Custom inserter for std::copy

Question

Given a std::vector which holds objects of MyClass. How can I create another vector which holds just data of one member of MyClass using std::copy?

Answer 1

You want to use std::transform not std::copy and std::bind to bind to a pointer to a member variable:

#include 
#include 
#include 
#include 
#include 

struct foo {
  int a;
};

int main() {
  const std::vector f = {{0},{1},{2}};
  std::vector out;

  out.reserve(f.size());
  std::transform(f.begin(), f.end(), std::back_inserter(out), 
                 std::bind(&foo::a, std::placeholders::_1));

  // Print to prove it worked:
  std::copy(out.begin(), out.end(), std::ostream_iterator(std::cout, "\n"));
}

My example is C++11, but if you skip the handy vector initalization and use boost::bind instead this works just as well without C++11.