C++ Cookbook

3.1. Converting a String to a Numeric Type

Problem

You have numbers in string format, but you need to convert them to a numeric type, such as an int or float.

Solution

You can do this in one of two ways, with standard library functions or with the lexical_cast class in Boost (written by Kevlin Henney). The standard library functions are cumbersome and unsafe, but they are standard, and in some cases, you need them, so I present them as the first solution. lexical_cast is safer, easier to use, and just more fun, so I present it in the discussion.

The functions strtol, strtod, and strtoul, defined in <cstdlib>, convert a null-terminated character string to a long int, double, or unsigned long. You can use them to convert numeric strings of any base to a numeric type. The code in Example 3-1 demonstrates a function, hex2int, that you can use for converting a hexadecimal string to a long.

Example 3-1. Converting number strings to numbers

#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;

long hex2int(const string& hexStr) {
   char *offset;
   if (hexStr.length() > 2) {
      if (hexStr[0] == '0' && hexStr[1] == 'x') {
         return strtol(hexStr.c_str(), &offset, 0);
      }
   }
   return strtol(hexStr.c_str(), &offset, 16);
}

int main() {
   string str1 = "0x12AB";
   cout << hex2int(str1) << endl;
   string str2 = "12AB";
   cout << hex2int(str2) << endl;
   string str3 = "QAFG";
   cout << hex2int(str3) << endl;
}

Here’s the output from this program:

4779
4779
0

The first two strings both contain the hexadecimal ...