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?

should it detect those characters by itself or indent should be forced by call of some function? Is it enough to change behavior when single character is output on stream? — Marek R, Jan 31 '22 at 16:52
@MarekR The only restriction is that normal formatting of std::ostream should continue working. — Frank Puck, Jan 31 '22 at 16:56
[boost iostreams](https://www.boost.org/doc/libs/1_78_0/libs/iostreams/doc/guide/filtering_streams.html) might help — Alan Birtles, Jan 31 '22 at 17:41

score 1 · Accepted Answer · answered Jan 31 '22 at 18:03

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> > >";
}

As far as I can see in the boost sources, the newLine() method is not called from the outside. Does it need to have a bool return type? — Frank Puck, Apr 08 '22 at 19:49

score 0 · Answer 2 · answered Jan 31 '22 at 17:08

std::ostream? The IOStream design is rather complicated (and a tad slow), so I can see why you'd think this. The name ostream sounds like a good hint, but it really doesn't do much. It really works by binding std::ostreambuf and operator<< overloads together. The derived ostream classes provide concrete streambufs.

What you'd want is likely a streambuf filter. You grab the underlying streambuf, and use that as an output, but you insert some extra spaces when you see a >

make std::ostream automatically ident when encountering special characters

2 Answers2