URI Parser in C++

uri-parser.cpp

/* Original code: http://stackoverflow.com/questions/2616011/easy-way-to-parse-a-url-in-c-cross-platform */
#include <string>
struct url {
  public:
    url(const std::string& url_s) {
      this->parse(url_s);
    }
    std::string protocol_, host_, path_, query_;
  private:
    void parse(const std::string& url_s);
};

#include <string>
#include <algorithm>
#include <cctype>
#include <functional>
#include <iostream>

using namespace std;

// ctors, copy, equality, ...

void url::parse(const string& url_s)
{
  const string prot_end("://");
  string::const_iterator prot_i = search(url_s.begin(), url_s.end(),
      prot_end.begin(), prot_end.end());
  protocol_.reserve(distance(url_s.begin(), prot_i));
  transform(url_s.begin(), prot_i,
      back_inserter(protocol_),
      ptr_fun<int,int>(tolower)); // protocol is icase
  if( prot_i == url_s.end() )
    return;
  advance(prot_i, prot_end.length());
  string::const_iterator path_i = find(prot_i, url_s.end(), '/');
  host_.reserve(distance(prot_i, path_i));
  transform(prot_i, path_i,
      back_inserter(host_),
      ptr_fun<int,int>(tolower)); // host is icase
  string::const_iterator query_i = find(path_i, url_s.end(), '?');
  path_.assign(path_i, query_i);
  if( query_i != url_s.end() )
    ++query_i;
  query_.assign(query_i, url_s.end());
}

int main() {
  url u("https://github.com/redcarrottt?query=1");
  cout << u.protocol_ << '\n'
    << u.host_ << '\n'
    << u.path_ << '\n'
    << u.query_ << '\n';
}