Boost::tokenizer point separated, but also keeping empty fields

Question

I have seen this question and mine is very similar to it, but it is different, so please do not mark it as duplicate.

My question is: How do I get the empty fields from a string?

I have a string like std::string s = "This.is..a.test"; and I want to get the fields <This> <is> <> <a> <test>.

I have tried also

typedef boost::char_separator<char> ChSep;
typedef boost::tokenizer<ChSep> TknChSep;
ChSep sep(".", ".", boost::keep_empty_tokens);
TknChSep tok(s, sep);
for (TknChSep::iterator beg = tok.begin(); beg != tok.end(); ++beg)
{
  std::cout << "<" << *beg << "> ";
}

but I get <This> <.> <is> <.> <> <.> <a> <test>.

Answer 1

The second argument to Boost.Tokenizer's char_separator is the kept_delims parameter. It is used to specify a delimiters that will show up as tokens. The original code is specifying that "." should be kept as a token. To resolve this, change:

ChSep sep(".", ".", boost::keep_empty_tokens);

to:

ChSep sep(".", "", boost::keep_empty_tokens);
            // ^-- no delimiters will show up as tokens.

---

Here is a complete example:

#include <iostream>
#include <string>
#include <boost/foreach.hpp>
#include <boost/tokenizer.hpp>

int main()
{
  std::string str = "This.is..a.test";
  typedef boost::tokenizer<boost::char_separator<char> > tokenizer;
  boost::char_separator<char> sep(
      ".", // dropped delimiters
      "",  // kept delimiters
      boost::keep_empty_tokens); // empty token policy

  BOOST_FOREACH(std::string token, tokenizer(str, sep))
  {
    std::cout << "<" << token << "> ";
  }
  std::cout << std::endl;
}

Which produces the desired output:

<This> <is> <> <a> <test> 

Answer 2

I think I'd skip Boost::tokenizer, and just use a standard regex to do the splitting:

#include <iterator>
#include <regex>
#include <string>
#include <iostream>

int main() {     
    std::string s = "This.is..a.test";

    std::regex sep{ "\\." };

    std::copy(std::sregex_token_iterator(s.begin(), s.end(), sep, -1),
        std::sregex_token_iterator(), 
        std::ostream_iterator<std::string>(std::cout, "\n"));
}

Result:

This
is

a
test