6 Statements [stmt.stmt]

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

• (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 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 size or an array of incomplete type, the program is ill-formed;

  • (1.3.2)

    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 ([basic.lookup.classref]), and if either (or both) finds 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 are looked up in the associated namespaces ([basic.lookup.argdep]). [ Note: Ordinary unqualified lookup ([basic.lookup.unqual]) is not performed.  — end note ]

[ Example:

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

 — 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.