What is "for (x : y)"?

C++11 introduced a new iteration statement, the so-called range-based for loop. It differs from the ordinary for loop in that it only gives you access to the members of a range, without requiring you to name the range itself explicitly and without using proxy iterator objects. Specifically, you are not supposed to mutate the range during the iteration, so this new loop documents the intent to "look at each range element" and not do anything complicated with the range itself.

The syntax is this: for (decl x : r) { /* body */ }, where decl stands for some declaration and r is an arbitrary expression. This is functionally mostly equivalent to the following traditional loop:

{
    auto && __r = r;

    using std::begin;
    using std::end;

    for (auto __it = begin(__r), __e = end(__r); __it != __e; ++__it)
    {
        decl x = *it;
        /* body */
    }
}

As a special case, arrays and braced lists are also supported natively.

It is a C++11 range-based loop, requiring a ranged expression which may be:

• an array or
• braced lists
• a class having either
  • Member functions begin() and end() or
  • available free functions begin() and end() (via ADL)

This

for ( for_range_declaration : expression ) statement;

expands to

range_init = (expression)
{
  auto && __range = range_init;
  for ( auto __begin = begin_expr,
  __end = end_expr;
  __begin != __end;
  ++__begin ) {
    for_range_declaration = *__begin;
    statement;
  }
}

Where begin_expr and end_expr are obtained via array inspection or begin() / end() pair. (The statement may be a compund statement in curly braces.)