How to parse a string to an int in C++?

This is a safer C way than atoi()

const char* str = "123";
int i;

if(sscanf(str, "%d", &i)  == EOF )
{
   /* error */
}

C++ with standard library stringstream: (thanks CMS )

int str2int (const string &str) {
  stringstream ss(str);
  int num;
  if((ss >> num).fail())
  { 
      //ERROR 
  }
  return num;
}

With boost library: (thanks jk)

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

try
{
    std::string str = "123";
    int number = boost::lexical_cast< int >( str );
}
catch( const boost::bad_lexical_cast & )
{
    // Error
}

Edit: Fixed the stringstream version so that it handles errors. (thanks to CMS's and jk's comment on original post)

The good 'old C way still works. I recommend strtol or strtoul. Between the return status and the 'endPtr', you can give good diagnostic output. It also handles multiple bases nicely.

You can use Boost's lexical_cast, which wraps this in a more generic interface. lexical_cast<Target>(Source) throws bad_lexical_cast on failure.

You can use the a stringstream from the C++ standard libraray:

stringstream ss(str);
int x;
ss >> x;

if(ss) { // <-- error handling
  // use x
} else {
  // not a number
}

The stream state will be set to fail if a non-digit is encountered when trying to read an integer.

See Stream pitfalls for pitfalls of errorhandling and streams in C++.

You can use stringstream's

int str2int (const string &str) {
  stringstream ss(str);
  int num;
  ss >> num;
  return num;
}