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
                  init-declarator-list , init-declarator
                  declarator initializeropt

2 The  two  components  of  a  declaration  are  the  specifiers  (decl-
  specifier-seq; _dcl.spec_) 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 _dcl.init_
  and _class.init_.

3 Each  init-declarator in a declaration is analyzed separately as if it
  was in a declaration by itself.1)

  1) 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

4 Declarators have the syntax
                  ptr-operator declarator
                  direct-declarator ( parameter-declaration-clause ) cv-qualifier-seqopt exception-specificationopt
                  direct-declarator [ constant-expressionopt ]
                  ( declarator )
                  * cv-qualifier-seqopt
                  ::opt nested-name-specifier * cv-qualifier-seqopt
                  cv-qualifier cv-qualifier-seqopt
                  ::opt id-expression
                  ::opt nested-name-specifieropt type-name
  A class-name has special meaning in a declaration of the class of that
  name and when qualified by that name using the scope resolution opera­
  tor :: (_expr.prim_, _class.ctor_, _class.dtor_).

  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
                  type-specifier-seq abstract-declaratoropt
                  type-specifier type-specifier-seqopt
                  ptr-operator abstract-declaratoropt
                  direct-abstract-declaratoropt ( parameter-declaration-clause ) cv-qualifier-seqopt exception-specificationopt
                  direct-abstract-declaratoropt [ constant-expressionopt ]
                  ( abstract-declarator )
  It is possible to identify uniquely  the  location  in  the  abstract-
  declarator  where the identifier would appear if the construction were
  a declarator in a declaration.  The named type is then the same as the
  type of the hypothetical identifier.  [Example:
          int                 // int i
          int *               // int *pi
          int *[3]            // int *p[3]
          int (*)[3]          // int (*p3i)[3]
          int *()             // int *f()
          int (*)(double)     // int (*pf)(double)
  name  respectively  the  types  int,"  "pointer  to  int," "array of 3

  pointers to int," "pointer to array of 3 int," "function of (no param­
  eters)  returning  pointer to int," and "pointer to a function of dou­
  ble) returning int.''  ]

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

  8.2  Ambiguity resolution                              [dcl.ambig.res]

1 The  ambiguity  arising  from  the similarity between a function-style
  cast and a declaration mentioned in _stmt.ambig_ 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
  _stmt.ambig_,  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 indi­
  cate initialization or by removing the  redundant  parentheses  around
  the parameter name.  ] [Example:
          struct S {
          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
   --end example]

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 consid­
  ered 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)
   --end example]

7 Another ambiguity arises in a parameter-declaration-clause of a  func­
  tion  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.
          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 ptr
                  f(g);    // OK
  For another example,
          class C { };
          void h(int *(C[10]));  // void h(int *(*_fp)(C _parm[10]));
                                 // not: void h(int *C[10]);
   --end example]

  8.3  Meaning of declarators                              [dcl.meaning]

1 A  list  of  declarators  appears  after an optional (_dcl.dcl_) decl-
  specifier-seq (_dcl.spec_).   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 (_class.conv_, _class.dtor_,

  _over.oper_) and for the declaration of  template  specializations  or
  partial  specializations  (_temp.spec_).  A declarator-id shall not be
  qualified  except  for   the   definition   of   a   member   function
  (_class.mfct_)  or static data member (_class.static_) or nested class
  (_class.nest_) 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
  (_class.friend_).   When  the declarator-id is qualified, the declara­
  tion shall refer to a previously  declared  member  of  the  class  or
  namespace  to  which the qualifier refers.  [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 defini­
  tion.  [Example:
          namespace A {
                  struct B {
                          void f();
                  void A::B::f() { } // ill-formed: the declarator must not be
                                     // qualified with A::
   --end example]

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
  where T is a decl-specifier-seq and D is a declarator.  The  following
  subclauses  give a recursive procedure for determining the type speci­
  fied for the contained declarator-id by such a declaration.

4 First, the decl-specifier-seq determines a type.  In a declaration
          T D
  the decl-specifier-seq T determines the type  T."   [Example:  in  the
          int unsigned i;
  the  type  specifiers  int  unsigned  determine the type unsigned int"
  (_dcl.type.simple_).  ]

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 )
  the  type  of  the  contained declarator-id is the same as that of the
  contained declarator-id in the declaration

          T D1
  Parentheses do not alter the type of the embedded  declarator-id,  but
  they can alter the binding of complex declarators.

  8.3.1  Pointers                                              [dcl.ptr]

1 In a declaration T D where D has the form
          * cv-qualifier-seqopt D1
  and  the  type  of the identifier in the declaration T D1 is "derived-
  declarator-type-list T," then the type  of  the  identifier  of  D  is
  "derived-declarator-type-list cv-qualifier-seq pointer to T."  The cv-
  qualifiers apply to the pointer and not to the object pointed to.

2 [Example: the declarations
          const int ci = 10, *pc = &ci, *const cpc = pc, **ppc;
          int i, *p, *const cp = &i;
  declare ci, a constant integer; pc, a pointer to a  constant  integer;
  cpc,  a  constant  pointer  to a constant integer, ppc, a pointer to a
  pointer to a constant integer; i, an integer; p, a pointer to integer;
  and  cp,  a constant pointer to integer.  The value of ci, cpc, and cp
  cannot be changed after  initialization.   The  value  of  pc  can  be
  changed,  and  so  can  the object pointed to by cp.  Examples of some
  correct operations are
          i = ci;
          *cp = ci;
          pc = cpc;
          pc = p;
          ppc = &pc;
  Examples of ill-formed operations are
          ci = 1;      // error
          ci++;        // error
          *pc = 2;     // error
          cp = &ci;    // error
          cpc++;       // error
          p = pc;      // error
          ppc = &p;    // error
  Each is unacceptable because it would either change the  value  of  an
  object  declared  const  or  allow  it  to  be  changed  through a cv-
  unqualified pointer later, for example:
          *ppc = &ci;  // okay, but would make p point to ci ...
                       // ... because of previous error
          *p = 5;      // clobber ci
   --end example]

3 See also _expr.ass_ and _dcl.init_.

4 [Note: there are no pointers to references; see _dcl.ref_.  Since  the
  address  of  a  bit-field (_class.bit_) 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
  and the type of the identifier in the declaration T  D1  is  "derived-
  declarator-type-list  T,"  then  the  type  of  the identifier of D is
  "derived-declarator-type-list reference to  T."   Cv-qualified  refer­
  ences  are  ill-formed  except  when  the cv-qualifiers are introduced
  through the use of a typedef (_dcl.typedef_) or  of  a  template  type
  argument  (_temp.arg_),  in  which case the cv-qualifiers are ignored.
  [Example: in
          typedef int& A;
          const A aref = 3; // ill-formed;
                            // non-const reference initialized with rvalue
  the type of aref is "reference to int", not "const reference to  int".
  ]  [Note:  a  reference can be thought of as a name of an object.  ] A
  declarator that specifies the type "reference  to  cv  void"  is  ill-

2 [Example:
          void f(double& a) { a += 3.14; }
          // ...
          double d = 0;
  declares  a to be a reference parameter of f so the call f(d) will add
  3.14 to d.
          int v[20];
          // ...
          int& g(int i) { return v[i]; }
          // ...
          g(3) = 7;
  declares the function g() to return  a  reference  to  an  integer  so
  g(3)=7  will  assign  7  to  the  fourth  element of the array v.  For
  another example,
          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;
  declares p to be a reference to a pointer to link so h(q) will leave q
  with the value zero.  See also _dcl.init.ref_.  ]

3 It  is  unspecified  whether  or  not  a  reference  requires  storage

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 (_dcl.init.ref_) except  when  the  declaration
  contains  an  explicit extern specifier (_dcl.stc_), is a class member
  (_class.mem_) declaration within a class declaration, or is the decla­
  ration  of  a parameter or a return type (_dcl.fct_); see _basic.def_.
  A reference shall be initialized to refer to a valid object  or  func­
  tion.   [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
  _class.bit_, 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
  and the nested-name-specifier names a class, and the type of the iden­
  tifier in the declaration T D1  is  "derived-declarator-type-list  T,"
  then  the type of the identifier of D is "derived-declarator-type-list
  cv-qualifier-seq pointer to member of class  nested-name-specifier  of
  type T."

2 [Example:
          class X {
              void f(int);
              int a;
          class Y;

          int X::* pmi = &X::a;
          void (X::* pmf)(int) = &X::f;
          double X::* pmd;
          char Y::* pmc;
  declares  pmi,  pmf,  pmd  and pmc to be a pointer to a member of X of
  type int, a pointer to a member of X of type void(int), a pointer to a
  member of X of type double and a pointer to a member of Y of type char
  respectively.  The declaration of pmd is well-formed even though X has
  no members of type double.  Similarly, the declaration of pmc is well-
  formed even though Y is an incomplete type.  pmi and pmf can  be  used
  like this:
          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
   --end example]

3 A  pointer  to  member  shall  not point to a static member of a class
  (_class.static_), a member with reference type, or "cv void."   [Note:
  see also _expr.unary_ and _expr.mptr.oper_.  The type "pointer to mem­
  ber" 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]
  and  the  type  of the identifier in the declaration T D1 is "derived-
  declarator-type-list T," then the type of the identifier of  D  is  an
  array  type.   T is called the array element type; this type shall not
  be a reference type, the type void, a function  type  or  an  abstract
  class  type.  If the constant-expression (_expr.const_) is present, it
  shall be an integral  constant  expression  and  its  value  shall  be
  greater  than  zero.   The  constant expression specifies the bound of
  (number of elements in) the array.   If  the  value  of  the  constant
  expression  is  N, the array has N elements numbered 0 to N-1, and the
  type of the identifier of D is "derived-declarator-type-list array  of
  N  T."  If the constant expression is omitted, the type of the identi­
  fier of D is "derived-declarator-type-list array of unknown  bound  of
  T," an incomplete object type.  The type "derived-declarator-type-list
  array of N T" is a different type from the  type  "derived-declarator-
  type-list  array  of unknown bound of T," see _basic.types_.  Any type
  of the form "cv-qualifier-seq array of N T" is adjusted to "array of N
  cv-qualifier-seq  T," and similarly for "array of unknown bound of T."
  An object of array type contains a  contiguously  allocated  non-empty
  set of N sub-objects of type T.  [Example:
          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''
   --end example]

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 multidimen­
  sional 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 (_dcl.init_).  In this case the bound is  calculated  from
  the  number  of  initial elements (say, N) supplied (_dcl.init.aggr_),
  and the type of the identifier of D is "array of N T."

4 [Example:
          float fa[17], *afp[17];

  declares an array of float numbers and an array of pointers  to  float
  numbers.  For another example,
          static int x3d[3][5][7];
  declares  a  static  three-dimensional  array  of  integers, with rank
  3×5×7.  In complete detail, x3d is an array of three items; each  item
  is  an  array of five arrays; each of the latter arrays is an array of
  seven integers.   Any  of  the  expressions  x3d,  x3d[i],  x3d[i][j],
  x3d[i][j][k] can reasonably appear in an expression.  ]

5 [Note:  conversions  affecting  lvalues of array type are described in
  _conv.array_.   Objects  of  array  types  cannot  be  modified,   see
  _basic.lval_.  ]

6 Except  where  it has been declared for a class (_over.sub_), the sub­
  script operator [] is interpreted in such a way that E1[E2] is identi­
  cal 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, sub­
  scripting 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 expres­
  sion 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 con­
  verted into a pointer.

8 [Example: consider
          int x[3][5];
  Here x is a 3×5 array of integers.  When x appears in  an  expression,
  it  is  converted  to  a pointer to (the first of three) five-membered
  arrays of integers.  In the expression x[i], which  is  equivalent  to
  *(x+i),  x  is  first converted to a pointer as described; then x+i is
  converted to the type of x, which involves multiplying i by the length
  of  the  object  to  which  the  pointer  points,  namely five integer
  objects.  The results are added and indirection applied  to  yield  an
  array  (of  five integers), which in turn is converted to a pointer to
  the first of the integers.  If there is  another  subscript  the  same
  argument applies again; this time the result is an integer.  ]

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
  and the type of the contained declarator-id in the declaration T D1 is
  "derived-declarator-type-list  T,"  the type of the declarator-id in D
  is "derived-declarator-type-list function  of  (parameter-declaration-
  clause)  cv-qualifier-seqopt  returning  T";  a type of this form is a

  function type2).
                  parameter-declaration-listopt ...opt
                  parameter-declaration-list , ...
                  parameter-declaration-list , parameter-declaration
                  decl-specifier-seq declarator
                  decl-specifier-seq declarator = assignment-expression
                  decl-specifier-seq abstract-declaratoropt
                  decl-specifier-seq abstract-declaratoropt = 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 _expr.call_.  ] 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  parame­
  ter-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*, ...);
  declares a function that can be called with varying numbers and  types
  of arguments.
          printf("hello world");
          printf("a=%d b=%d", a, b);
  However, the first argument must be of a type that can be converted to
  a const char*.  ] [Note: the  standard  header  <cstdarg>  contains  a
  mechanism  for  accessing  arguments  passed  using  the ellipsis (see
  _expr.call_ and _lib.support.runtime_).  ]

3 A single name can be used for several different functions in a  single
  scope;  this is function overloading (_over_).  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 follow­
  ing 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,  sev­
  eral  transformations  take  place  upon  these types to determine the
  function  type.   Any  cv-qualifier  modifying  a  parameter  type  is
  deleted;  e.g.,  the type void(const int) becomes void(int).  Such cv-
  qualifiers only affect the definition of the parameter within the body
  2) As indicated by the syntax, cv-qualifiers are a significant  compo­
  nent in function return types.

  of  the function; they do not affect the function type.  If a storage-
  class-specifier modifies a parameter type, the specifier  is  deleted;
  e.g.,  register  char*  becomes  char*.  Such storage-class-qualifiers
  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.   A
  cv-qualifier-seq  shall  only  be part of the function type for a non­
  static member function, the function type to which a pointer to member
  refers,  or the top-level function type of a function typedef declara­
  tion.  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;
   --end example] The return type, the parameter type list and  the  cv-
  qualifier-seq,  but  not  the default arguments (_dcl.fct.default_) or
  the exception specification (_except.spec_), are part of the  function
  type.   [Note:  function  types are checked during the assignments and
  initializations of pointer-to-functions,  reference-to-functions,  and
  pointer-to-member-functions.  ]

4 [Example: the declaration
          int fseek(FILE*, long, int);
  declares a function taking three arguments of the specified types, and
  returning int (_dcl.type_).  ]

5 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.3) 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.

6 A typedef of function type may be used to declare a function but shall
  not be used to define a function.  [Example:
          typedef void F();
          F  fv;    // ok: equivalent to void fv();
          F  fv { } // ill-formed
          void fv() { } // ok: definition of fv
   --end example] A typedef of a function type whose declarator includes
  3) 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 pa­
  rameter is a pointer to function or pointer to member function then to
  its parameters also, etc.

  a cv-qualifier-seq shall be used only to declare the function type for
  a nonstatic member function, to declare the function type to  which  a
  pointer to member refers, or to declare the top-level function type of
  another function typedef declaration.  [Example:
          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
   --end example]

7 An identifier can optionally be provided as a parameter name; if  pre­
  sent  in  a  function definition (_dcl.fct.def_), 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 (_basic.scope_).  ]

8 [Example: the declaration
          int i,
              (*pif)(const char*, const char*);
  declares an integer i, a pointer pi to an integer, a function f taking
  no  arguments and returning an integer, a function fpi taking an inte­
  ger argument and returning a pointer to an integer, a pointer pif to a
  function  which  takes two pointers to constant characters and returns
  an integer, a function fpif taking an integer argument and returning a
  pointer  to  a  function that takes an integer argument and returns an
  integer.  It is especially useful to compare fpi and pif.  The binding
  of *fpi(int) is *(fpi(int)), so the declaration suggests, and the same
  construction in an expression requires, the calling of a function fpi,
  and  then  using  indirection through the (pointer) result to yield an
  integer.  In the declarator  (*pif)(const  char*,  const  char*),  the
  extra parentheses are necessary to indicate that indirection through a
  pointer to a function yields a function,  which  is  then  called.   ]
  [Note:  typedefs  are  sometimes  convenient when the return type of a
  function is complex.  For example, the function fpif above could  have
  been declared
          typedef int  IFUNC(int);
          IFUNC*  fpif(int);
   --end note]

  8.3.6  Default arguments                             [dcl.fct.default]

1 If  an expression is specified in a parameter declaration this expres­
  sion 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);
  declares  a  function  that can be called with zero, one, or two argu­
  ments of type int.  It can be called in any of these ways:
          point(1,2);  point(1);  point();
  The last two  calls  are  equivalent  to  point(1,4)  and  point(3,4),
  respectively.  ]

3 A  default  argument expression shall be specified only in the parame­
  ter-declaration-clause of a function declaration  or  in  a  template-
  parameter   (_temp.param_).   If  it  is  specified  in  a  parameter-
  declaration-clause,  it  shall  not  occur  within  a  declarator   or
  abstract-declarator of a parameter-declaration.4)

4 For  non-template  functions,  default arguments can be added in later
  declarations of a function in the same scope.  Declarations in differ­
  ent  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)
    --end  example]  For  a  given  inline function defined in different
  translation units, the accumulated sets of default  arguments  at  the
  end of the translation units shall be the same; see _basic.def.odr_.

  4)  This  means  that default arguments cannot appear, for example, in
  declarations of pointers to functions,  references  to  functions,  or
  typedef declarations.

5 Default  argument  expressions  have their names bound and their types
  checked at the point of declaration.  [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))
    --end  example]  [Note:  in  member  function declarations, names in
  default  argument  expressions  are  looked   up   as   described   in
  _basic.lookup.unqual_.   Access  checking  applies to names in default
  argument expressions as described in _class.access_.  ]

6 The default arguments in a member  function  definition  that  appears
  outside  of the class definition are added to the set of default argu­
  ments provided by the member function declaration in the class defini­
  tion.  [Example:
          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
   --end example]

7 Local  variables  shall  not  be used in default argument expressions.
          void f()
              int i;
              extern void g(int x = i);   // error
              // ...
   --end example]

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
   --end example]

9 Default  arguments  are  evaluated  at each point of call before entry
  into a function.  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
    --end  example] Similarly, a nonstatic member shall not be used in a
  default argument expression, even if it is not  evaluated,  unless  it
  appears  as  the  id-expression  of  a  class member access expression
  (_expr.ref_) or unless  it  is  used  to  form  a  pointer  to  member
  (_expr.unary.op_).  [Example: the declaration of X::mem1() in the fol­
  lowing example is ill-formed because no object  is  supplied  for  the
  nonstatic member X::a used as an initializer.
          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 b;
  The  declaration  of X::mem2() is meaningful, however, since no object
  is needed to access the static member  X::b.   Classes,  objects,  and
  members are described in _class_.  ] A default argument is not part of
  the type of a function.  [Example:
          int f(int = 0);

          void h()
              int j = f(1);
              int k = f();      // fine, means f(0)

          int (*p1)(int) = &f;
          int (*p2)() = &f;     // error: type mismatch
   --end example] When a declaration of a function is introduced by  way
  of  a  using  declaration  (_namespace.udecl_),  any  default argument
  information associated with the declaration is imported as well.

  +-------                      BEGIN BOX 1                     -------+
  Can additional default arguments be added to the  function  thereafter
  by  way  of redeclarations of the function?  Can the function be rede­
  clared in the namespace with added default arguments, and if  so,  are
  those  added arguments visible to those who have imported the function
  via using?
  +-------                       END BOX 1                      -------+

10A virtual function call (_class.virtual_) 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()
   --end example]

  8.4  Function definitions                                [dcl.fct.def]

1 Function definitions have the form
                  decl-specifier-seqopt declarator ctor-initializeropt function-body
                  decl-specifier-seqopt declarator function-try-block

  The declarator in a function-definition shall have the form
          D1 ( parameter-declaration-clause ) cv-qualifier-seqopt exception-specificationopt
  as described in _dcl.fct_.  A function shall be defined only in names­
  pace or class scope.

2 The  parameters  are  in the scope of the outermost block of the func­

3 [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;
  Here int is the decl-specifier-seq; max(int a, int b, int  c)  is  the
  declarator; { /* ... */ } is the function-body.  ]

4 A ctor-initializer is used only in a constructor; see _class.ctor_ and

5 A cv-qualifier-seq can be part of a non-static member function  decla­
  ration,  non-static  member  function definition, or pointer to member
  function only; see _class.this_.  It is part of the function type.

6 [Note: unused parameters need not be named.  For example,
          void print(int a, int)
              printf("a = %d\n",a);
   --end note]

  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 ini­
  tialized.  The process of initialization described in the remainder of
  this  subclause (_dcl.init_) applies also to initializations specified
  by other syntactic contexts, such as the  initialization  of  function
  parameters  with argument expressions (_expr.call_) or the initializa­
  tion of return values (_stmt.return_).
                  = initializer-clause
                  ( expression-list )
                  { initializer-list ,opt }
                  { }
                  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);
   --end example]

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

4 The  order  of  initialization  of  static  objects  is  described  in
  _basic.start_ and _stmt.dcl_.  ]

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

  --if T is a scalar type (_basic.types_), 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 member5) is
  5) This member must not be static, by virtue of  the  requirements  in


  --if T is an array  type,  the  storage  for  each  element  is  zero-

  --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 (_class_), the default constructor for
    T is called (and the initialization is ill-formed if T has no acces­
    sible default constructor);

  --if T is an array type, each element is default-initialized;

  --otherwise, the storage for the object is zero-initialized.

  Default-initialization   uses   the   direct-initialization  semantics
  described below.

6 The memory occupied by any object of static storage duration shall  be
  zero-initialized.  Furthermore, if no initializer is explicitly speci­
  fied in the declaration of the object and the  object  is  of  non-POD
  class  type  (or  array thereof), then default initialization shall be
  performed.  If no initializer is specified for an  object  with  auto­
  matic  or  dynamic storage duration, the object and its subobjects, if
  any, have an indeterminate initial value.6)

7 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
   --end example]

8 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.

9 [Note: since () is not permitted by the syntax for initializer,
          X a();
  6) This does not apply to aggregate objects with automatic storage du­
  ration initialized with an incomplete brace-enclosed initializer-list;
  see _dcl.init.aggr_.

  is not the declaration of an object of class X, but the declaration of
  a function taking no argument and returning an X.  The form () is per­
  mitted  in  certain   other   initialization   contexts   (_expr.new_,
  _expr.type.conv_, _class.base.init_).  ]

10The  initialization  that occurs in argument passing, function return,
  throwing  an  exception  (_except.throw_),   handling   an   exception
  (_except.handle_),     and     brace-enclosed     initializer    lists
  (_dcl.init.aggr_) is called copy-initialization and is  equivalent  to
  the form
          T x = a;
  The  initialization  that  occurs  in  new  expressions  (_expr.new_),
  static_cast expressions (_expr.static.cast_), functional notation type
  conversions  (_expr.type.conv_),  and  base  and  member  initializers
  (_class.base.init_) is called direct-initialization and is  equivalent
  to the form
          T x(a);

11If T is a scalar type, then a declaration of the form
          T x = { a };
  is equivalent to
          T x = a;

12The 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 _dcl.init.ref_.

  --If  the  destination  type  is an array of characters or an array of
    wchar_t,  and   the   initializer   is   a   string   literal,   see

  --Otherwise, if the destination type is an array, see _dcl.init.aggr_.

  --If the destination type is a (possibly cv-qualified) class type:

    --If the class is an aggregate (_dcl.init.aggr_), and  the  initial­
      izer is a brace-enclosed list, see _dcl.init.aggr_.

    --If  the initialization is direct-initialization, or if it is copy-
      initialization where the cv-unqualified version of the source type
      is the same class as, or a derived class of, the class of the des­
      tination, constructors are considered.  The  applicable  construc­
      tors  are enumerated (_over.match.ctor_), and the best one is cho­
      sen through overload resolution (_over.match_).   The  constructor
      so  selected is called to initialize the object, with the initial­
      izer expression(s) as its argument(s).  If no constructor applies,
      or  the  overload  resolution  is ambiguous, the initialization is

    --Otherwise (i.e., for the remaining copy-initialization  cases),  a

      temporary  is created.  User-defined conversion sequences that can
      convert from the source type to the destination type or a  derived
      class thereof are enumerated (_over.match.copy_), and the best one
      is chosen through overload resolution (_over.match_).   The  user-
      defined  conversion  so selected is called to convert the initial­
      izer expression into a temporary, whose type is the type  returned
      by  the call of the user-defined conversion function, with the cv-
      qualifiers of the destination type.  If the conversion  cannot  be
      done  or  is  ambiguous,  the  initialization  is ill-formed.  The
      object being initialized is then direct-initialized from the  tem­
      porary according to the rules above.7) In certain cases, an imple­
      mentation  is permitted to eliminate the temporary by initializing
      the object directly; see _class.temporary_.

  --Otherwise, if the source type is  a  (possibly  cv-qualified)  class
    type,  conversion  functions are considered.  The applicable conver­
    sion functions are enumerated (_over.match.conv_), and the best  one
    is  chosen  through  overload  resolution (_over.match_).  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 initializer expression.  Standard
    conversions (clause _conv_) will be used, if necessary,  to  convert
    the initializer expression to the cv-unqualified version of the des­
    tination 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]

  8.5.1  Aggregates                                      [dcl.init.aggr]

1 An  aggregate  is  an array or a class (_class_) with no user-declared
  constructors (_class.ctor_), no private or protected  non-static  data
  members  (_class.access_),  no  base classes (_class.derived_), and no
  virtual functions (_class.virtual_).

2 When an aggregate is initialized the initializer can  be  an  initial­
  izer-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.
  7)  Because  the  type of the temporary is the same as the type of the
  object being initialized, or is a derived class thereof, this  direct-
  initialization,   if   well-formed,   will   use  a  copy  constructor
  (_class.copy_) to copy the temporary.

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

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

4 An array of unknown size initialized with  a  brace-enclosed  initial­
  izer-list  containing  n  initializers,  where n shall be greater than
  zero, is defined as having n elements (_dcl.array_).  [Example:
          int x[] = { 1, 3, 5 };
  declares and initializes x as a one-dimensional array that  has  three
  elements since no size was specified and there are three initializers.
  ] An empty initializer list {} shall not be used  as  the  initializer
  for an array of unknown bound.8)

5 Static data members are not considered members of the class  for  pur­
  poses of aggregate initialization.  [Example:
          struct A {
                  int i;
                  static int s;
                  int j;
          } a = { 1, 2 };
  Here, the second initializer 2 initializes a.j and not the static data
  member A::s.  ]

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
  is ill-formed.  ]

7 If  there are fewer initializers in the list than there are members in
  the aggregate, then each member not explicitly  initialized  shall  be
  initialized  with  a value of the form T() (_expr.type.conv_), where T
  represents the type of the uninitialized member.  [Example:
          struct S { int a; char* b; int c; };
          S ss = { 1, "asdf" };
  initializes ss.a with 1, ss.b with "asdf", and ss.c with the value  of
  an expression of the form int(), that is, 0.  ]

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

  8)  The  syntax  provides for empty initializer-lists, but nonetheless
  C++ does not have zero length arrays.

          struct S { };
          struct A {
                  S s;
                  int i;
          } a = { { } , 3 };
   --end example] An empty initializer-list can be  used  to  initialize
  any aggregate.  If the aggregate is not an empty class, then each mem­
  ber of the aggregate shall be initialized with a value of the form T()
  (_expr.type.conv_),  where  T represents the type of the uninitialized

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

10When initializing a multi-dimensional array, the initializers initial­
  ize the elements with the last (rightmost) index of the array  varying
  the fastest (_dcl.array_).  [Example:
          float y[4][3] = {
              { 1 }, { 2 }, { 3 }, { 4 }
  initializes  the  first  column  of  y  (regarded as a two-dimensional
  array) and leaves the rest zero.  ]

11Braces can be elided in an initializer-list as follows.  If  the  ini­
  tializer-list  begins  with  a  left brace, then the succeeding comma-
  separated list of initializers initializes the members of a  subaggre­
  gate;  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 },
  is a completely-braced initialization: 1,  3,  and  5  initialize  the
  first  row  of  the  array y[0], namely y[0][0], y[0][1], and y[0][2].
  Likewise the next two lines initialize y[1] and y[2].  The initializer
  ends early and therefore y[3]'s elements are initialized as if explic­
  itly initialized with an expression of the form float(), that is,  are
  initialized  with  0.0.   In the following example, braces in the ini­
  tializer-list are elided; however the initializer-list  has  the  same
  effect as the completely-braced initializer-list of the above example,
          float y[4][3] = {
              1, 3, 5, 2, 4, 6, 3, 5, 7
  The initializer for y begins with a left brace, but the one  for  y[0]
  does  not,  therefore three elements from the list are used.  Likewise
  the next three are taken successively for y[1] and y[2].   --end exam­

12All  implicit type conversions (_conv_) are considered when initializ­
  ing 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 initial­
  ization 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 };
  Braces are elided around the initializer for b.a1.i.  b.a1.i  is  ini­
  tialized  with  4, b.a2 is initialized with a, b.z is initialized with
  whatever a.operator int() returns.  ]

13[Note: An aggregate array or an aggregate class may contain members of
  a  class  type  with a user-declared constructor (_class.ctor_).  Ini­
  tialization   of   these   aggregate   objects   is    described    in
  _class.expl.init_.  ]

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

15When  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
    --end example] [Note: as described above, the braces around the ini­
  tializer for a union member can be omitted if the union is a member of
  another aggregate.  ]

  8.5.2  Character arrays                              [dcl.init.string]

1 A char array (whether plain char, signed, or unsigned) can be initial­
  ized by a string; a wchar_t array can be initialized by a wide  string
  literal; successive characters of the string initialize the members of
  the array.  [Example:
          char msg[] = "Syntax error on line %s\n";

  shows a character array whose members are initialized with  a  string.
  Note  that  because  '\n' is a single character and because a trailing
  '\0' is appended, sizeof(msg) is 25.  ]

2 There shall not be more initializers than there  are  array  elements.
          char cv[4] = "asdf";  // error
  is  ill-formed  since there is no space for the implied trailing '\0'.

  8.5.3  References                                       [dcl.init.ref]

1 A variable declared to  be  a  T&,  that  is  "reference  to  type  T"
  (_dcl.ref_), 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]'
   --end example]

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

3 The  initializer  can  be  omitted for a reference only in a parameter
  declaration (_dcl.fct_), in the declaration of a function return type,
  in  the  declaration  of  a  class member within its class declaration
  (_class.mem_), and where the  extern  specifier  is  explicitly  used.
          int& r1;         // error: initializer missing
          extern int& r2;  // ok
   --end example]

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 _over.ics.rank_).

  In all cases where the reference-related or reference-compatible rela­
  tionship 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 (_class.access_)
  or ambiguous (_class.member.lookup_) 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 not an lvalue for a
    bit-field), and

    --"cv1 T1" is reference-compatible with "cv2 T2," or

    --T2 is a class type, and the initializer expression can be  implic­
      itly converted to an lvalue of type "cv3 T3," where T3 is the same
      type as T2, or is a derived class thereof, and cv3 is the same cv-
      qualification as, or lesser cv-qualification than,  cv1  9)  (this
      conversion  is  selected  by enumerating the applicable conversion
      functions (_over.match.ref_) and choosing  the  best  one  through
      overload resolution (_over.match_)), then

7   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.  [Note: the usual lvalue-to-
    rvalue (_conv.lval_), array-to-pointer (_conv.array_), and function-
    to-pointer  (_conv.func_)  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]

  9)  This requires a conversion function (_class.conv.fct_) returning a
  reference type.

    --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  implementa­

      --The reference is bound directly to the object represented by the
        rvalue (see _basic.lval_) or to a sub-object within that object.

      --A  temporary  of type "cv1 T2" [sic] is created, and a copy con­
        structor 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.10)

9     The  appropriate  copy constructor must 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 directly 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 (_dcl.init_).  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]

11  [Note: _class.temporary_ describes the lifetime of temporaries bound
    to references.  ]

  10) Clearly, if the reference initialization being  processed  is  one
  for  the  first argument of a copy constructor call, an implementation
  must eventually choose the direct-binding alternative to  avoid  infi­
  nite recursion.