8 Statements [stmt.stmt]

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 1:

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.

— end example]

A trivially empty iteration statement is an iteration statement matching one of the following forms:

(3.1)
while ( expression ) ;
(3.2)
while ( expression ) { }
(3.3)
do ; while ( expression ) ;
(3.4)
do { } while ( expression ) ;
(3.5)
for ( init-statement expression $_{o p t}$ ; ) ;
(3.6)
for ( init-statement expression $_{o p t}$ ; ) { }

The controlling expression of a trivially empty iteration statement is the expression of a while, do, or for statement (or true, if the for statement has no expression).

A trivial infinite loop is a trivially empty iteration statement for which the converted controlling expression is a constant expression, when interpreted as a constant-expression ([expr.const]), and evaluates to true.

The statement of a trivial infinite loop is replaced with a call to the function std::this_thread::yield ([thread.thread.this]); it is implementation-defined whether this replacement occurs on freestanding implementations.

[Note 2:

In a freestanding environment, concurrent forward progress is not guaranteed; such systems therefore require explicit cooperation.

A call to yield can add implicit cooperation where none is otherwise intended.

— end note]

8.6.2 The while statement [stmt.while]

In the while statement, the substatement is executed repeatedly until the value of the condition ([stmt.pre]) becomes false.

The test takes place before each execution of the substatement.

A while statement is equivalent to

label :
{
if ( condition ) {
statement
goto label ;
}
}

[Note 1:

The variable created in the condition is destroyed and created with each iteration of the loop.

[Example 1: 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.

— end example]

— end note]

8.6.3 The do statement [stmt.do]

The expression is contextually converted to bool; if that conversion is ill-formed, the program is ill-formed.

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.

8.6.4 The for statement [stmt.for]

The for statement

for ( init-statement condition

_{o p t}

; expression

_{o p t}

) statement

is equivalent to

{
init-statement
while ( condition ) {
statement
expression ;
}
}

except that the init-statement is in the same scope as the condition, and except that a continue in statement (not enclosed in another iteration statement) will execute expression before re-evaluating condition.

[Note 1:

Thus the first statement specifies initialization for the loop; the condition ([stmt.pre]) specifies a test, sequenced before each iteration, such that the loop is exited when the condition becomes false; the expression often specifies incrementing that is sequenced after each iteration.

— end note]

Either or both of the condition and the expression can be omitted.

A missing condition makes the implied while clause equivalent to while(true).

8.6.5 The range-based for statement [stmt.ranged]

The range-based for statement

for ( init-statement

_{o p t}

for-range-declaration : for-range-initializer ) statement

is equivalent to

{
init-statement

_{o p t}

auto &&range = for-range-initializer ;
auto begin = begin-expr ;
auto end = end-expr ;
for ( ; begin != end; ++begin ) {
for-range-declaration = * begin ;
statement
}
}

where

(1.1)
if the for-range-initializer is an expression, it is regarded as if it were surrounded by parentheses (so that a comma operator cannot be reinterpreted as delimiting two init-declarators);
(1.2)
range, begin, and end are variables defined for exposition only; and
(1.3)
begin-expr and end-expr are determined as follows:
- (1.3.1)
  if the type of range is a reference to an array type R, begin-expr and end-expr are range and range + N, respectively, where N is the array bound.
  
  If R is an array of unknown bound or an array of incomplete type, the program is ill-formed;
- (1.3.2)
  if the type of range is a reference to a class type C, and searches in the scope of C ([class.member.lookup]) for the names begin and end each find at least one declaration, begin-expr and end-expr are range.begin() and range.end(), respectively;
- (1.3.3)
  otherwise, begin-expr and end-expr are begin(range) and end(range), respectively, where begin and end undergo argument-dependent lookup ([basic.lookup.argdep]).
  
  [Note 1:
  Ordinary unqualified lookup ([basic.lookup.unqual]) is not performed.
  — end note]

[Example 1: int array[5] = { 1, 2, 3, 4, 5 }; for (int& x : array) x *= 2; — end example]

[Note 2:

The lifetime of some temporaries in the for-range-initializer is extended to cover the entire loop ([class.temporary]).

— end note]

[Example 2: using T = std::list<int>; const T& f1(const T& t) { return t; } const T& f2(T t) { return t; } T g(); void foo() { for (auto e : f1(g())) {} // OK, lifetime of return value of g() extended for (auto e : f2(g())) {} // undefined behavior } — end example]

In the decl-specifier-seq of a for-range-declaration, each decl-specifier shall be either a type-specifier or constexpr.

The decl-specifier-seq shall not define a class or enumeration.