# Tutorial

## 2.2.7. UNIX-to-DOS Filters

Suppose you want to write a Filter to convert `UNIX` line endings to `DOS` line-endings. The basic idea is simple: you process the characters in a sequence one at a time, and whenever you encounter the character `'\n'` you replace it with the two-character sequence `'\r'`, `'\n'`. In the following sections I'll implement this algorithm as a `stdio_filter`, an InputFilter and an OutputFilter. The source code can be found in the header <`libs/iostreams/example/unix2dos_filter.hpp`>

#### `unix2dos_stdio_filter`

You can express a `UNIX`-to-`DOS` Filter as a `stdio_filter` by deriving from `stdio_filter` and overriding the `private` `virtual` function do_filter as follows:

    #include <cstdio>    // EOF
    #include <iostream>  // cin, cout
    #include <boost/iostreams/filter/stdio.hpp>

    namespace boost { namespace iostreams { namespace example {

    class unix2dos_stdio_filter : public stdio_filter {
    private:
        void do_filter()
        {
            int c;
            while ((c = std::cin.get()) != EOF) {
                if (c == '\n')
                    std::cout.put('\r');
                std::cout.put(c);
            }
        }
    };

    } } } // End namespace boost::iostreams:example

The function `do_filter` consists of a straightforward implementation of the algorithm I described above: it reads characters from standard input and writes them to standard output unchanged, except that when it encounters `'\n'` it writes `'\r'`, `'\n'`.

#### `unix2dos_input_filter`

Now, let's express a `UNIX`-to-`DOS` Filter as an InputFilter.

    #include <boost/iostreams/categories.hpp> // input_filter_tag
    #include <boost/iostreams/operations.hpp> // get

    namespace boost { namespace iostreams { namespace example {

    class unix2dos_input_filter {
    public:
        typedef char              char_type;
        typedef input_filter_tag  category;

        unix2dos_input_filter() : has_linefeed_(false) { }

        template<typename Source>
        int get(Source& src)
        {
            // Handle unfinished business
            if (has_linefeed_) {
                has_linefeed_ = false;
                return '\n';
            }

            // Forward all characters except '\n'
            int c;
            if ((c = iostreams::get(src)) == '\n') {
                has_linefeed_ = true;
                return '\r';
            }

            return c;
        }

        template<typename Source>
        void close(Source&);
    private:
        bool has_linefeed_;
    };

    } } } // End namespace boost::iostreams:example

The implementation of `get` can be described as follows. Most of the time, you simply read a character from `src` and return it. The special values `EOF` and `WOULD_BLOCK` are treated the same way: they are simply forwarded as-is. The exception is when `iostreams::get` returns `'\n'`. In this case, you return `'\r'` instead and make a note to return `'\n'` the next time `get` is called.

As usual, the member function `close` reset's the Filter's state:

        template<typename Source>
        void close(Source&) { skip_ = false; }

#### `unix2dos_output_filter`

You can express a `UNIX`-to-`DOS` Filter as an OutputFilter as follows:

    #include <boost/iostreams/concepts.hpp>   // output_filter
    #include <boost/iostreams/operations.hpp> // put

    namespace boost { namespace iostreams { namespace example {

    class unix2dos_output_filter : public output_filter {
    public:
        unix2dos_output_filter() : has_linefeed_(false) { }

        template<typename Sink>
        bool put(Sink& dest, int c);

        template<typename Sink>
        void close(Sink&) { has_linefeed_ = false; }
    private:
        template<typename Sink>
        bool put_char(Sink& dest, int c);

        bool has_linefeed_;
    };

    } } } // End namespace boost::iostreams:example

Here I've derived from the helper class `output_filter`, which provides a member type `char_type` equal to `char` and a category tag convertible to `output_filter_tag` and to `closable_tag`.

Let's look first at the helper function `put_char`:

        template<typename Sink>
        bool put_char(Sink& dest, int c)
        {
            bool result;
            if ((result = iostreams::put(dest, c)) == true) {
                has_linefeed_ =
                    c == '\r' ?
                        true :
                        c == '\n' ?
                            false :
                            has_linefeed_;
            }
            return result;
        }

This function attempts to write a single character to the Sink dest, returning `true` for success. If successful, it updates the flag `has_linefeed_`, which indicates that an attempt to write a `DOS` line ending sequence failed after the first character was written.

Using `put_char` you can implement `put` as follows:

        bool put(Sink& dest, int c)
        {
            if (c == '\n')
                return has_linefeed_ ?
                    put_char(dest, '\n') :
                    put_char(dest, '\r') ?
                        this->put(dest, '\n') :
                        false;
            return iostreams::put(dest, c);
        }

The implementation works like so:

1. If you're at the beginning of a `DOS` line-ending sequence — that is, if `c` is `'n'` and `has_line_feed_` is `false` — you attempt to write `'\r'` and then `'\n'` to `dest`.
2. If you're in the middle of a `DOS` line-ending sequence — that is, if `c` is `'n'` and `has_line_feed_` is `true` — you attempt to complete it by writing `'\n'`.
3. Otherwise, you attempt to write `c` to `dest`.

There are two subtle points. First, why does `c == 'n'` and `has_line_feed_ == true` mean that you're in the middle of a `DOS` line-ending sequence? Because when you attempt to write `'\r'`, `'\n'` but only the first character succeeds, you set `has_line_feed_` and return `false`. This causes the user of the Filter to resend the character `'\n'` which triggered the line-ending sequence. Second, note that to write the second character of a line-ending sequence you call `put` recursively instead of calling `put_char`.

Comparing the implementations of `unix2dos_input_filter` and `unix2dos_output_filter`, you can see that this a case where a filtering algorithm is much easier to express as an Input than as an OutputFilter. If you wanted to avoid the complexity of the above definition, you could use the class template `inverse` to construct an OutputFilter from `unix2dos_input_filter`:

    #include <boost/iostreams/invert.hpp>   // inverse

    namespace io = boost::iostreams;
    namespace ex = boost::iostreams::example;

    typedef io::inverse<ex::unix2dos_input_filter> unix2dos_output_filter;

Even this is more work than necessary, however, since line-ending conversions can be handled easily with the built-in component `newline_filter`.

© Copyright 2008 CodeRage, LLC
© Copyright 2004-2007 Jonathan Turkanis

Use, modification, and distribution are subject to the Boost Software License, Version 2.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)