boost::regex_iterator

As an introduction to some of the most interesting boost libraries you can read "Beyond the C++ Standard Library: An Introduction to Boost", by Björn Karlsson, an Addison Wesley Professional book. That's what I'm actually doing, and these are a few notes that I'm jotting down in the meanwhile.

Instead of looping on regex_search to search all the occurrence of a regex in a string, we could use regex_iterator (or sregex_iterator for std::string) coupled with a for_each() call to scan the string.

Here is an example that scan in string all the comma separated numbers and then add them using a functor:

#include <iostream>
#include <string>
#include <functional>
#include "boost/regex.hpp"

using std::cout;
using std::endl;
using std::string;
using std::for_each;
using boost::regex;
using boost::sregex_iterator;
using boost::regex_search;

namespace
{
    class Adder
    {
        int sum_;
    public:
        Adder() : sum_(0) {}

        template <typename T> void operator()(const T& what)
        {
            sum_ += atoi(what[1].str().c_str());
        }

        int sum() const
        {
            return sum_;
        }
    };
}

void r06() {
    regex reg("(\\d+),?");
    string s = "1,1,2,3,5,8,13,21,34,55";

    sregex_iterator it(s.begin(), s.end(), reg);
    sregex_iterator end;

    int sum = for_each(it, end, Adder()).sum();
    cout << "summing " << s << ": " << sum << endl;
}