[stmt.ranged]

The range-based for statement

{o p t for-range-declaration : for-range-initializer) 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}}}_{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]