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

Question

What\'s the C++ way of parsing a string (given as char *) into an int? Robust and clear error handling is a plus (instead of returning zero).

Answer 1

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++.

Answer 2

From C++17 onwards you can use std::from_chars from the <charconv> header as documented here.

For example:

#include <iostream>
#include <charconv>
#include <array>

int main()
{
    char const * str = "42";
    int value = 0;

    std::from_chars_result result = std::from_chars(std::begin(str), std::end(str), value);

    if(result.error == std::errc::invalid_argument)
    {
      std::cout << "Error, invalid format";
    }
    else if(result.error == std::errc::result_out_of_range)
    {
      std::cout << "Error, value too big for int range";
    }
    else
    {
      std::cout << "Success: " << result;
    }
}

As a bonus, it can also handle other bases, like hexadecimal.

Answer 3

In C, you can use int atoi (const char * str),

Parses the C-string str interpreting its content as an integral number, which is returned as a value of type int.

Answer 4

You can use stringstream's

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

Answer 5

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)

Answer 6

You could use this defined method.

#define toInt(x) {atoi(x.c_str())};

And if you were to convert from String to an Integer, you would just do the following.

int main()
{
string test = "46", test2 = "56";
int a = toInt(test);
int b = toInt(test2);
cout<<a+b<<endl;
}

The output would be 102.