause 7) decl-specifier-seq (7.1). Each declarator contains exactly one declarator-id; it names the identifier that is declared. The id-expression of a declarator-id shall be a simple identifier except for the declaration of some special functions (12.3, 12.4, 13.5) and for the declaration of template specializations or partial specializations (14.7). A declarator-id shall not be qualified except for the definition of a member function (9.3) or static data member (9.4) or nested class (9.7) outside of its class, the definition or explicit instantiation of a function, variable or class member of a namespace outside of its namespace, or the definition of a previously declared explicit specialization outside of its namespace, or the declaration of a friend function that is a member of another class or namespace (11.4). When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier refers, and the member shall not have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier of the declarator-id. [Note: if the qualifier is the global :: scope resolution operator, the declarator-id refers to a name declared in the global namespace scope. ] In the qualified declarator-id for a class or namespace member definition that appears outside of the member's class or namespace, the nested-name-specifier shall not name any of the namespaces that enclose the member's definition. [Example:

namespace A {
          struct B {
                     void f();
          };
          void A::B::f() { }             // ill-formed: the declarator must not be
                                         // qualified with A::
}

2 An auto, static, extern, register, mutable, friend, inline, virtual, or typedef specifier applies directly to each declarator-id in a init-declarator-list; the type specified for each declarator-id depends on both the decl-specifier-seq and its declarator.

3 Thus, a declaration of a particular identifier has the form

T D

4 First, the decl-specifier-seq determines a type. In a declaration

T D

int unsigned i;

5 In a declaration T D where D is an unadorned identifier the type of this identifier is ``T.''

6 In a declaration T D where D has the form

( D1 )

T D1

8.3.1 Pointers [dcl.ptr]

1 In a declaration T D where D has the form

* cv-qualifier-seqopt D1

2 [Example: the declarations

const int ci = 10, *pc = &ci, *const cpc = pc, **ppc;
int i, *p, *const cp = &i;

i = ci;
*cp = ci;
pc++;
pc = cpc;
pc = p;
ppc = &pc;

ci = 1;                                  // error
ci++;                                    // error
*pc = 2;                                 // error
cp = &ci;                                // error
cpc++;                                   // error
p = pc;                                  // error
ppc = &p;                                // error

*ppc = &ci;                              // OK, but would make p point to ci ...
                                         // ... because of previous error
*p = 5;                                  // clobber ci

3 See also 5.17 and 8.5.

4 [Note: there are no pointers to references; see 8.3.2. Since the address of a bit-field (9.6) cannot be taken, a pointer can never point to a bit-field. ]

8.3.2 References [dcl.ref]

1 In a declaration T D where D has the form

& D1

typedef int& A;
const A aref = 3;                        // ill-formed;
                                         // non-const  reference initialized with rvalue

2 [Example:

void f(double& a) { a += 3.14; }
// ...
double d = 0;
f(d);

int v[20];
// ...
int& g(int i) { return v[i]; }
// ...
g(3) = 7;

struct link {
     link* next;
};

link* first;

void h(link*& p)                         // p is a reference to pointer
{
     p->next = first;
     first = p;
     p = 0;
}

void k()
{
          link* q = new link;
          h(q);
}

3 It is unspecified whether or not a reference requires storage (3.7).

4 There shall be no references to references, no arrays of references, and no pointers to references. The declaration of a reference shall contain an initializer (8.5.3) except when the declaration contains an explicit extern specifier (7.1.1), is a class member (9.2) declaration within a class declaration, or is the declaration of a parameter or a return type (8.3.5); see 3.1. A reference shall be initialized to refer to a valid object or function. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the ``object'' obtained by dereferencing a null pointer, which causes undefined behavior. As described in 9.6, a reference cannot be bound directly to a bit-field. ]

8.3.3 Pointers to members [dcl.mptr]

1 In a declaration T D where D has the form

::opt nested-name-specifier * cv-qualifier-seqopt D1

2 [Example:

class X {
public:
     void f(int);
     int a;
};
class Y;

int X::* pmi = &X::a;
void (X::* pmf)(int) = &X::f;
double X::* pmd;
char Y::* pmc;

X obj;
//...
obj.*pmi = 7;                             // assign 7 to an integer
                                          // member of obj
(obj.*pmf)(7);                            // call a function member of obj
                                          // with the argument 7

3 A pointer to member shall not point to a static member of a class (9.4), a member with reference type, or ``cv void.'' [Note: see also 5.3 and 5.5. The type ``pointer to member'' is distinct from the type ``pointer'', that is, a pointer to member is declared only by the pointer to member declarator syntax, and never by the pointer declarator syntax. There is no ``reference-to-member'' type in C++. ]

8.3.4 Arrays [dcl.array]

1 In a declaration T D where D has the form

D1 [constant-expressionopt]

typedef int A[5], AA[2][3];
typedef const A CA;                        // type is ``array of 5 const int''
typedef const AA CAA;                      // type is ``array of 2 array of 3 const int''

2 An array can be constructed from one of the fundamental types (except void), from a pointer, from a pointer to member, from a class, from an enumeration type, or from another array.

3 When several ``array of'' specifications are adjacent, a multidimensional array is created; the constant expressions that specify the bounds of the arrays can be omitted only for the first member of the sequence. [Note: this elision is useful for function parameters of array types, and when the array is external and the definition, which allocates storage, is given elsewhere. ] The first constant-expression can also be omitted when the declarator is followed by an initializer (8.5). In this case the bound is calculated from the number of initial elements (say, N) supplied (8.5.1), and the type of the identifier of D is ``array of N T.''

4 [Example:

float fa[17], *afp[17];

static int x3d[3][5][7];

5 [Note: conversions affecting lvalues of array type are described in 4.2. Objects of array types cannot be modified, see 3.10. ]

6 Except where it has been declared for a class (13.5.5), the subscript operator [] is interpreted in such a way that E1[E2] is identical to *((E1)+(E2)). Because of the conversion rules that apply to +, if E1 is an array and E2 an integer, then E1[E2] refers to the E2-th member of E1. Therefore, despite its asymmetric appearance, subscripting is a commutative operation.

7 A consistent rule is followed for multidimensional arrays. If E is an n-dimensional array of rank i× j× ^{. . .} × k, then E appearing in an expression is converted to a pointer to an (n -1 )-dimensional array with rank j× ^{. . .} × k. If the * operator, either explicitly or implicitly as a result of subscripting, is applied to this pointer, the result is the pointed-to (n -1 )-dimensional array, which itself is immediately converted into a pointer.

8 [Example: consider

int x[3][5];

9 [Note: it follows from all this that arrays in C++ are stored row-wise (last subscript varies fastest) and that the first subscript in the declaration helps determine the amount of storage consumed by an array but plays no other part in subscript calculations. ]

8.3.5 Functions [dcl.fct]

1 In a declaration T D where D has the form

D1 (   parameter-declaration-clause ) cv-qualifier-seqopt exception-specificationopt

parameter-declaration-clause:

parameter-declaration-listopt ...opt
parameter-declaration-list , ...

parameter-declaration-list:

parameter-declaration
parameter-declaration-list , parameter-declaration

parameter-declaration:

decl-specifier-seq declarator
decl-specifier-seq declarator = assignment-expression
decl-specifier-seq abstract-declaratoropt
decl-specifier-seq abstract-declarator_opt = assignment-expression

2 The parameter-declaration-clause determines the arguments that can be specified, and their processing, when the function is called. [Note: the parameter-declaration-clause is used to convert the arguments specified on the function call; see 5.2.2. ] If the parameter-declaration-clause is empty, the function takes no arguments. The parameter list (void) is equivalent to the empty parameter list. Except for this special case, void shall not be a parameter type (though types derived from void, such as void*, can). If the parameter-declaration-clause terminates with an ellipsis, the number of arguments shall be equal to or greater than the number of parameters specified. Where syntactically correct, ``, ...'' is synonymous with ``...''. [Example: the declaration

int printf(const char*, ...);

printf("hello world");
printf("a=%d b=%d", a, b);

3 A single name can be used for several different functions in a single scope; this is function overloading (clause 13). All declarations for a function with a given parameter list shall agree exactly both in the type of the value returned and in the number and type of parameters; the presence or absence of the ellipsis is considered part of the function type. The type of a function is determined using the following rules. The type of each parameter is determined from its own decl-specifier-seq and declarator. After determining the type of each parameter, any parameter of type ``array of T'' or ``function returning T'' is adjusted to be ``pointer to T'' or ``pointer to function returning T,'' respectively. After producing the list of parameter types, several transformations take place upon these types to determine the function type. Any cv-qualifier modifying a parameter type is deleted. [Example: the type void(*)(const int) becomes void(*)(int) ---end example] Such cv-qualifiers affect only the definition of the parameter within the body of the function; they do not affect the function type. If a storage-class-specifier modifies a parameter type, the specifier is deleted. [Example: register char* becomes char* ---end example] Such storage-class-specifiers affect only the definition of the parameter within the body of the function; they do not affect the function type. The resulting list of transformed parameter types is the function's parameter type list.

4 A cv-qualifier-seq shall only be part of the function type for a nonstatic member function, the function type to which a pointer to member refers, or the top-level function type of a function typedef declaration. The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type, i.e., it does not create a cv-qualified function type. In fact, if at any time in the determination of a type a cv-qualified function type is formed, the program is ill-formed. [Example:

typedef void F();
struct S {
          const F f;                       // ill-formed:
                                           // not equivalent to: void f() const;
};

5 [Example: the declaration

int fseek(FILE*, long, int);

6 If the type of a parameter includes a type of the form ``pointer to array of unknown bound of T'' or ``reference to array of unknown bound of T,'' the program is ill-formed.⁸⁷⁾ Functions shall not have a return type of type array or function, although they may have a return type of type pointer or reference to such things. There shall be no arrays of functions, although there can be arrays of pointers to functions. Types shall not be defined in return or parameter types. The type of a parameter or the return type for a function declaration that is not a definition may be an incomplete class type.

7 A typedef of function type may be used to declare a function but shall not be used to define a function (8.4). [Example:

typedef void F();
F   fv;                                    // OK: equivalent to void fv();
F   fv { }                                 // ill-formed
void fv() { }                              // OK: definition of fv

typedef int FIC(int) const;
FIC f;                                     // ill-formed: does not declare a member function
struct S {
          FIC f;                           // OK
};
FIC S::*pm = &S::f;                        // OK

8 An identifier can optionally be provided as a parameter name; if present in a function definition (8.4), it names a parameter (sometimes called ``formal argument''). [Note: in particular, parameter names are also optional in function definitions and names used for a parameter in different declarations and the definition of a function need not be the same. If a parameter name is present in a function declaration that is not a definition, it cannot be used outside of the parameter-declaration-clause since it goes out of scope at the end of the function declarator (3.3). ]

9 [Example: the declaration

int i,
     *pi,
     f(),
     *fpi(int),
     (*pif)(const char*, const char*);
     (*fpif(int))(int);

typedef int      IFUNC(int);
IFUNC*     fpif(int);

86) As indicated by the syntax, cv-qualifiers are a significant component in function return types. [back to text]

87) This excludes parameters of type ``ptr-arr-seq T2'' where T2 is ``pointer to array of unknown bound of T'' and where ptr-arr-seq means any sequence of ``pointer to'' and ``array of'' derived declarator types. This exclusion applies to the parameters of the function, and if a parameter is a pointer to function or pointer to member function then to its parameters also, etc. [back to text]

8.3.6 Default arguments [dcl.fct.default]

1 If an expression is specified in a parameter declaration this expression is used as a default argument. Default arguments will be used in calls where trailing arguments are missing.

2 [Example: the declaration

void point(int = 3, int = 4);

point(1,2);      point(1);      point();

3 A default argument expression shall be specified only in the parameter-declaration-clause of a function declaration or in a template-parameter (14.1). If it is specified in a parameter-declaration-clause, it shall not occur within a declarator or abstract-declarator of a parameter-declaration.⁸⁸⁾

4 For non-template functions, default arguments can be added in later declarations of a function in the same scope. Declarations in different scopes have completely distinct sets of default arguments. That is, declarations in inner scopes do not acquire default arguments from declarations in outer scopes, and vice versa. In a given function declaration, all parameters subsequent to a parameter with a default argument shall have default arguments supplied in this or previous declarations. A default argument shall not be redefined by a later declaration (not even to the same value). [Example:

void f(int, int);
void f(int, int = 7);
void h()
{
     f(3);                              // OK, calls f(3, 7)
     void f(int = 1, int);              // error: does not use default
                                        // from surrounding scope
}

void m()
{
     void f(int, int);                  // has no defaults
     f(4);                              // error: wrong number of arguments
     void f(int, int = 5);              // OK
     f(4);                              // OK, calls f(4, 5);
     void f(int, int = 5);              // error: cannot redefine, even to
                                        // same value
}
void n()
{
     f(6);                              // OK, calls f(6, 7)
}

5 A default argument expression is implicitly converted (clause 4) to the parameter type. The default argument expression has the same semantic constraints as the initializer expression in a declaration of a variable of the parameter type, using the copy-initialization semantics (8.5). The names in the expression are bound, and the semantic constraints are checked, at the point where the default argument expression appears. Name lookup and checking of semantic constraints for default arguments in function templates and in member functions of class templates are performed as described in 14.7.1. [Example: in the following code, g will be called with the value f(1):

int a = 1;
int f(int);
int g(int x = f(a));                    // default argument: f(::a)

void h() {
     a = 2;
     {
          int a = 3;
          g();                          // g(f(::a))
     }
}

6 The default arguments in a member function definition that appears outside of the class definition are added to the set of default arguments provided by the member function declaration in the class definition. [Exam-ple:

class C {
          void f(int i = 3);
          void g(int i, int j = 99);
};
void C::f(int i = 3)                    // error: default argument already
{ }                                     // specified in class scope
void C::g(int i = 88, int j)            // in this translation unit,
{ }                                     // C::g can be called with no argument

7 Local variables shall not be used in default argument expressions. [Example:

void f()
{
     int i;
     extern void g(int x = i);          // error
     // ...
}

8 The keyword this shall not be used in a default argument of a member function. [Example:

class A {
     void f(A* p = this) { }            // error
};

9 Default arguments are evaluated each time the function is called. The order of evaluation of function arguments is unspecified. Consequently, parameters of a function shall not be used in default argument expressions, even if they are not evaluated. Parameters of a function declared before a default argument expression are in scope and can hide namespace and class member names. [Example:

int a;
int f(int a, int b = a);                          // error: parameter a
                                                  // used as default argument
typedef int I;
int g(float I, int b = I(2));                     // error: parameter I found
int h(int a, int b = sizeof(a));                  // error, parameter a used
                                                  // in default argument

int b;
class X {
     int a;
     int mem1(int i = a);               // error: nonstatic member a
                                        // used as default argument
     int mem2(int i = b);               // OK;  use X::b
     static int b;
};

int f(int = 0);

void h()
{
     int j = f(1);
     int k = f();                       // OK, means f(0)
}

int (*p1)(int) = &f;
int (*p2)() = &f;                       // error: type mismatch

10 A virtual function call (10.3) uses the default arguments in the declaration of the virtual function determined by the static type of the pointer or reference denoting the object. An overriding function in a derived class does not acquire default arguments from the function it overrides. [Example:

struct A {
     virtual void f(int a = 7);
};
struct B : public A {
     void f(int a);
};
void m()
{
     B* pb = new B;
     A* pa = pb;
     pa->f();                           // OK, calls pa->B::f(7)
     pb->f();                           // error: wrong number of arguments for B::f()
}

88) This means that default arguments cannot appear, for example, in declarations of pointers to functions, references to functions, or typedef declarations. [back to text]

8.4 Function definitions [dcl.fct.def]

1 Function definitions have the form

function-definition:

decl-specifier-seqopt declarator ctor-initializeropt function-body
decl-specifier-seqopt declarator function-try-block

function-body:

compound-statement

D1 (  parameter-declaration-clause ) cv-qualifier-seqopt exception-specificationopt

2 [Example: a simple example of a complete function definition is

int max(int a, int b, int c)
{
     int m = (a > b) ? a : b;
     return (m > c) ? m : c;
}

3 A ctor-initializer is used only in a constructor; see 12.1 and 12.6.

4 A cv-qualifier-seq can be part of a non-static member function declaration, non-static member function definition, or pointer to member function only; see 9.3.2. It is part of the function type.

5 [Note: unused parameters need not be named. For example,

void print(int a, int)
{
     printf("a = %d\n",a);
}

8.5 Initializers [dcl.init]

1 A declarator can specify an initial value for the identifier being declared. The identifier designates an object or reference being initialized. The process of initialization described in the remainder of 8.5 applies also to initializations specified by other syntactic contexts, such as the initialization of function parameters with argument expressions (5.2.2) or the initialization of return values (6.6.3).

initializer:

= initializer-clause
( expression-list )

initializer-clause:

assignment-expression
{ initializer-list ,opt }
{ }

initializer-list:

initializer-clause
initializer-list , initializer-clause

2 Automatic, register, static, and external variables of namespace scope can be initialized by arbitrary expressions involving literals and previously declared variables and functions. [Example:

int f(int);
int a = 2;
int b = f(a);
int c(b);

3 [Note: default argument expressions are more restricted; see 8.3.6.

4 The order of initialization of static objects is described in 3.6 and 6.7. ]

5 To zero-initialize storage for an object of type T means:

• if T is a scalar type (3.9), the storage is set to the value of 0 (zero) converted to T;
• if T is a non-union class type, the storage for each nonstatic data member and each base-class subobject is zero-initialized;
• if T is a union type, the storage for its first data member⁸⁹⁾ is zero-initialized;
• if T is an array type, the storage for each element is zero-initialized;
• if T is a reference type, no initialization is performed. To default-initialize an object of type T means:
  • if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
  • if T is an array type, each element is default-initialized;
  • otherwise, the storage for the object is zero-initialized.
  A program that calls for default-initialization of an entity of reference type is ill-formed. If T is a cv-qualified type, the cv-unqualified version of T is used for these definitions of zero-initialization and default-initialization.

6 The memory occupied by any object of static storage duration shall be zero-initialized at program startup before any other initialization takes place. [Note: in some cases, additional initialization is done later. ]

7 An object whose initializer is an empty set of parentheses, i.e., (), shall be default-initialized.

8 [Note: since () is not permitted by the syntax for initializer,

X a();

9 If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for an object, the object and its subobjects, if any, have an indeterminate initial value⁹⁰⁾; if the object or any of its subobjects are of const-qualified type, the program is ill-formed.

10 An initializer for a static member is in the scope of the member's class. [Example:

int a;

struct X {
     static int a;
     static int b;
};

int X::a = 1;
int X::b = a;                              // X::b = X::a

11 The form of initialization (using parentheses or =) is generally insignificant, but does matter when the entity being initialized has a class type; see below. A parenthesized initializer can be a list of expressions only when the entity being initialized has a class type.

12 The initialization that occurs in argument passing, function return, throwing an exception (15.1), handling an exception (15.3), and brace-enclosed initializer lists (8.5.1) is called copy-initialization and is equivalent to the form

T x = a;

T x(a);

13 If T is a scalar type, then a declaration of the form

T x = { a };

T x = a;

14 The semantics of initializers are as follows. The destination type is the type of the object or reference being initialized and the source type is the type of the initializer expression. The source type is not defined when the initializer is brace-enclosed or when it is a parenthesized list of expressions.

• If the destination type is a reference type, see 8.5.3.
• If the destination type is an array of characters or an array of wchar_t, and the initializer is a string lit eral, see 8.5.2.
• Otherwise, if the destination type is an array, see 8.5.1.
• If the destination type is a (possibly cv-qualified) class type:
  • If the class is an aggregate (8.5.1), and the initializer is a brace-enclosed list, see 8.5.1.
  • If the initialization is direct-initialization, or if it is copy-initialization where the cv-unqualified ver sion of the source type is the same class as, or a derived class of, the class of the destination, constructors are considered. The applicable constructors are enumerated (13.3.1.3), and the best one is chosen through overload resolution (13.3). The constructor so selected is called to initialize the object, with the initializer expression(s) as its argument(s). If no constructor applies, or the overload resolution is ambiguous, the initialization is ill-formed.
  • Otherwise (i.e., for the remaining copy-initialization cases), user-defined conversion sequences that can convert from the source type to the destination type or (when a conversion function is used) to a derived class thereof are enumerated as described in 13.3.1.4, and the best one is chosen through overload resolution (13.3). If the conversion cannot be done or is ambiguous, the initialization is ill-formed. The function selected is called with the initializer expression as its argument; if the function is a constructor, the call initializes a temporary of the destination type. The result of the call (which is the temporary for the constructor case) is then used to direct-initialize, according to the rules above, the object that is the destination of the copy-initialization. In certain cases, an implementation is permitted to eliminate the copying inherent in this direct-initialization by constructing the intermediate result directly into the object being initialized; see 12.2, 12.8.
• Otherwise, if the source type is a (possibly cv-qualified) class type, conversion functions are considered. The applicable conversion functions are enumerated (13.3.1.5), and the best one is chosen through overload resolution (13.3). The user-defined conversion so selected is called to convert the initializer expression into the object being initialized. If the conversion cannot be done or is ambiguous, the initialization is ill-formed.
• Otherwise, the initial value of the object being initialized is the (possibly converted) value of the initial izer expression. Standard conversions (clause 4) will be used, if necessary, to convert the initializer expression to the cv-unqualified version of the destination type; no user-defined conversions are considered. If the conversion cannot be done, the initialization is ill-formed. [Note: an expression of type ``cv1 T'' can initialize an object of type ``cv2 T'' independently of the cv-qualifiers cv1 and cv2.

  int a;
  const int b = a;
  int c = b;
  
  

  ---end note]

89) This member must not be static, by virtue of the requirements in 9.5. [back to text]

90) This does not apply to aggregate objects with automatic storage duration initialized with an incomplete brace-enclosed initializer-list; see 8.5.1. [back to text]

8.5.1 Aggregates [dcl.init.aggr]

1 An aggregate is an array or a class (clause 9) with no user-declared constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10), and no virtual functions (10.3).

2 When an aggregate is initialized the initializer can be an initializer-clause consisting of a brace-enclosed, comma-separated list of initializers for the members of the aggregate, written in increasing subscript or member order. If the aggregate contains subaggregates, this rule applies recursively to the members of the subaggregate. [Example:

struct A {
          int x;
          struct B {
                     int i;
                     int j;
          } b;
} a = { 1, { 2, 3 } };

3 An aggregate that is a class can also be initialized with a single expression not enclosed in braces, as described in 8.5.

4 An array of unknown size initialized with a brace-enclosed initializer-list containing n initializers, where n shall be greater than zero, is defined as having n elements (8.3.4). [Example:

int x[] = { 1, 3, 5 };

5 Static data members are not considered members of the class for purposes of aggregate initialization. [Example:

struct A {
          int i;
          static int s;
          int j;
} a = { 1, 2 };

6 An initializer-list is ill-formed if the number of initializers exceeds the number of members or elements to initialize. [Example:

char cv[4] = { 'a', 's', 'd', 'f', 0 };                       // error

7 If there are fewer initializers in the list than there are members in the aggregate, then each member not explicitly initialized shall be default-initialized (8.5). [Example:

struct S { int a; char* b; int c; };
S ss = { 1, "asdf" };

8 An initializer for an aggregate member that is an empty class shall have the form of an empty initializer-list {}. [Example:

struct S { };
struct A {
           S s;
           int i;
} a = { { } , 3 };

9 If an incomplete or empty initializer-list leaves a member of reference type uninitialized, the program is ill-formed.

10 When initializing a multi-dimensional array, the initializers initialize the elements with the last (rightmost) index of the array varying the fastest (8.3.4). [Example:

int x[2][2] = { 3, 1, 4, 2 };

float y[4][3] = {
     { 1 }, { 2 }, { 3 }, { 4 }
};

11 Braces can be elided in an initializer-list as follows. If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializers initializes the members of a subaggregate; it is erroneous for there to be more initializers than members. If, however, the initializer-list for a subaggregate does not begin with a left brace, then only enough initializers from the list are taken to initialize the members of the subaggregate; any remaining initializers are left to initialize the next member of the aggregate of which the current subaggregate is a member. [Example:

float y[4][3] = {
     { 1, 3, 5 },
     { 2, 4, 6 },
     { 3, 5, 7 },
};

float y[4][3] = {
     1, 3, 5, 2, 4, 6, 3, 5, 7
};

12 All implicit type conversions (clause 4) are considered when initializing the aggregate member with an initializer from an initializer-list. If the initializer can initialize a member, the member is initialized. Otherwise, if the member is itself a non-empty subaggregate, brace elision is assumed and the initializer is considered for the initialization of the first member of the subaggregate. [Example:

struct A {
     int i;
     operator int();
};
struct B {
     A a1, a2;
     int z;
};
A a;
B b = { 4, a, a };

13 [Note: An aggregate array or an aggregate class may contain members of a class type with a user-declared constructor (12.1). Initialization of these aggregate objects is described in 12.6.1. ]

14 When an aggregate with static storage duration is initialized with a brace-enclosed initializer-list, if all the member initializer expressions are constant expressions, and the aggregate is a POD type, the initialization shall be done during the static phase of initialization (3.6.2); otherwise, it is unspecified whether the initialization of members with constant expressions takes place during the static phase or during the dynamic phase of initialization.

15 When a union is initialized with a brace-enclosed initializer, the braces shall only contain an initializer for the first member of the union. [Example:

union u { int a; char* b; };

u a = { 1 };
u b = a;
u c = 1;                                 // error
u d = { 0, "asdf" };                     // error
u e = { "asdf" };                        // error

91) The syntax provides for empty initializer-lists, but nonetheless C++ does not have zero length arrays. [back to text]

8.5.2 Character arrays [dcl.init.string]

1 A char array (whether plain char, signed char, or unsigned char) can be initialized by a string-literal (optionally enclosed in braces); a wchar_t array can be initialized by a wide string-literal (optionally enclosed in braces); successive characters of the string-literal initialize the members of the array. [Example:

char msg[] = "Syntax error on line %s\n";

2 There shall not be more initializers than there are array elements. [Example:

char cv[4] = "asdf";                     // error

8.5.3 References [dcl.init.ref]

1 A variable declared to be a T&, that is ``reference to type T'' (8.3.2), shall be initialized by an object, or function, of type T or by an object that can be converted into a T. [Example:

int g(int);
void f()
{
     int i;
     int& r = i;                          // r  refers to i
     r = 1;                               //  the value of i becomes 1
     int* p = &r;                         // p  points to i
     int& rr = r;                         // rr  refers to what r refers to, that is, to i
     int (&rg)(int) = g;                  // rg  refers to the function g
     rg(i);                               //  calls function g
     int a[3];
     int (&ra)[3] = a;                    // ra  refers to the array a
     ra[1] = i;                            // modifies a[1]
}

2 A reference cannot be changed to refer to another object after initialization. Note that initialization of a reference is treated very differently from assignment to it. Argument passing (5.2.2) and function value return (6.6.3) are initializations.

3 The initializer can be omitted for a reference only in a parameter declaration (8.3.5), in the declaration of a function return type, in the declaration of a class member within its class declaration (9.2), and where the extern specifier is explicitly used. [Example:

int& r1;                                  //  error: initializer missing
extern int& r2;                           //  OK

4 Given types ``cv1 T1'' and ``cv2 T2,'' ``cv1 T1'' is reference-related to ``cv2 T2'' if T1 is the same type as T2, or T1 is a base class of T2. ``cv1 T1'' is reference-compatible with ``cv2 T2'' if T1 is reference-related to T2 and cv1 is the same cv-qualification as, or greater cv-qualification than, cv2. For purposes of overload resolution, cases for which cv1 is greater cv-qualification than cv2 are identified as reference-compatible with added qualification (see 13.3.3.2). In all cases where the reference-related or referencecompatible relationship of two types is used to establish the validity of a reference binding, and T1 is a base class of T2, a program that necessitates such a binding is ill-formed if T1 is an inaccessible (clause 11) or ambiguous (10.2) base class of T2.

5 A reference to type ``cv1 T1'' is initialized by an expression of type ``cv2 T2'' as follows:

• If the initializer expression
  • is an lvalue (but is not a bit-field), and ``cv1 T1'' is reference-compatible with ``cv2 T2,'' or
  • has a class type (i.e., T2 is a class type) and can be implicitly converted to an lvalue of type ``cv3 T3,'' where ``cv1 T1'' is reference-compatible with ``cv3 T3'' ⁹²⁾ (this conversion is selected by enumerating the applicable conversion functions (13.3.1.6) and choosing the best one through overload resolution (13.3)),
  then the reference is bound directly to the initializer expression lvalue in the first case, and the reference is bound to the lvalue result of the conversion in the second case. In these cases the reference is said to bind directly to the initializer expression. [Note: the usual lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are not needed, and therefore are suppressed, when such direct bindings to lvalues are done. ] [Example:

  double d = 2.0;
  double& rd = d;                           // rd refers to d
  const double& rcd = d;                    // rcd  refers to d
  
  struct A { };
  struct B : public A { } b;
  A& ra = b;                                // ra refers to A sub-object in b
  const A& rca = b;                         // rca  refers to A sub-object in b
  
  

  ---end example]
• Otherwise, the reference shall be to a non-volatile const type (i.e., cv1 shall be const). [Example:

  double& rd2 = 2.0;                        // error: not an lvalue and reference not const
  int   i = 2;
  double& rd3 = i;                          // error: type mismatch and reference not const
  
  

  ---end example]
  • If the initializer expression is an rvalue, with T2 a class type, and ``cv1 T1'' is reference-compatible with ``cv2 T2,'' the reference is bound in one of the following ways (the choice is implementationdefined):
    • The reference is bound to the object represented by the rvalue (see 3.10) or to a sub-object within that object.
    • A temporary of type ``cv1 T2'' [sic] is created, and a constructor is called to copy the entire rvalue object into the temporary. The reference is bound to the temporary or to a sub-object within the temporary.⁹³⁾
    The constructor that would be used to make the copy shall be callable whether or not the copy is actually done. [Example:

    struct A { };
    struct B : public A { } b;
    extern B f();
    const A& rca = f();                       //  Either bound to the A sub-object of the B rvalue,
                                              //   or the entire B object is copied and the reference
                                              //  is bound to the A sub-object of the copy
    
    

    ---end example]
  • Otherwise, a temporary of type ``cv1 T1'' is created and initialized from the initializer expression using the rules for a non-reference copy initialization (8.5). The reference is then bound to the temporary. If T1 is reference-related to T2, cv1 must be the same cv-qualification as, or greater cv-qualification than, cv2; otherwise, the program is ill-formed. [Example:

    const double& rcd2 = 2;                   // rcd2  refers to temporary with value 2.0
    const volatile int cvi = 1;
    const int& r = cvi;                       //  error: type qualifiers dropped
    
    

    ---end example]

6 [Note: 12.2 describes the lifetime of temporaries bound to references. ]

92) This requires a conversion function (12.3.2) returning a reference type. [back to text]

93) Clearly, if the reference initialization being processed is one for the first argument of a copy constructor call, an implementation must eventually choose the first alternative (binding without copying) to avoid infinite recursion. [back to text]