c++ stringstream is too slow, how to speed up?

Possible Duplicate:
Fastest way to read numerical values from text file in C++ (double in this case)

#include <ctime>
#include <cstdlib>
#include <string>
#include <sstream>
#include <iostream>
#include <limits>

using namespace std;

static const double NAN_D = numeric_limits<double>::quiet_NaN();

void die(const char *msg, const char *info)
{
    cerr << "** error: " << msg << " \"" << info << '\"';
    exit(1);
}

double str2dou1(const string &str)
{
    if (str.empty() || str[0]=='?') return NAN_D;
    const char *c_str = str.c_str();
    char *err;
    double x = strtod(c_str, &err);
    if (*err != 0) die("unrecognized numeric data", c_str);
    return x;
}

static istringstream string_to_type_stream;

double str2dou2(const string &str)
{
    if (str.empty() || str[0]=='?') return NAN_D;
    string_to_type_stream.clear();
    string_to_type_stream.str(str);
    double x = 0.0;
    if ((string_to_type_stream >> x).fail())
        die("unrecognized numeric data", str.c_str());
    return x;
}

int main()
{
    string str("12345.6789");

    clock_t tStart, tEnd;

    cout << "strtod: ";
    tStart=clock();

    for (int i=0; i<1000000; ++i)
        double x = str2dou1(str);

    tEnd=clock();
    cout << tEnd-tStart << endl;

    cout << "sstream: ";
    tStart=clock();

    for (int i=0; i<1000000; ++i)
        double x = str2dou2(str);

    tEnd=clock();
    cout << tEnd-tStart << endl;

    return 0;
}

strtod: 405
sstream: 1389

update: remove undersocres, env: win7+vc10