6.5.4 The range-based for statement [stmt.ranged]
1 For a range-based for statement of the form
for ( for-range-declaration : expression ) statement
let range-init be equivalent to the expression surrounded by parentheses86
( expression )
and for a range-based for statement of the form
for ( for-range-declaration : braced-init-list ) statement
let range-init be equivalent to the braced-init-list. In each case, a range-based for statement is equivalent
to
{
auto && __range = range-init;
for ( auto __begin = begin-expr,
__end = end-expr;
__begin != __end;
++__begin ) {
for-range-declaration = *__begin;
statement
86) this ensures that a top-level comma operator cannot be reinterpreted as a delimiter between init-declarators in the declaration
of __range.
§ 6.5.4 128

c ISO/IEC N3337
}
}
where __range, __begin, and __end are variables defined for exposition only, and _RangeT is the type
of the expression, and begin-expr and end-expr are determined as follows:
— if _RangeT is an array type, begin-expr and end-expr are __range and __range + __bound, respectively,
where __bound is the array bound. If _RangeT is an array of unknown size or an array of
incomplete type, the program is ill-formed;
— if _RangeT is a class type, the unqualified-ids begin and end are looked up in the scope of class _RangeT
as if by class member access lookup (3.4.5), and if either (or both) finds at least one declaration, beginexpr
and end-expr are __range.begin() and __range.end(), respectively;
— otherwise, begin-expr and end-expr are begin(__range) and end(__range), respectively, where begin
and end are looked up with argument-dependent lookup (3.4.2). For the purposes of this name lookup,
namespace std is an associated namespace.