______________________________________________________________________

  4   Standard conversions                                        [conv]

  ______________________________________________________________________

1 Standard conversions are implicit  conversions  defined  for  built-in
  types.   Clause _conv_ enumerates the full set of such conversions.  A
  standard conversion sequence is a sequence of standard conversions  in
  the following order:

  --Zero or one conversion from the following set: lvalue-to-rvalue con-
    version, array-to-pointer conversion, and  function-to-pointer  con-
    version.

  --Zero  or one conversion from the following set: integral promotions,
    floating point promotion, integral conversions, floating point  con-
    versions,   floating-integral   conversions,   pointer  conversions,
    pointer to member conversions, and boolean conversions.

  --Zero or one qualification conversion.

  [Note: a standard conversion sequence can be empty, i.e., it can  con-
  sist  of  no  conversions.   ]  A standard conversion sequence will be
  applied to an expression if necessary to convert it to a required des-
  tination type.

2 [Note:  expressions  with a given type will be implicitly converted to
  other types in several contexts:

  --When used as operands of operators.  The operator's requirements for
    its operands dictate the destination type (clause _expr_).

  --When used in the condition of an if statement or iteration statement
    (_stmt.select_, _stmt.iter_).  The destination type is bool.

  --When used in the expression of a switch statement.  The  destination
    type is integral (_stmt.select_).

  --When  used  as  the  source  expression for an initialization (which
    includes use as an argument in  a  function  call  and  use  as  the
    expression  in  a  return  statement).  The type of the entity being
    initialized is (generally) the destination  type.   See  _dcl.init_,
    _dcl.init.ref_.

   --end note]

3 An expression e can be implicitly converted to a type T if and only if
  the declaration T t=e;" is well-formed, for  some  invented  temporary

  variable t (_dcl.init_).  The effect of the implicit conversion is the
  same as performing the declaration and initialization and  then  using
  the temporary variable as the result of the conversion.  The result is
  an lvalue if T is a reference type (_dcl.ref_), and an  rvalue  other-
  wise.   The  expression e is used as an lvalue if and only if the ini-
  tialization uses it as an lvalue.

4 [Note: For user-defined types, user-defined conversions are considered
  as  well;  see  _class.conv_.   In  general,  an  implicit  conversion
  sequence (_over.best.ics_) consists of a standard conversion  sequence
  followed  by  a  user-defined  conversion followed by another standard
  conversion sequence.

5 There are some contexts where certain conversions are suppressed.  For
  example, the lvalue-to-rvalue conversion is not done on the operand of
  the unary & operator.  Specific exceptions are given in  the  descrip-
  tions of those operators and contexts.  ]

  4.1  Lvalue-to-rvalue conversion                           [conv.lval]

1 An  lvalue  (_basic.lval_)  of a non-function, non-array type T can be
  converted to an rvalue.  If T is an incomplete type,  a  program  that
  necessitates  this  conversion  is ill-formed.  If the object to which
  the lvalue refers is not an object of type T and is not an object of a
  type derived from T, or if the object is uninitialized, a program that
  necessitates this conversion has undefined behavior.  If T is  a  non-
  class type, the type of the rvalue is the cv-unqualified version of T.
  Otherwise, the type of the rvalue is T.  1)

2 The value contained in the object  indicated  by  the  lvalue  is  the
  rvalue  result.  When an lvalue-to-rvalue conversion occurs within the
  operand of sizeof (_expr.sizeof_) the value contained  in  the  refer-
  enced  object  is  not accessed, since that operator does not evaluate
  its operand.

3 [Note: See also _basic.lval_.  ]

  4.2  Array-to-pointer conversion                          [conv.array]

1 An lvalue or rvalue of type "array of N T" or "array of unknown  bound
  of  T"  can  be  converted  to  an rvalue of type "pointer to T."  The
  result is a pointer to the first element of the array.

2 A string literal (_lex.string_) that is not a wide string literal  can
  be  converted  to  an  rvalue of type "pointer to char"; a wide string
  literal can be converted to an rvalue of type  "pointer  to  wchar_t".
  In  either  case,  the result is a pointer to the first element of the
  array.  This conversion is considered only when there is  an  explicit
  appropriate  pointer target type, and not when there is a general need
  _________________________
  1) In C++ class rvalues can have cv-qualified types (because they  are
  objects).   This  differs  from ISO C, in which non-lvalues never have
  cv-qualified types.

  to convert from an lvalue to an rvalue.   [Note:  this  conversion  is
  deprecated.   See Annex _depr_.  ] For the purpose of ranking in over-
  load resolution (_over.ics.scs_), this  conversion  is  considered  an
  array-to-pointer  conversion  followed  by  a qualification conversion
  (_conv.qual_).  [Example: "abc" is  converted  to  "pointer  to  const
  char" as an array-to-pointer conversion, and then to "pointer to char"
  as a qualification conversion.  ]

  4.3  Function-to-pointer conversion                        [conv.func]

1 An lvalue of function type T can be converted to  an  rvalue  of  type
  "pointer to T."  The result is a pointer to the function.2)

2 [Note:  See  _over.over_  for  additional rules for the case where the
  function is overloaded.  ]

  4.4  Qualification conversions                             [conv.qual]

1 An rvalue of type "pointer to cv1 T" can be converted to an rvalue  of
  type  "pointer to cv2 T" if "cv2 T" is more cv-qualified than "cv1 T."

2 An rvalue of type "pointer to member of X of type cv1 T" can  be  con-
  verted  to an rvalue of type "pointer to member of X of type cv2 T" if
  "cv2 T" is more cv-qualified than "cv1 T."

3 [Note: Function types (including those used in pointer to member func-
  tion types) are never cv-qualified; see _dcl.fct_ .  ]

4 A  conversion  can add cv-qualifiers at levels other than the first in
  multi-level pointers, subject to the following rules:3)
    Two pointer types T1 and T2 are similar if there exists a type T and
    integer n>0 such that:

            T1 is cv1,0 pointer to cv1,1 pointer to ... cv1,n-1 pointer to cv1,n T
    and

            T2 is cv2,0 pointer to cv2,1 pointer to ... cv2,n-1 pointer to cv2,n T
    where each cvi,j is const, volatile,  const  volatile,  or  nothing.
    The  n-tuple  of  cv-qualifiers  after  the first in a pointer type,
    e.g., cv1,1, cv1,2,  ... , cv1,n in the pointer type T1,  is  called
    the  cv-qualification  signature of the pointer type.  An expression
    of type T1 can be converted to type T2 if and only if the  following
    conditions are satisfied:

      --the pointer types are similar.

      --for  every j>0, if const is in cv1,j then const is in cv2,j, and
  _________________________
  2) This conversion never applies to nonstatic member functions because
  an lvalue that refers to a nonstatic member  function  cannot  be  ob-
  tained.
  3) These rules ensure that const-safety is preserved  by  the  conver-
  sion.

        similarly for volatile.

      --if the cv1,j and cv2,j are different, then  const  is  in  every
        cv2,k for 0<k<j.
    [Note:  if a program could assign a pointer of type T** to a pointer
    of type const T** (that is, if line //1 below was allowed),  a  pro-
    gram  could  inadvertently  modify  a const object (as it is done on
    line //2).  For example,
      int main() {
              const char c = 'c';
              char* pc;
              const char** pcc = &pc;         //1: not allowed
              *pcc = &c;
              *pc = 'C';              //2: modifies a const object
      }
     --end note]

5 A multi-level pointer to member type, or a multi-level  mixed  pointer
  and pointer to member type has the form:

            cv0P0 to cv1P1 to ... cvn-1Pn-1 to cvn T
  where Pi is either a pointer or pointer to member and where T is not a
  pointer type or pointer to member type.

6 Two multi-level pointer to  member  types  or  two  multi-level  mixed
  pointer  and  pointer  to  member types T1 and T2 are similar if there
  exists a type T and integer n>0 such that:

            T1 is cv1,0P0 to cv1,1P1 to ... cv1,n-1Pn-1 to cv1,n T
  and

            T2 is cv2,0P0 to cv2,1P1 to ... cv2,n-1Pn-1 to cv2,n T

7 For similar multi-level pointer to member  types  and  similar  multi-
  level  mixed pointer and pointer to member types, the rules for adding
  cv-qualifiers are the same as those used for similar pointer types.

  4.5  Integral promotions                                   [conv.prom]

1 An rvalue of type char, signed char,  unsigned  char,  short  int,  or
  unsigned  short  int  can be converted to an rvalue of type int if int
  can represent all the values of the source type; otherwise, the source
  rvalue can be converted to an rvalue of type unsigned int.

2 An rvalue of type wchar_t (_basic.fundamental_) or an enumeration type
  (_dcl.enum_) can be converted to an rvalue of the first of the follow-
  ing  types  that  can represent all the values of its underlying type:
  int, unsigned int, long, or unsigned long.

3 An rvalue for an integral bit-field (_class.bit_) can be converted  to
  an  rvalue of type int if int can represent all the values of the bit-
  field; otherwise, it can be converted to unsigned int if unsigned  int
  can  represent  all  the values of the bit-field.  If the bit-field is
  larger yet, no integral promotion applies to it.  If the bit-field has

  an  enumerated type, it is treated as any other value of that type for
  promotion purposes.

4 An rvalue of type bool can be converted to an rvalue of type int, with
  false becoming zero and true becoming one.

5 These conversions are called integral promotions.

  4.6  Floating point promotion                            [conv.fpprom]

1 An  rvalue of type float can be converted to an rvalue of type double.
  The value is unchanged.

2 This conversion is called floating point promotion.

  4.7  Integral conversions                              [conv.integral]

1 An rvalue of an integer type can be converted to an rvalue of  another
  integer type.  An rvalue of an enumeration type can be converted to an
  rvalue of an integer type.

2 If the destination type is unsigned, the resulting value is the  least
  unsigned integer congruent to the source integer (modulo 2n where n is
  the number of bits used to represent the unsigned type).  [Note: In  a
  two's  complement  representation,  this  conversion is conceptual and
  there is no change in the bit pattern (if there is no truncation).  ]

3 If the destination type is signed, the value is unchanged if it can be
  represented  in the destination type (and bit-field width); otherwise,
  the value is implementation-defined.

4 If the destination type is bool, see _conv.bool_.  If the source  type
  is  bool,  the  value false is converted to zero and the value true is
  converted to one.

5 The conversions allowed as integral promotions are excluded  from  the
  set of integral conversions.

  4.8  Floating point conversions                          [conv.double]

1 An  rvalue  of  floating  point  type can be converted to an rvalue of
  another floating point type.  If the source value can be exactly  rep-
  resented in the destination type, the result of the conversion is that
  exact representation.  If the source value  is  between  two  adjacent
  destination  values,  the  result  of the conversion is an unspecified
  choice of either of those values.  Otherwise, the  behavior  is  unde-
  fined.

2 The conversions allowed as floating point promotions are excluded from
  the set of floating point conversions.

  4.9  Floating-integral conversions                        [conv.fpint]

1 An rvalue of a floating point type can be converted to an rvalue of an
  integer  type.  The conversion truncates; that is, the fractional part
  is discarded.  The behavior is undefined if the truncated value cannot
  be  represented  in  the  destination type.  [Note: If the destination
  type is bool, see _conv.bool_.  ]

2 An rvalue of an integer type or of an enumeration  type  can  be  con-
  verted  to an rvalue of a floating point type.  The result is exact if
  possible.  Otherwise, it is an unspecified choice of either  the  next
  lower  or higher representable value.  [Note: loss of precision occurs
  if the integral value cannot be represented exactly as a value of  the
  floating  type.  ] If the source type is bool, the value false is con-
  verted to zero and the value true is converted to one.

  4.10  Pointer conversions                                   [conv.ptr]

1 A  null  pointer  constant  is   an   integral   constant   expression
  (_expr.const_)  rvalue of integer type that evaluates to zero.  A null
  pointer constant can be converted to a pointer type; the result is the
  null  pointer  value  of  that  type and is distinguishable from every
  other value of pointer to object or pointer  to  function  type.   Two
  null pointer values of the same type shall compare equal.  The conver-
  sion of a null pointer constant to a pointer to cv-qualified type is a
  single  conversion,  and not the sequence of a pointer conversion fol-
  lowed by a qualification conversion (_conv.qual_).

2 An rvalue of type "pointer to cv T," where T is an object type, can be
  converted  to  an  rvalue of type "pointer to cv void."  The result of
  converting a "pointer to cv T" to a "pointer to cv void" points to the
  start  of  the storage location where the object of type T resides, as
  if the object is a most derived  object  (_intro.object_)  of  type  T
  (that is, not a base class subobject).

3 An  rvalue  of type "pointer to cv D," where D is a class type, can be
  converted to an rvalue of type "pointer to cv B," where B  is  a  base
  class  (clause _class.derived_) of D.  If B is an inaccessible (clause
  _class.access_) or ambiguous (_class.member.lookup_) base class of  D,
  a program that necessitates this conversion is ill-formed.  The result
  of the conversion is a pointer to the base  class  sub-object  of  the
  derived class object.  The null pointer value is converted to the null
  pointer value of the destination type.

  4.11  Pointer to member conversions                         [conv.mem]

1 A null pointer constant (_conv.ptr_) can be converted to a pointer  to
  member  type.   The  result is a value (called the null member pointer
  value of that type) distinguishable from any  pointer  to  member  not
  created  from a null pointer constant.  Two null member pointer values
  of the same type shall  compare  equal.   The  conversion  of  a  null
  pointer constant to a pointer to member of cv-qualified type is a sin-
  gle conversion, and not the sequence of a pointer to member conversion
  followed by a qualification conversion (_conv.qual_).

2 An  rvalue of type "pointer to member of B of type cv T," where B is a
  class type, can be converted to an rvalue of type "pointer  to  member
  of   D   of   type   cv T,"   where  D  is  a  derived  class  (clause
  _class.derived_)  of   B.    If   B   is   an   inaccessible   (clause
  _class.access_),    ambiguous   (_class.member.lookup_)   or   virtual
  (_class.mi_) base class of D, a program that necessitates this conver-
  sion  is  ill-formed.  The result of the conversion refers to the same
  member as the pointer to member before the conversion took place,  but
  it  refers  to  the  base  class  member as if it were a member of the
  derived class.  The result refers to the member in D's instance of  B.
  Since  the  result  has type "pointer to member of D of type cv T," it
  can be dereferenced with a D object.  The result is the same as if the
  pointer  to  member of B were dereferenced with the B sub-object of D.
  The null member pointer value is converted to the null member  pointer
  value of the destination type.4)

  4.12  Boolean conversions                                  [conv.bool]

1 An rvalue of arithmetic, enumeration, pointer, or  pointer  to  member
  type  can  be converted to an rvalue of type bool.  A zero value, null
  pointer value, or null member pointer value is converted to false; any
  other value is converted to true.

  _________________________
  4)  The  rule  for  conversion of pointers to members (from pointer to
  member of base to pointer to member of derived) appears inverted  com-
  pared  to the rule for pointers to objects (from pointer to derived to
  pointer to base) (_conv.ptr_, clause _class.derived_).  This inversion
  is  necessary to ensure type safety.  Note that a pointer to member is
  not a pointer to object or a pointer to function  and  the  rules  for
  conversions  of such pointers do not apply to pointers to members.  In
  particular, a pointer to member cannot be converted to a void*.