Converting a string to a double c++ - c++

Question

This code reads the first line of some data I have in .txt file,
this first line value is '6.0', im reading the rest of the data later on in a loop, my question is how can I convert 'line' (a string) to a double (I would call 'instances') so that I can use it later in the program? It needs to be a double so I can do some calculations later with other double types. Probably a very simple solution, thanks in advance.
NOTE: I have tried using:
#include <stdlib.h>
double instances = atof(line.c_str());
But it returns 0 not the correct value '6.0'
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
int main()
{
string line;
ifstream infile ("data2.txt");
if (infile.is_open())
{
getline (infile,line);
cout << line << '\n';
infile.close();
}
...
}

Answer 1

You have lots of options. These all have different ways of checking the output for validity. Some of them throw an exception if line doesn't contain a double, some give a return value that you could check. Whichever one is best for you depends entirely on your program.
C++11 introduced std::stod:
double instances = std::stod(line);
Before that, there's the confusingly named std::atof that still returns a double:
double instances = std::atof(line.c_str());
Or you could use a std::istringstream's operator>> to extract a double:
std::istringstream iss(line);
double instances;
iss >> instances;
Or you could use std::sscanf:
double instances;
std::sscanf(line.c_str(), "%lf", &instances);
Or you could use Boost.Lexical_Cast:
double instances = boost::lexical_cast<double>(line);