You wouldn't use std::for_each, but rather std::transform (you're transforming a point into a single number.)
For example:
#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; }
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:
std::vector<int> xs; xs.reserve(points.size()); std::transform(points.begin(), points.end(), std::back_inserter(xs), point_to_int);
And with boost::lambda along with boost::bind:
#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; }
Removes the need to specify a function elsewhere. This keeps the code close to the calling site, and generally improves readability.
In C++0x, it will simply be:
std::transform(points.begin(), points.end(), std::back_inserter(xs), [](const Point& p){ return p.x; } );
(To the best of my knowledge, anyway)
