[stmt.iter.general]

iteration-statement:
while ( condition ) statement
do statement while ( expression ) ;
for ( init-statement condition

{o p t; expression {o p t) statement for (init-statement {o p t for-range-declaration : for-range-initializer) statement}_{o p t for-range-declaration : for-range-initializer) statement}}_{o p t) statement for (init-statement {o p t for-range-declaration : for-range-initializer) statement}_{o p t for-range-declaration : for-range-initializer) statement}}}_{o p t; expression {o p t) statement for (init-statement {o p t for-range-declaration : for-range-initializer) statement}_{o p t for-range-declaration : for-range-initializer) statement}}_{o p t) statement for (init-statement {o p t for-range-declaration : for-range-initializer) statement}_{o p t for-range-declaration : for-range-initializer) statement}}}

[Note 1:

An init-statement ends with a semicolon.

— end note]

The substatement in an iteration-statement implicitly defines a block scope which is entered and exited each time through the loop.

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 ${o p t;);}_{o p t;);}$
(3.6)
for ( init-statement expression ${o p t;) {}}_{o p t;) {}}$

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]