The range-based for statement
for ( for-range-declaration : for-range-initializer ) statement
is equivalent to
{
auto &&__range = for-range-initializer ;
auto __begin = begin-expr ;
auto __end = end-expr ;
for ( ; __begin != __end; ++__begin ) {
for-range-declaration = *__begin;
statement
}
}
where
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);
__range, __begin, and __end are variables defined for exposition only; and
begin-expr and end-expr are determined as follows:
if the for-range-initializer is an expression of array type R, begin-expr and end-expr are __range and __range + __bound, respectively, where __bound is the array bound. If R is an array of unknown bound or an array of incomplete type, the program is ill-formed;
if the for-range-initializer is an expression of class type C, the unqualified-ids begin and end are looked up in the scope of C as if by class member access lookup, and if either (or both) finds at least one declaration, begin-expr 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 in the associated namespaces. [ Note: Ordinary unqualified lookup is not performed. — end note ]
[ Example:
int array[5] = { 1, 2, 3, 4, 5 };
for (int& x : array)
x *= 2;
— end example ]
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