6   Statements                                             [stmt.stmt]


1 Except as indicated, statements are executed in sequence.

  6.1  Labeled statement                                    [stmt.label]

1 A statement can be labeled.
          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
          expressionopt ;
  The expression is evaluated and its value is discarded.   The  lvalue-
  to-rvalue  (_conv.lval_),  array-to-pointer  (_conv.array_), and func-
  tion-to-pointer (_conv.func_) standard conversions are not applied  to
  the  expression.   All  side  effects from an expression statement are
  completed before the next statement is executed.  An expression state-
  ment  with  the expression 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.
           { statement-seqopt }
          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.
          if ( condition ) statement
          if ( condition ) statement else statement
          switch ( condition ) statement
          type-specifier-seq declarator = assignment-expression
  In clause _stmt_, 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_).
  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 com-
  pound-statement containing the original substatement.  [Example:
  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-seq shall not con-
  tain 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-

  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

  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-

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.
          while ( condition ) statement
          do statement  while ( expression ) ;
          for ( for-init-statement conditionopt ; expressionopt ) statement
  [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 [Note:  The  requirements  on  conditions  in iteration statements are
  described in _stmt.select_.   --end note]

  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
  {                               // start of condition scope
      T t = x;
      if (t) {
          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

          while ( condition ) {
                  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

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.
          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 type
  void, a constructor (_class.ctor_), or a destructor (_class.dtor_).  A
  return  statement with an expression of non-void type can be used only
  in functions returning  a  value;  the  value  of  the  expression  is
  returned  to the caller of the function.  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 behav-
  ior in a value-returning function.

3 A return statement with an expression of type "cv void"  can  be  used
  only  in  functions  with  a return type of cv void; the expression is
  evaluated just before the function returns to its caller.

  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

  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
      // ...
      X a = 1;
      // ...
      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   of   POD   type
  (_basic.types_)  with  static  storage  duration initialized with con-
  stant-expressions is initialized before its block  is  first  entered.
  An  implementation  is  permitted  to  perform early initialization of
  other local objects with static storage duration under the same condi-
  tions  that an implementation is permitted to statically initialize an
  object   with   static   storage   duration   in    namespace    scope
  (_basic.start.init_).   Otherwise  such  an  object is initialized the
  first time control passes through its declaration; such an  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 control
  enters the declaration.  If control re-enters the declaration  (recur-
  sively)  while  the object is being initialized, the behavior is unde-
  fined.  [Example:

  2) The transfer from the condition of a switch statement to a case la-
  bel is considered a jump in this respect.

  int foo(int i)
      static int s = foo(2*i);    // recursive call - undefined
      return i+1;
   --end example]

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 decla-
  ration.  [Note: To disambiguate, the whole statement might have to  be
  examined  to  determine if it is an expression-statement or a declara-
  tion.  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
  In  the last example above, g, which is a pointer to T, is initialized
  to double(3).  This is of course ill-formed for semantic reasons,  but
  that does not affect the syntactic analysis.   --end example]

2 The remaining cases are declarations.  [Example:
          class T {
                  // ...
                  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-
  names  or  not, is not generally used in or changed by the disambigua-
  tion.  Class templates are instantiated as necessary to determine if a

  qualified name is a type-name.  Disambiguation precedes parsing, and a
  statement disambiguated as a declaration may be an ill-formed declara-
  tion.   If,  during  parsing,  a name in a template parameter is bound
  differently than it would be bound during a trial parse,  the  program
  is ill-formed.  No diagnostic is required.  [Note: This can occur only
  when the name is declared earlier in the declaration.  ] [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() {
          // disambiguation 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]