C++ Cookbook

3.6. Converting Between Numeric Types

Problem

You have number of one type and you need to convert it to another, such as an int to a short or a vice versa, but you want to catch any overflow or underflow errors at runtime.

Solution

Use Boost’s numeric_cast class template. It performs runtime checks that throw an exception of type bad_numeric_cast if you will overflow or underflow the variable where you are putting a value. Example 3-8 shows you how to do this.

Example 3-8. Safe numeric conversions

#include <iostream>
#include <boost/cast.hpp>

using namespace std;
using boost::numeric_cast;
using boost::bad_numeric_cast;

int main() {

   // Integer sizes
   try {
      int i = 32767;
      short s = numeric_cast<short>(i);

      cout << "s = " << s << endl;

      i++; // Now i is out of range (if sizeof(short) is 2)
      s = numeric_cast<short>(i);
   }
   catch (bad_numeric_cast& e) {
      cerr << e.what() << endl;
   }

   try {
      int i = 300;
      unsigned int ui = numeric_cast<unsigned int>(i);

      cout << ui << endl; // Fine

      i *= -1;
      ui = numeric_cast<unsigned int>(i); // i is negative!
   }
   catch (bad_numeric_cast& e) {
      cerr << e.what() << endl;
   }

   try {
      double d = 3.14;
      int i = numeric_cast<int>(d);

      i = numeric_cast<int>(d); // This shaves off the 0.14!

      cout << i << endl;  // i = 3

   }
   catch (bad_numeric_cast& e) {
      cerr << e.what() << endl;
   }
}

Discussion

You are probably aware of the fact that the basic C++ types have different sizes. The C++ standard has strict specifications for the relative size of types—an int is always at least as big as a