C++ Cookbook

3.3. Testing Whether a String Contains a Valid Number

Problem

You have a string and you need to find out if it contains a valid number.

Solution

You can use the Boost lexical_cast function template to test for a valid number. Using this approach, a valid number can include a preceding minus sign, or a preceding plus sign, but not whitespace. I give a few examples of the kinds of formats that work with lexical_cast in Example 3-5.

Example 3-5. Validating a string number

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

using namespace std;
using boost::lexical_cast;
using boost::bad_lexical_cast;

template<typename T>
bool isValid(const string& num) {

   bool res = true;

   try {
      T tmp = lexical_cast<T>(num);
   }
   catch (bad_lexical_cast &e) {
      res = false;
   }

   return(res);
}

void test(const string& s) {

   if (isValid<int>(s))
      cout << s << " is a valid integer." << endl;
   else
      cout << s << " is NOT a valid integer." << endl;

   if (isValid<double>(s))
      cout << s << " is a valid double." << endl;
   else
      cout << s << " is NOT a valid double." << endl;

   if (isValid<float>(s))
      cout << s << " is a valid float." << endl;
   else
      cout << s << " is NOT a valid float." << endl;
}

int main() {

   test("12345");
   test("1.23456");
   test("-1.23456");
   test(" - 1.23456");
   test("+1.23456");
   test("  1.23456  ");
   test("asdf");
}

Here’s the output from this example:

12345 is a valid integer. 12345 is a valid double. 12345 is a valid float. 1.23456 is NOT a valid integer. 1.23456 is a valid double. 1.23456 is a valid float. -1.23456 ...