I thought that for_each() goes while (it != end). How does for_each() 'know' what end is here? It doesn't seem to be initialized to anything.

Thanks much,
Graham

class regex_callback
{
    int m_sum;
public:
    regex_callback() : m_sum(0) {}

    template <typename T> void operator()(const T& what)
    { m_sum += boost::lexical_cast<int>(what[1].str().c_str()); }

    int sum() const { return m_sum; }
};
int main()
{
    boost::regex("(\\d+),?");
    string s = "1,1,2,3,5,8,13,21";

    boost::sregex_iterator it(s.begin(), s.end(), reg);
    // Is end initialized to anything here?
    boost::sregex_iterator end;

    regex_callback c;

    // How is end valid here?
    int sum = for_each(it, end, c).sum();

    return 0;
}