5 Statements [stmt]

[ Note: This clause is presented as a set of differences to apply to ISO/IEC 14882:2014 §[stmt.ranged] to allow differently typed begin and end iterators, as in C++17.  — end note ]

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 parentheses

( 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
  }
}

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

• (2.1)

  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);

• (2.2)

  __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:

• (2.3)

  begin-expr and end-expr are determined as follows:

  • (2.3.1)

    if _RangeTthe 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 _RangeTR is an array of unknown sizebound or an array of incomplete type, the program is ill-formed;

  • (2.3.2)

    if _RangeTthe for-range-initializer is aan expression of class type C, the unqualified-ids begin and end are looked up in the scope of class _RangeTC 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;

  • (2.3.3)

    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.