______________________________________________________________________

  6   Statements                                             [stmt.stmt]

  ______________________________________________________________________

1 Except as indicated, statements are executed in sequence.
          statement:
                  labeled-statement
                  expression-statement
                  compound-statement
                  selection-statement
                  iteration-statement
                  jump-statement
                  declaration-statement
                  try-block

  6.1  Labeled statement                                    [stmt.label]

1 A statement can be labeled.
          labeled-statement:
                  identifier : statement
                  case constant-expression : statement
                  default : statement
  An identifier label declares the identifier.  The only use of an iden­
  tifier  label is as the target of a goto.  The scope of a label is the
  function in which it appears.  Labels shall not be redeclared within a
  function.   A label can be used in a goto statement before its defini­
  tion.  Labels have their own name space  and  do  not  interfere  with
  other identifiers.

2 Case  labels and default labels shall occur only in switch statements.

  6.2  Expression statement                                  [stmt.expr]

1 Expression statements have the form
          expression-statement:
                  expressionopt ;
  All side effects from an expression statement are completed before the
  next  statement is executed.  An expression statement with the expres­
  sion missing is called a null statement.  [Note: Most  statements  are
  expression  statements--usually  assignments or function calls. A null
  statement is useful to carry a label just before the } of  a  compound
  statement  and to supply a null body to an iteration statement such as
  a while statement (_stmt.while_).  ]

  6.3  Compound statement or block                          [stmt.block]

1 So that several statements can be used where one is expected, the com­
  pound  statement (also, and equivalently, called "block") is provided.
          compound-statement:
                   { statement-seqopt }
          statement-seq:
                  statement
                  statement-seq statement
  A compound statement defines a local scope (_basic.scope_).  [Note:  a
  declaration is a statement (_stmt.dcl_).  ]

  6.4  Selection statements                                [stmt.select]

1 Selection statements choose one of several flows of control.
          selection-statement:
                  if ( condition ) statement
                  if ( condition ) statement else statement
                  switch ( condition ) statement
          condition:
                  expression
                  type-specifier-seq declarator = assignment-expression
  In  this  clause, the term substatement refers to the contained state­
  ment or statements that appear in the syntax notation.  The  substate­
  ment in a selection-statement (both substatements, in the else form of
  the if statement) implicitly defines a  local  scope  (_basic.scope_).
  [Example:  If  the  substatement  in a selection-statement is a single
  statement and not a compound-statement, it is as if it  was  rewritten
  to be a compound-statement containing the original substatement.
          if (x)
              int i;
  can be equivalently rewritten as
          if (x) {
              int i;
          }
  Thus after the if statement, i is no longer in scope.  ]

2 The rules for conditions apply both to selection-statements and to the
  for and while statements  (_stmt.iter_).   The  declarator  shall  not
  specify  a function or an array.  The type-specifier shall not contain
  typedef and shall not declare a new class or enumeration.

3 A name introduced by a declaration in a condition  (either  introduced
  by  the  type-specifier-seq  or the declarator of the condition) is in
  scope from its point of declaration until the end of the substatements
  controlled by the condition.  If the name is re-declared in the outer­
  most block of a substatement controlled by the condition, the declara­
  tion that re-declares the name is ill-formed.  [Example:
          if (int x = f()) {
                  int x; // ill-formed, redeclaration of 'x'
          }
          else {
                  int x; // ill-formed, redeclaration of 'x'
          }

   --end example]

4 The  value  of  a  condition  that  is an initialized declaration in a
  statement other than a switch statement is the value of  the  declared
  variable  implicitly  converted  to  type bool.  If that conversion is
  ill-formed, the program is ill-formed.  The value of a condition  that
  is  an  initialized  declaration in a switch statement is the value of
  the declared variable if it has integral or enumeration  type,  or  of
  that  variable  implicitly  converted  to integral or enumeration type
  otherwise.  The value of a condition that  is  an  expression  is  the
  value  of  the expression, implicitly converted to bool for statements
  other than switch; if that conversion is ill-formed,  the  program  is
  ill-formed.   The value of the condition will be referred to as simply
  "the condition" where the usage is unambiguous.

5 If a condition can be syntactically resolved as either  an  expression
  or  the  declaration  of a local name, it is interpreted as a declara­
  tion.

  6.4.1  The if statement                                      [stmt.if]

1 If the condition (_stmt.select_) yields true the first substatement is
  executed.   If the else part of the selection statement is present and
  the condition yields false, the second substatement is  executed.   In
  the second form of if statement (the one including else), if the first
  substatement is also an if statement  then  that  inner  if  statement
  shall contain an else part.1)

  6.4.2  The switch statement                              [stmt.switch]

1 The switch statement causes control to be transferred to one  of  sev­
  eral statements depending on the value of a condition.

2 The  condition  shall  be  of integral type, enumeration type, or of a
  class type for which a single conversion function to integral or  enu­
  meration  type  exists  (_class.conv_).   If the condition is of class
  type, the condition is converted by calling that conversion  function,
  and the result of the conversion is used in place of the original con­
  dition for the remainder of this  section.   Integral  promotions  are
  performed.   Any  statement within the switch statement can be labeled
  with one or more case labels as follows:
          case constant-expression :
  where the constant-expression shall be  an  integral  constant-expres­
  sion.   The  integral constant-expression (_expr.const_) is implicitly
  converted to the promoted type of the switch condition.  No two of the
  case constants in the same switch shall have the same value after con­
  version to the promoted type of the switch condition.

3 There shall be at most one label of the form
          default :
  _________________________
  1)  In  other  words, the else is associated with the nearest un-elsed
  if.

  within a switch statement.

4 Switch statements can be nested; a case or default label is associated
  with the smallest switch enclosing it.

5 When  the switch statement is executed, its condition is evaluated and
  compared with each case constant.  If one of  the  case  constants  is
  equal  to  the value of the condition, control is passed to the state­
  ment following the matched case label.  If no  case  constant  matches
  the  condition, and if there is a default label, control passes to the
  statement labeled by the default label.  If no  case  matches  and  if
  there  is no default then none of the statements in the switch is exe­
  cuted.

6 case and default labels in themselves do not alter the  flow  of  con­
  trol,  which  continues  unimpeded across such labels.  To exit from a
  switch, see break, _stmt.break_.   [Note:  usually,  the  substatement
  that  is  the  subject  of  a  switch is compound and case and default
  labels appear on the top-level statements contained within  the  (com­
  pound)  substatement,  but  this  is  not  required.  Declarations can
  appear in the substatement of a switch-statement.  ]

  6.5  Iteration statements                                  [stmt.iter]

1 Iteration statements specify looping.
          iteration-statement:
                  while ( condition ) statement
                  do statement  while ( expression ) ;
                  for ( for-init-statement conditionopt ; expressionopt ) statement
          for-init-statement:
                  expression-statement
                  simple-declaration
  [Note: a for-init-statement ends with a semicolon.  ]

2 The substatement in an iteration-statement implicitly defines a  local
  scope  (_basic.scope_)  which  is entered and exited each time through
  the loop.

3 If the substatement in an iteration-statement is  a  single  statement
  and  not  a  compound-statement,  it is as if it was rewritten to be a
  compound-statement containing the original statement.  [Example:
          while (--x >= 0)
              int i;
  can be equivalently rewritten as
          while (--x >= 0) {
              int i;
          }
  Thus after the while statement, i is no longer in scope.  ]

4 The requirements on conditions are  the  same  as  for  if  statements
  (_stmt.if_).

  6.5.1  The while statement                                [stmt.while]

1 In  the  while statement the substatement is executed repeatedly until
  the value of the condition (_stmt.select_) becomes  false.   The  test
  takes place before each execution of the substatement.

2 When the condition of a while statement is a declaration, the scope of
  the variable that is declared extends from its  point  of  declaration
  (_basic.scope.pdecl_)  to  the  end  of  the while statement.  A while
  statement of the form
          while (T t = x) statement
  is equivalent to
          label:
          {            // start of condition scope
              T t = x;
              if (t) {
                  statement
                  goto label;
              }
          }            // end of condition scope
  The object created in a condition is destroyed and created  with  each
  iteration of the loop.  [Example:
          struct A {
              int val;
              A(int i) : val(i) { }
              ~A() { }
              operator bool() { return val != 0; }
          };
          int i = 1;
          while (A a = i) {
              //...
              i = 0;
          }
  In  the  while-loop,  the  constructor  and destructor are each called
  twice, once for the condition that succeeds and once for the condition
  that fails.  ]

  6.5.2  The do  statement                                     [stmt.do]

1 The  expression is implicitly converted to bool; if that is not possi­
  ble, the program is ill-formed.

2 In the do statement the substatement is executed repeatedly until  the
  value  of  the  expression  becomes false.  The test takes place after
  each execution of the statement.

  6.5.3  The for statement                                    [stmt.for]

1 The for statement
          for ( for-init-statement conditionopt ; expressionopt ) statement
  is equivalent to

          {
                  for-init-statement
                  while ( condition ) {
                          statement
                          expression ;
                  }
          }
  except that names declared in the for-init-statement are in  the  same
  declarative-region as those declared in the condition, and except that
  a continue in statement (not enclosed in another iteration  statement)
  will  execute  expression before re-evaluating condition.  [Note: Thus
  the first statement specifies initialization for the loop; the  condi­
  tion  (_stmt.select_)  specifies  a  test, made before each iteration,
  such that the loop is exited when the  condition  becomes  false;  the
  expression often specifies incrementing that is done after each itera­
  tion.  ]

2 Either or both of the condition and the expression can be omitted.   A
  missing  condition  makes  the  implied  while  clause  equivalent  to
  while(true).

3 If the for-init-statement is a declaration, the scope of  the  name(s)
  declared extends to the end of the for-statement.  [Example:
          int i = 42;
          int a[10];

          for (int i = 0; i < 10; i++)
                  a[i] = i;

          int j = i;        // j = 42
   --end example]

  6.6  Jump statements                                       [stmt.jump]

1 Jump statements unconditionally transfer control.
          jump-statement:
                  break ;
                  continue ;
                  return expressionopt ;
                  goto identifier ;

2 On   exit   from   a   scope   (however   accomplished),   destructors
  (_class.dtor_) are called for all constructed objects  with  automatic
  storage  duration  (_basic.stc.auto_)  (named  objects or temporaries)
  that are declared in that scope, in the reverse order of their  decla­
  ration.   Transfer out of a loop, out of a block, or back past an ini­
  tialized  variable  with  automatic  storage  duration  involves   the
  destruction  of  variables with automatic storage duration that are in
  scope at the point transferred from but not at the  point  transferred
  to.   (See _stmt.dcl_ for transfers into blocks).  [Note: However, the
  program can be terminated  (by  calling  exit()  or  abort()(_lib.sup­
  port.start.term_),  for example) without destroying class objects with
  automatic storage duration.  ]

  6.6.1  The break statement                                [stmt.break]

1 The break statement shall occur only in an  iteration-statement  or  a
  switch  statement  and  causes  termination  of the smallest enclosing
  iteration-statement or switch statement; control passes to the  state­
  ment following the terminated statement, if any.

  6.6.2  The continue statement                              [stmt.cont]

1 The  continue statement shall occur only in an iteration-statement and
  causes control to pass to the loop-continuation portion of the  small­
  est  enclosing  iteration-statement,  that is, to the end of the loop.
  More precisely, in each of the statements
      while (foo) {       do {                for (;;) {
       {                   {                   {
        // ...              // ...              // ...
       }                   }                   }
      contin: ;           contin: ;           contin: ;
      }                   } while (foo);      }
  a continue not contained in an enclosed iteration statement is equiva­
  lent to goto contin.

  6.6.3  The return statement                              [stmt.return]

1 A function returns to its caller by the return statement.

2 A return statement without an expression can be used only in functions
  that do not return a value, that is, a function with the return  value
  type   void,   a   constructor   (_class.ctor_),   or   a   destructor
  (_class.dtor_).  A return statement with an  expression  can  be  used
  only  in  functions  returning a value; the value of the expression is
  returned to the caller of the function.  If required,  the  expression
  is implicitly converted to the return type of the function in which it
  appears.  A return statement can involve the construction and copy  of
  a  temporary  object  (_class.temporary_).   Flowing  off the end of a
  function is equivalent to a return with  no  value;  this  results  in
  undefined behavior in a value-returning function.

  6.6.4  The goto statement                                  [stmt.goto]

1 The  goto statement unconditionally transfers control to the statement
  labeled  by  the  identifier.   The  identifier  shall  be   a   label
  (_stmt.label_) located in the current function.

  6.7  Declaration statement                                  [stmt.dcl]

1 A  declaration statement introduces one or more new identifiers into a
  block; it has the form
          declaration-statement:
                  block-declaration
  If an identifier introduced by a declaration was  previously  declared
  in  an  outer block, the outer declaration is hidden for the remainder
  of the block, after which it resumes its force.

2 Variables with automatic storage duration (_basic.stc.auto_) are  ini­
  tialized each time their declaration-statement is executed.  Variables
  with automatic storage duration declared in the block are destroyed on
  exit from the block (_stmt.jump_).

3 It  is  possible  to  transfer  into  a  block,  but not in a way that
  bypasses  declarations  with  initialization.   A program that jumps2)
  from a point where a local variable with automatic storage duration is
  not  in scope to a point where it is in scope is ill-formed unless the
  variable has POD type (_basic.types_) and is declared without an  ini­
  tializer (_dcl.init_).  [Example:
          void f()
          {
              // ...
              goto lx;    // ill-formed: jump into scope of `a'
              // ...
          ly:
              X a = 1;
              // ...
          lx:
              goto ly;    // ok, jump implies destructor
                          // call for `a' followed by construction
                          // again immediately following label ly
          }
   --end example]

4 The  zero-initialization (_dcl.init_) of all local objects with static
  storage duration (_basic.stc.static_) is performed  before  any  other
  initialization  takes place.  A local object with static storage dura­
  tion  (_basic.stc.static_)  initialized  with  an  integral  constant-
  expression  is initialized before its block is first entered.  A local
  object with static storage duration not initialized with  an  integral
  constant-expression  is  initialized  the  first  time  control passes
  through its declaration; such object is  considered  initialized  upon
  the  completion of its initialization.  If the initialization exits by
  throwing an exception, the initialization is not complete, so it  will
  be tried again the next time the function is called.

5 The destructor for a local object with static storage duration will be
  executed  if  and  only  if  the  variable  was  constructed.   [Note:
  _basic.start.term_  describes  the  order  in which local objects with
  static storage duration are destroyed.  ]

  6.8  Ambiguity resolution                                 [stmt.ambig]

1 There is an ambiguity in the grammar  involving  expression-statements
  and   declarations:  An  expression-statement  with  a  function-style
  explicit type conversion (_expr.type.conv_) as its leftmost subexpres­
  sion  can  be  indistinguishable  from  a  declaration where the first
  declarator starts with a  (.   In  those  cases  the  statement  is  a
  _________________________
  2) The transfer from the condition of a switch statement to a case la­
  bel is considered a jump in this respect.

  declaration.   [Note:  To disambiguate, the whole statement might have
  to be examined to determine if it is an expression-statement or a dec­
  laration.   This disambiguates many examples.  [Example: assuming T is
  a simple-type-specifier (_dcl.type_),
          T(a)->m = 7;       // expression-statement
          T(a)++;            // expression-statement
          T(a,5)<<c;         // expression-statement
          T(*d)(int);        // declaration
          T(e)[5];           // declaration
          T(f) = { 1, 2 };   // declaration
          T(*g)(double(3));  // declaration
   --end example] In the last example above, g, which is a pointer to T,
  is  initialized to double(3).  This is of course ill-formed for seman­
  tic reasons, but that does not affect the syntactic analysis.

2 The remaining cases are declarations.  [Example:
          class T {
                  // ...
          public:
                  T();
                  T(int);
                  T(int, int);
          };
          T(a);         // declaration
          T(*b)();      // declaration
          T(c)=7;       // declaration
          T(d),e,f=3;   // declaration
          extern int h;
          T(g)(h,2);    // declaration
   --end example]  --end note]

3 The disambiguation is purely syntactic; that is, the  meaning  of  the
  names  occurring in such a statement, beyond whether they are type-ids
  or not, is not used in or changed by the disambiguation.   Disambigua­
  tion  precedes parsing, and a statement disambiguated as a declaration
  may be an ill-formed declaration.  [Note: because  the  disambiguation
  is purely syntactic, template instantiation does not take place during
  the disambiguation step.  ] [Example:
          struct T1 {
                  T1 operator()(int x) { return T1(x); }
                  int operator=(int x) { return x; }
                  T1(int) { }
          };
          struct T2 { T2(int){ } };
          int a, (*(*b)(T2))(int), c, d;

          void f() {
                  // dismabiguation requires this to be parsed
                  // as a declaration
                  T1(a) = 3,
                  T2(4),                  // T2 will be declared as
                  (*(*b)(T2(c)))(int(d)); // a variable of type T1
                                          // but this will not allow
                                          // the last part of the
                                          // declaration to parse
                                          // properly since it depends
                                          // on T2 being a type-name
          }
   --end example]

4 A slightly different ambiguity between expression-statements and  dec­
  larations is resolved by requiring a type-id for function declarations
  within a block (_stmt.block_).  [Example:
          void g()
          {
              int f();   // declaration
              int a;     // declaration
              f();       // expression-statement
              a;         // expression-statement
          }
   --end example]