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C++ Cookbook
book

C++ Cookbook

by D. Ryan Stephens, Christopher Diggins, Jonathan Turkanis, Jeff Cogswell
November 2005
Beginner to intermediate content levelBeginner to intermediate
594 pages
16h 23m
English
O'Reilly Media, Inc.
Content preview from 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 ...
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Publisher Resources

ISBN: 0596007612Supplemental ContentErrata Page