String to float using stringstream

Question

I found this code online as a template for doing a string to float/int/double conversion. It's only here so I have something to reference for the question....

I want to have a user enter a number as a string, convert it to a float, test it for success and drop out if entry was 'Q' or print "Invalid input" if it wasn't the 'Q'uit character and return for more input.

What's the syntax for a conversion fail test? Would it be ss.fail() ?

// using stringstream constructors.
#include <iostream>
#include <sstream>
using namespace std;

int main () {

  int val;
  stringstream ss (stringstream::in | stringstream::out);

  ss << "120 42 377 6 5 2000";

  /* Would I insert an 

     if(ss.fail())
       { 
        // Deal with conversion error }
       }

    in here?! */


  for (int n=0; n<6; n++)
  {
    ss >> val;
    cout << val*2 << endl;
  }

  return 0;
}

Answer 1

Your code isn't very helpful. But if I understand you right do it like this

string str;
if (!getline(cin, str))
{
  // error: didn't get any input
}
istringstream ss(str);
float f;
if (!(ss >> f))
{
  // error: didn't convert to a float
}

There's no need to use fail.

Answer 2

Actually, the simplest way to do string to float conversion is probably boost::lexical_cast

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

int main() {
    std::string const s = "120.34";

    try {
        float f = boost::lexical_cast<float>(s);
    } catch(boost::bad_lexical_cast const&) {
        // deal with error
    }
}

Obviously, in most cases, you just don't catch the exception right away and let it bubble up the call chain, so the cost is much reduced.