std::stringstream ss;
ss << "0.1234";
float f;
ss >> f;

#include <iostream>
#include <sstream>

using namespace std;

int main()
{
    string str = "0.1234";
    float f;
    f = atof(str.c_str());
    cout << f << endl;
    stringstream ss;
    ss << "0.12345";
    ss >> f;
    cout << f << endl;
    return 0;
}