8 Statements [stmt.stmt]

8.6 Iteration statements [stmt.iter]

8.6.4 The range-based for statement [stmt.ranged]

The range-based for statement is equivalent to
{
   init-statement
   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 + 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;
    • 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 both find 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 ([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.