Chapter 52. Boost.Foreach

Boost.Foreach provides a macro that simulates the range-based for loop from C++11. You can use the macro BOOST_FOREACH, defined in boost/foreach.hpp, to iterate over a sequence without using iterators. If your development environment supports C++11, you can ignore Boost.Foreach.

Example 52.1. Using BOOST_FOREACH and BOOST_REVERSE_FOREACH

#include <boost/foreach.hpp>
#include <array>
#include <iostream>
int main()
{
  std::array<int, 4> a{{0, 1, 2, 3}};
  BOOST_FOREACH(int &i, a)
    i *= i;
  BOOST_REVERSE_FOREACH(int i, a)
  {
    std::cout << i << '\n';
  }
}

BOOST_FOREACH expects two parameters. The first parameter is a variable or reference, and the second is a sequence. The type of the first parameter needs to match the type of the elements in the sequence.

Anything offering iterators, such as containers from the standard library, classifies as a sequence. Boost.Foreach uses Boost.Range instead of directly accessing the member functions begin() and end(). However, because Boost.Range is based on iterators, anything providing iterators is compatible with BOOST_FOREACH.

Example 52.1 iterates over an array of type std::array with BOOST_FOREACH. The first parameter passed is a reference so that you can both read and modify the elements in the array. In Example 52.1, the first loop multiplies each number by itself.

The second loop uses the macro BOOST_REVERSE_FOREACH, which works the same as BOOST_FOREACH, but iterates backwards over a sequence. The loop writes the numbers 9, 4, 1, and 0 in that order to the standard output stream.

As usual, curly brackets can be omitted if the block only consists of one statement.

Please note that you should not use operations that invalidate the iterator inside the loop. For example, elements should not be added or removed while iterating over a vector. BOOST_FOREACH and BOOST_REVERSE_FOREACH require iterators to be valid throughout the whole iteration.