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  6   Statements                                   [stmt.stmt]

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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 cannot 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 can occur only in switch statements.

  6.2  Expression statement                                  [stmt.expr]

1 Most statements are expression statements, which have the form
          expression-statement:
                  expressionopt ;
  Usually  expression statements are assignments or function calls.  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; it 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 while (_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_).

2 Note that 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
  The  statement  in a selection-statement (both statements, in the else
  form  of  the  if  statement)  implicitly  defines   a   local   scope
  (_basic.scope_).   That  is, if the statement 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 state­
  ment.  For example,
          if (x)
              int i;
  may 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 may not spec­
  ify 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 is in scope from its
  point of declaration until the end of the statements controlled by the
  condition.   The  value of a condition that is an initialized declara­
  tion is the value of the initialized variable; the value of  a  condi­
  tion  that is an expression is the value of the expression.  The value
  of the condition will be referred to as simply the condition where the
  usage is unambiguous.

4 A  variable, constant, etc. in the outermost block of a statement con­
  trolled by a condition may not have the same name as a variable,  con­
  stant, etc. declared in the condition.

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 The  condition is converted to type bool; if that is not possible, the
  program is ill-formed.  If it yields true the  first  substatement  is
  executed.   If else is used and the condition yields false, the second
  substatement is executed.  The else ambiguity is resolved by  connect­
  ing an else with the last encountered else-less if.

  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 must be of integral type or of a class type for which an
  unambiguous  conversion to integral type exists (_class.conv_).  Inte­
  gral promotion is performed.  Any statement within the  statement  may
  be labeled with one or more case labels as follows:
          case constant-expression :
  where  the constant-expression (_expr.const_) is converted to the pro­
  moted type of the switch condition.  No two of the case  constants  in
  the same switch may have the same value.

3 There may be at most one label of the form
          default :
  within a switch statement.

4 Switch statements may 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_.

7 Usually,  the  statement  that is the subject of a switch is compound.
  Declarations may appear in the statement 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
                  declaration-statement

2 Note that a for-init-statement ends with a semicolon.

3 The statement in an iteration-statement  implicitly  defines  a  local
  scope  (_basic.scope_)  which  is entered and exited each time through
  the loop.  That is, if the statement 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  state­
  ment.  For example,
          while (--x >= 0)
              int i;
  may be equivalently rewritten as
          while (--x >= 0) {
              int i;
          }
  Thus after the while statement, i is no longer in scope.

4 See _stmt.select_ for the rules on conditions.

  6.5.1  The while statement                                [stmt.while]

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

2 The condition is converted to bool (_conv.bool_).

  6.5.2  The do  statement                                     [stmt.do]

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

2 The condition is converted to bool (_conv.bool_).

  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 a continue in statement (not enclosed in another iteration
  statement) will execute  expression  before  re-evaluating  condition.

  Thus  the  first  statement specifies initialization for the loop; the
  condition 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 iteration.  The  condi­
  tion is converted to bool (_conv.bool_).

2 Either  or both of the condition and the expression may be dropped.  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.  For example:
          int i = 42;
          int a[10];

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

          int j = i;        // j = 42

  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).  However, the program
  may  be terminated (by calling exit() or abort(), for example) without
  destroying class objects with automatic storage duration.

  6.6.1  The break statement                                [stmt.break]

1 The break statement may 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  may 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 converted, as in an initialization (_dcl.init_), to the return type
  of the function in which it appears.  A return statement  may  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 func­
  tion.

  6.6.4  The goto statement                                  [stmt.goto]

1 The goto statement unconditionally transfers control to the  statement
  labeled   by   the   identifier.   The  identifier  must  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:
                  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 jumps 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  pointer  or  arithmetic  type  or   is   an   aggregate
  (_dcl.init.aggr_),    and   is   declared   without   an   initializer
  (_dcl.init_).  For 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
          }

4 The default initialization to zero (_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  duration  (_basic.stc.static_)  initialized  with a constant-
  expression is initialized before its block is first entered.  A  local
  object  with  static storage duration not initialized with a constant-
  expression is initialized the first  time  control  passes  completely
  through  its  declaration.  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.  The  destructor
  must  be  called  either immediately before or as part of the calls of
  the atexit() functions (_basic.start.term_).  Exactly when is unspeci­
  fied.

  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.

2 To disambiguate, the whole statement may have to be examined to deter­
  mine  if  it is an expression-statement or a declaration.  This disam­
  biguates many examples.  For 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)[];            // 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.

3 The remaining cases are declarations.  For example,
          T(a);         // declaration
          T(*b)();      // declaration
          T(c)=7;       // declaration
          T(d),e,f=3;   // declaration
          T(g)(h,2);    // declaration

4 The  disambiguation  is  purely syntactic; that is, the meaning of the
  names, beyond whether they are type-ids or not, is  not  used  in  the
  disambiguation.

5 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_).  For example,
          void g()
          {
              int f();   // declaration
              int a;     // declaration
              f();       // expression-statement
              a;         // expression-statement
          }