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

possible duplicate:
fastest way read numerical values text file in c++ (double in 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();      (int i=0; i<1000000; ++i)         double x = str2dou1(str);      tend=clock();     cout << tend-tstart << endl;      cout << "sstream: ";     tstart=clock();      (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

c/c++ text number formatting slow. streams horribly slow c number parsing slow because it's quite difficult correct down last precision bit.

in production application reading speed important , data known have @ 3 decimal digits , no scientific notation got vast improvement hand-coding floating parsing function handling sign, integer part , number of decimals (by "vast" mean 10x faster compared strtod).

if don't need exponent , precision of function enough code of parser similar 1 wrote then. on pc it's 6.8 times faster strtod , 22.6 times faster sstream.

double parsefloat(const std::string& input) {     const char *p = input.c_str();     if (!*p || *p == '?')         return nan_d;     int s = 1;     while (*p == ' ') p++;      if (*p == '-') {         s = -1; p++;     }      double acc = 0;     while (*p >= '0' && *p <= '9')         acc = acc * 10 + *p++ - '0';      if (*p == '.') {         double k = 0.1;         p++;         while (*p >= '0' && *p <= '9') {             acc += (*p++ - '0') * k;             k *= 0.1;         }     }     if (*p) die("invalid numeric format");     return s * acc; }