performance of map in MSVC 10b2

I use map a lot in my daily work, so the performance of map is very
important to me. today i did a very simple test on MSVC10b2, using
std::map, std::unordered_map and boost::unordered_map, and found that
the test result is suprise to me.

here comes test code first:
MSVC10b2, WinXP
//-----------------------------------------------
#include <iostream>
#include <string>
#include <map>
#include <unordered_map>

#include <boost/unordered_map.hpp>
#include <boost/progress.hpp>

using namespace std;

int main()
{
    typedef std::unordered_map<int, string> map_t;
    //typedef boost::unordered_map<int, string> map_t;
    //typedef std::map<int, string> map_t;
    const int size = 1E7;
    map_t imap;
    {
        boost::progress_timer t1;
        boost::progress_display prog_bar(size);
        for(int i=0; i<size; ++i) {
            imap[i] = "test";
            ++prog_bar;
        }
    }
    {
        boost::progress_timer t2;
        string buff;
        for(int i=0; i<size; ++i) {
            buff = imap[i];
        }
    }
    return 0;

}
//-----------------------------------------------------------

and the test result:

MAP INSERT FIND
std::map 7.73s 1.98s
boost::unorder_map 5.47s 0.80s
std::unorder_map 9.91s 2.98s

am i wrong with the usage? or something i didn't catch up here?

thanks,
jimzshi

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