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copied!<p>You wouldn't use <a href="http://www.cplusplus.com/reference/algorithm/for_each/" rel="nofollow noreferrer"><code>std::for_each</code></a>, but rather <a href="http://www.cplusplus.com/reference/algorithm/transform/" rel="nofollow noreferrer"><code>std::transform</code></a> (you're transforming a point into a single number.)</p> <p>For example:</p> <pre><code>#include <algorithm> // transform resides here #include <iostream> #include <iterator> #include <vector> struct Point { int x; int y; Point(int x, int y) : x(x), y(y) { } }; int point_to_int(const Point& p) { return p.x; } int main() { std::vector<Point> points; points.push_back(Point(1, 2)); points.push_back(Point(4, 6)); std::vector<int> xs; std::transform(points.begin(), points.end(), std::back_inserter(xs), point_to_int); std::copy(xs.begin(), xs.end(), std::ostream_iterator<int>(std::cout, " ")); std::cout << std::endl; return 0; } </code></pre> <p>Because you know the size of the container you'll be transforming, you might get a slight performance improvement from the following. I also find it more readable:</p> <pre><code>std::vector<int> xs; xs.reserve(points.size()); std::transform(points.begin(), points.end(), std::back_inserter(xs), point_to_int); </code></pre> <p>And with <a href="http://www.boost.org/doc/libs/1_40_0/doc/html/lambda.html" rel="nofollow noreferrer"><code>boost::lambda</code></a> along with <a href="http://www.boost.org/doc/libs/1_40_0/libs/bind/bind.html" rel="nofollow noreferrer"><code>boost::bind</code></a>:</p> <pre><code>#include <algorithm> #include <iostream> #include <iterator> #include <vector> #include <boost/bind.hpp> #include <boost/lambda/lambda.hpp> struct Point { int x; int y; Point(int x, int y) : x(x), y(y) { } }; int main() { using namespace boost; std::vector<Point> points; points.push_back(Point(1, 2)); points.push_back(Point(4, 6)); std::vector<int> xs; xs.reserve(points.size()); std::transform(points.begin(), points.end(), std::back_inserter(xs), bind(&Point::x, lambda::_1)); std::copy(xs.begin(), xs.end(), std::ostream_iterator<int>(std::cout, " ")); std::cout << std::endl; return 0; } </code></pre> <p>Removes the need to specify a function elsewhere. This keeps the code close to the calling site, and generally improves readability.</p> <p>In C++0x, it will simply be:</p> <pre><code>std::transform(points.begin(), points.end(), std::back_inserter(xs), [](const Point& p){ return p.x; } ); </code></pre> <p>(To the best of my knowledge, anyway)</p>

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