make std::ostream automatically ident when encountering special characters

Question

I would like to have some facility which makes a std::ostream (or derived) automatically ident on encountering special characters (or special objects). Let's assume that the special characters are < and >. In this case the following input test0<test1<test2, test3<test4> > > should produce the following output:

test0<
    test1<
        test2,
        test3<
            test4
        >
    >
>

How would one go to implement this?

Answer 1

boost::iostreams makes this fairly easy, you can define filters then chain them together with an output stream to transform the input to the desired output:

#include <iostream>
#include <boost/iostreams/filtering_stream.hpp>

namespace io = boost::iostreams;

struct QuoteOutputFilter {
  typedef char                   char_type;
  typedef io::output_filter_tag  category;

  int indent = 0;

  template<typename Sink>
  bool newLine(Sink& snk)
  {
    std::string str = "\n" + std::string(indent * 4, ' ');
    return io::write(snk, str.c_str(), str.size());
  }

  template<typename Sink>
  bool put(Sink& snk, char c)
  {
    switch (c)
    {
      case '<':
        io::put(snk, c);
        indent += 1;
        return newLine(snk);
      case ',':
        io::put(snk, c);
        return newLine(snk);
      case '>':
        indent -= 1;
        newLine(snk);
        return io::put(snk, c);
      default:
        return io::put(snk, c);
    }
  }
};

int main()
{
  io::filtering_ostream out;
  out.push(QuoteOutputFilter());
  out.push(std::cout);

  out << "test0<test1<test2, test3<test4> > >";
}