std::string to float or double

Answer 1

Yes, with a lexical cast. Use a stringstream and the << operator, or use Boost, they've already implemented it.

Your own version could look like:

template<typename to, typename from>to lexical_cast(from const &x) {
  std::stringstream os;
  to ret;

  os << x;
  os >> ret;

  return ret;  
}

Answer 2

Lexical cast is very nice.

#include <boost/lexical_cast.hpp>
#include <iostream>
#include <string>

using std::endl;
using std::cout;
using std::string;
using boost::lexical_cast;

int main() {
    string str = "0.6";
    double dub = lexical_cast<double>(str);
    cout << dub << endl;
}

Answer 3

You can use boost lexical cast:

#include <boost/lexical_cast.hpp>

string v("0.6");
double dd = boost::lexical_cast<double>(v);
cout << dd << endl;

Note: boost::lexical_cast throws exception so you should be prepared to deal with it when you pass invalid value, try passing string("xxx")

Answer 4

std::string num = "0.6";
double temp = ::atof(num.c_str());

Does it for me, it is a valid C++ syntax to convert a string to a double.

You can do it with the stringstream or boost::lexical_cast but those come with a performance penalty.

---

Ahaha you have a Qt project ...

QString winOpacity("0.6");
double temp = winOpacity.toDouble();

Extra note:
If the input data is a const char*, QByteArray::toDouble will be faster.

Answer 5

You can use std::stringstream:

   #include <sstream>
   #include <string>
   template<typename T>
   T StringToNumber(const std::string& numberAsString)
   {
      T valor;

      std::stringstream stream(numberAsString);
      stream >> valor;
      if (stream.fail()) {
         std::runtime_error e(numberAsString);
         throw e;
      }
      return valor;
   }

Usage:

double number= StringToNumber<double>("0.6");

Answer 6

This answer is backing up litb in your comments. I have profound suspicions you are just not displaying the result properly.

I had the exact same thing happen to me once. I spent a whole day trying to figure out why I was getting a bad value into a 64-bit int, only to discover that printf was ignoring the second byte. You can't just pass a 64-bit value into printf like its an int.

Answer 7

Rather than dragging Boost into the equation, you could keep your string (temporarily) as a char[] and use sprintf().

*But of course if you're using Boost anyway, it's really not too much of an issue.

Answer 8

   double myAtof ( string &num){
      double tmp;
      sscanf ( num.c_str(), "%lf" , &tmp);
      return tmp;
   }