String Tokenizer Iterator Class

Introduction

As a part of a larger project I had to write some basic string utility functions and classes. One of the things needed was a flexible way of splitting strings into separate tokens.

As is often the case when it comes to programming, there are different ways to handle a problem like this. After reviewing my options I decided that an iterator based solution would be flexible enough for my needs.

Non-iterator based solutions to this particular problem often have the disadvantage of tying the user to a certain container type. With an iterator based tokenizer the programmer is free to chose any type of container (or no container at all). Many STL containers such as std::list and std::vector offer constructors that can populate the container from a set of iterators. This feature makes it very easy to use the tokenizer.

Example usage

std::vector<std::string> s(string_token_iterator("one two three"),
                             string_token_iterator());
std::copy(s.begin(),
          s.end(),
          std::ostream_iterator<std::string>(std::cout,"\n"));
// output:
// one
// two
// three

std::copy(string_token_iterator("one,two..,..three",",."),
          string_token_iterator(),
          std::ostream_iterator<std::string>(std::cout,"\n"));
// same output as above

The code has been tested with Visual C++.NET and GCC 3.

The Code

#include <string>
#include <iterator>

struct string_token_iterator 
  : public std::iterator<std::input_iterator_tag, std::string>
{
public:
  string_token_iterator() : str(0), start(0), end(0) {}
  string_token_iterator(const std::string & str_, const char * separator_ = " ") :
    separator(separator_),
    str(&str_),
    end(0)
  {
    find_next();
  }
  string_token_iterator(const string_token_iterator & rhs) :
    separator(rhs.separator),
    str(rhs.str),
    start(rhs.start),
    end(rhs.end)
  {
  }

  string_token_iterator & operator++()
  {
    find_next();
    return *this;
  }

  string_token_iterator operator++(int)
  {
    string_token_iterator temp(*this);
    ++(*this);
    return temp;
  }

  std::string operator*() const
  {
    return std::string(*str, start, end - start);
  }

  bool operator==(const string_token_iterator & rhs) const
  {
    return (rhs.str == str && rhs.start == start && rhs.end == end);
  }

  bool operator!=(const string_token_iterator & rhs) const
  {
    return !(rhs == *this);
  }

private:

  void find_next(void)
  {
    start = str->find_first_not_of(separator, end);
    if(start == std::string::npos)
    {
      start = end = 0;
      str = 0;
      return;
    }

    end = str->find_first_of(separator, start);
  }

  const char * separator;
  const std::string * str;
  std::string::size_type start;
  std::string::size_type end;
};