Range-based for-loops

Range-based for-loops are another syntactic convenience introduced in C++11. Range-based for-loops allow you to iterate through a sequence of values like arrays, containers, iterator ranges, and so on, without having to explicitly specify boundary conditions. It makes iterating less error-prone by obviating the need to specify boundary conditions.

The general syntax for range-based for-loop is:

for (range-declaration : sequence-expression) {
  statements;
}

The sequence expression identifies a sequence of values like an array or a container, that is to be iterated through. The range declaration identifies a variable that would represent each element from the sequence in successive iterations of the loop. Range-based for-loops automatically recognize arrays, brace-enclosed sequences of expressions, and containers with begin and end member functions that return forward iterators. To iterate through all elements in an array, you write this:

T arr[N];
...
for (const auto& elem : arr) {
  // do something on each elem
}

You can also iterate through a sequence of expressions enclosed in braces:

for (const auto& elem: {"Aragorn", "Gandalf", "Frodo Baggins"}) {
  // do something on each elem
}

Iterating through elements in a container that exposes forward iterators through begin and end member functions is not all that different:

std::vector<T> vector;
...
for (const auto& elem: vector) {
  // do something on each elem
}

The range expression declares a loop variable called elem using auto to deduce its type. This use of auto in range-based for-loops is idiomatic and common. To traverse sequences encapsulated within other kinds of objects, range-based for-loops require that two namespace-level methods, begin and end, be available and be resolved via Argument Dependent Lookup (see Chapter 2, The First Brush with Boost's Utilities). Range-based for-loops are great for traversing sequences whose lengths remain fixed during traversal.

Boost.Foreach

You can use the BOOST_FOREACH macro to emulate the basic uses of C++11's range-based for-loops:

#include <boost/foreach.hpp>

std::vector<std::string> names;
...
BOOST_FOREACH(std::string& name, names) {
  // process each elem
}

In the preceding example, we use the BOOST_FOREACH macro to iterate through the elements of a vector of strings called names, using a loop variable called name of type string. Using BOOST_FOREACH, you can iterate over arrays, containers with member functions begin and end that return forward iterators, iterator pairs, and null-terminated character arrays. Note that C++11 range-based for-loops do not readily support the last two types of sequences. On the other hand, with BOOST_FOREACH, you cannot deduce the type of the loop variable using the auto keyword.