C++ Converting a String to Double

问题:

I've been trying to find the solution for this all day! You might label this as re-post but what I'm really looking for is a solution without using boost lexical cast. A traditional C++ way of doing it would be great. I tried this code but it returns a set of gibberish numbers and letters.

string line; 
double lineconverted;

istringstream buffer(line);
lineconverted;
buffer >> lineconverted;

And I alse tried this, but it ALWAYS returns 0.

stringstream convert(line);
if ( !(convert >> lineconverted) ) {
    lineconverted  = 0;
}

Thanks in advance :)

EDIT: For the first solution I used (gibberish).. Here's a snapshot

回答1:

#include <sstream>

int main(int argc, char *argv[])
{
    double f = 0.0;

    std::stringstream ss;
    std::string s = "3.1415";

    ss << s;
    ss >> f;

    cout << f;
}

The good thing is, that this solution works for others also, like ints, etc.

If you want to repeatedly use the same buffer, you must do ss.clear in between.

There is also a shorter solution available where you can initialize the value to a stringstream and flush it to a double at the same time:

#include <sstream>
int main(int argc, char *argv[]){
   stringstream("3.1415")>>f ;
}

回答2:

Since C++11 you could use std::stod function:

string line; 
double lineconverted;

try
{
    lineconverted = std::stod(line);
}
catch(std::invalid_argument)
{
    // can't convert
}

But solution with std::stringstream also correct:

#include <sstream>
#include <string>
#include <iostream>

int main()
{
    std::string str;
    std::cin >> str;
    std::istringstream iss(str);
    double d = 0;
    iss >> d;
    std::cout << d << std::endl;
    return 0;
}

回答3:

If you want to store (to a vector for example) all the doubles of a line

#include <iostream>
#include <vector>
#include <iterator>

int main()
{

  std::istream_iterator<double> in(std::cin);
  std::istream_iterator<double> eof;
  std::vector<double> m(in,eof);

  //print
  std::copy(m.begin(),m.end(),std::ostream_iterator<double>(std::cout,"\n"));

}