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  4   Standard conversions                                        [conv]

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1 Standard conversions are implicit  conversions  defined  for  built-in
  types.  The full set of such conversions is enumerated in this clause.
  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.  See _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 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.  [Note: this conversion is deprecated.  See  Annex  _depr_.   ]
  For  the  purpose  of ranking in overload resolution (_over.ics.scs_),
  _________________________
  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.

  this conversion is considered an array-to-pointer conversion  followed
  by  a qualification conversion (_conv.qual_).  [Example: "abc" is con-
  verted 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
        similarly for volatile.

      --if  the  cv1,j  and  cv2,j are different, then const is in every
  _________________________
  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.

        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,
              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 An  integral constant expression (_expr.const_) rvalue of integer type
  that evaluates to zero (called a null pointer constant)  can  be  con-
  verted  to  a  pointer  type.   The result is a value (called the null
  pointer value of that type) distinguishable from every pointer  to  an
  object  or  function.   Two null pointer values of the same type shall
  compare equal.  The conversion of a null pointer constant to a pointer
  to cv-qualified type is a single conversion, and not the sequence of a
  pointer   conversion   followed   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   (_class.derived_)   of   D.    If   B   is   an   inaccessible
  (_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 (_class.derived_) of B.
  If  B  is  an  inaccessible  (_class.access_),  ambiguous (_class.mem-
  ber.lookup_) or virtual (_class.mi_) base class of D, a  program  that

  necessitates this conversion is ill-formed.  The result of the conver-
  sion 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_,  _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 conver-
  sions  of  such pointers do not apply to pointers to members.  In par-
  ticular, a pointer to member cannot be converted to a void*.