Use stable_partition and partition : stable_partition « STL Algorithms Modifying sequence operations « C++ Tutorial

1. C++ Tutorial
2. STL Algorithms Modifying sequence operations
3. stable_partition

/* The following code example is taken from the book
 * "The C++ Standard Library - A Tutorial and Reference"
 * by Nicolai M. Josuttis, Addison-Wesley, 1999
 *
 * (C) Copyright Nicolai M. Josuttis 1999.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */

#include <iostream>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <string>
#include <algorithm>
#include <iterator>
#include <functional>
#include <numeric>

/* PRINT_ELEMENTS()
 * - prints optional C-string optcstr followed by
 * - all elements of the collection coll
 * - separated by spaces
 */
template <class T>
inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="")
{
    typename T::const_iterator pos;

    std::cout << optcstr;
    for (pos=coll.begin(); pos!=coll.end(); ++pos) {
        std::cout << *pos << ' ';
    }
    std::cout << std::endl;
}

/* INSERT_ELEMENTS (collection, first, last)
 * - fill values from first to last into the collection
 * - NOTE: NO half-open range
 */
template <class T>
inline void INSERT_ELEMENTS (T& coll, int first, int last)
{
    for (int i=first; i<=last; ++i) {
        coll.insert(coll.end(),i);
    }
}

using namespace std;

int main()
{
   vector<int> coll1;
   vector<int> coll2;

   INSERT_ELEMENTS(coll1,1,9);
   INSERT_ELEMENTS(coll2,1,9);
   PRINT_ELEMENTS(coll1,"coll1: ");
   PRINT_ELEMENTS(coll2,"coll2: ");
   cout << endl;

   // move all even elements to the front
   vector<int>::iterator pos1, pos2;
   pos1 = partition(coll1.begin(), coll1.end(),         // range
                    not1(bind2nd(modulus<int>(),2)));   // criterion
   pos2 = stable_partition(coll2.begin(), coll2.end(),       // range
                           not1(bind2nd(modulus<int>(),2))); // crit.

   // print collections and first odd element
   PRINT_ELEMENTS(coll1,"coll1: ");
   cout << "first odd element: " << *pos1 << endl;
   PRINT_ELEMENTS(coll2,"coll2: ");
   cout << "first odd element: " << *pos2 << endl;
}

coll1: 1 2 3 4 5 6 7 8 9
coll2: 1 2 3 4 5 6 7 8 9

coll1: 8 2 6 4 5 3 7 1 9
first odd element: 5
coll2: 2 4 6 8 1 3 5 7 9
first odd element: 1