# Tutorial

## 2.2.3. Shell Comments Filters

Suppose you want to write a filter to remove shell-style comments. The basic algorithm is as follows: you examine characters one at a time, forwarding them unchanged, until you encounter a comment character, typically `'#'`. When you find a comment character, you examine and ignore characters until you encounter a newline character, at which point the algorithm begins again. Note that this algorithm consists of two subalgorithms: one algorithm for reading ordinary text, and one for reading comments.

In the next three sections, I'll express this algorithm as a `stdio_filter`, an InputFilter and an OutputFilter. The source code can be found in the header `<libs/iostreams/example/shell_comments_filter.hpp>`. These examples were inspired by James Kanze's `UncommentExtractor.hh` (see [Kanze]).

#### `shell_comments_stdio_filter`

You can express a shell comments Filter as a `stdio_filter` as follows:

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

    class shell_comments_stdio_filter : public stdio_filter {
    public:
        explicit shell_comments_stdio_filter(char comment_char = '#')
            : comment_char_(comment_char)
            { }
    private:
        void do_filter()
        {
            bool  skip = false;
            int   c;
            while ((c = std::cin.get()) != EOF) {
                skip = c == comment_char_ ?
                    true :
                    c == '\n' ?
                        false :
                        skip;
                if (!skip)
                    std::cout.put(c);
            }
        }
        char comment_char_;
    };

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

The implementation of the `virtual` function `do_filter` is straightforward: The local variable `skip` keeps track of whether you are currently processing a comment; the `while` loop reads a character `c` from `std::cin`, updates `skip` and writes `c` to `std::cout` unless `skip` is `true`.

Filters which derive from `stdio_filter` are DualUseFilters, which mean they can be used either for output or for input, but not both simultaneously. Therefore `unix2dos_stdio_filter` can be used in place of `shell_comments_input_filter` and `shell_comments_output_filter`, below.

#### `shell_comments_input_filter`

Next you will express a shell comments Filter as an InputFilter. A typical narrow-character InputFilter looks like this:

    #include <boost/iostreams/categories.hpp>  // input_filter_tag
    #include <boost/iostreams/char_traits.hpp> // EOF, WOULD_BLOCK
    #include <boost/iostreams/operations.hpp>  // get, read, putback

    namespace io = boost::iostreams;

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

        template<typename Source>
        int get(Source& src)
        {
            // Attempt to produce one character of filtered
            // data, reading from src as necessary. If successful,
            // return the character; otherwise return EOF to
            // indicate end-of-stream, or WOULD_BLOCK
        }

        /* Other members */
    };

The function `get` attempts to produce a single character of filtered output. It accesses the unfiltered character sequence though the provided Source `src`, using the fundamental i/o operations `get`, `read` and `putback`. If a character is produced, `get` returns it. Otherwise `get` returns one of the status codes `EOF` or `WOULD_BLOCK`. `EOF`, which indicates end-of-stream, is a macro defined in the standard header `<cstdio>`. `WOULD_BLOCK`, which indicates that input is temporarily unavailable, is a constant defined in the namespace `boost::iostreams`, in the header `<boost/iostreams/char_traits.hpp>`

You could also write the above example as follows:

    #include <boost/iostreams/concepts.hpp>  // input_filter

    class my_input_filter : public input_filter {
    public:
        template<typename Source>
        int get(Source& src);

        /* Other members */
    };

Here `input_filter` is a convenience base class which provides the member types `char_type` and `category`, as well as no-op implementations of member functions `close` and `imbue`. I will discuss `close` shortly.

You're now ready to express a shell Comments Filter as an InputFilter:

    #include <boost/iostreams/char_traits.hpp> // EOF, WOULD_BLOCK
    #include <boost/iostreams/concepts.hpp>    // input_filter
    #include <boost/iostreams/operations.hpp>  // get

    namespace boost { namespace iostreams { namespace example {

    class shell_comments_input_filter : public input_filter {
    public:
        explicit shell_comments_input_filter(char comment_char = '#')
            : comment_char_(comment_char), skip_(false)
            { }

        template<typename Source>
        int get(Source& src)
        {
            int c;
            while (true) {
                if ((c = boost::iostreams::get(src)) == EOF || c == WOULD_BLOCK)
                    break;
                skip_ = c == comment_char_ ?
                    true :
                    c == '\n' ?
                        false :
                        skip_;
                if (!skip_)
                    break;
            }
            return c;
        }

        template<typename Source>
        void close(Source&) { skip_ = false; }
    private:
        char comment_char_;
        bool skip_;
    };

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

Here the member variable `skip_` plays the same role as the local variable `skip` `shell_comments_stdio_filter::do_filter`. The implementation of `get` is very similar to that of `shell_comments_stdio_filter::do_filter`: the `while` loop reads a character `c`, updates `skip_` and returns `c` unless `skip_` is `true`. The main difference is that you have to handle the special value `WOULD_BLOCK`, which indicates that no input is currently available.

So you see that implementing an `InputFilter` from scratch is a bit more involved than deriving from `stdio_filter`. When writing an `InputFilter` you must be prepared to be interrupted at any point in the middle of the algorithm; when this happens, you must record enough information about the current state of the algorithm to allow you to pick up later exactly where you left off. The same is true for `OutputFilters`. In fact, many Inputfilters and OutputFilters can be seen as finite state machines; I will formalize this idea later. See Finite State Filters.

There's still one problem with `shell_comments_input_filter`: its instances can only be used once. That's because someone might close a stream while the `skip_` flag is set. If the stream were later reopened — with a fresh sequence of unfiltered data — the first line of text would be filtered out, regardless of whether it were commented.

The way to fix this is to make your Filter Closable. To do this, you must implement a member function `close`. You must also give your filter a category tag convertible to `closable_tag`, to tell the Iostream library that your filter implements `close`.

The improved Filter looks like this:

    namespace boost { namespace iostreams { namespace example {

    class shell_comments_input_filter : public input_filter {
    public:
        shell_comments_input_filter();

        template<typename Source>
        int get(Source& src);

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

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

Here I've derived from the helper class `input_filter`, which provides a member type `char_type` equal to `char` and a category tag convertible to `input_filter_tag` and to `closable_tag`. The implementation of `close` simply clears the `skip_` flag so that the Filter will be ready to be used again.

#### `shell_comments_output_filter`

Next, let's express a shell comments Filter as an OutputFilter. A typical narrow-character OutputFilter looks like this:

    #include <boost/iostreams/categories.hpp>
    #include <boost/iostreams/operations.hpp>  // put, write

    namespace io = boost::iostreams;

    class my_output_filter {
    public:
        typedef char               char_type;
        typedef output_filter_tag  category;

        template<typename Sink>
        bool put(Sink& dest, int c)
        {
            // Attempt to consume the given character of unfiltered
            // data, writing filtered data to dest as appropriate.
            // Return true if the character was successfully consumed.
        }

        /* Other members */
    };

The function `put` attempts to filter the single character `c`, writing filtered output to the Sink `dest`. It accesses `dest` using the fundamental i/o operations `put` and `write`. Both of these functions may fail: `iostreams::put` can return `false`, and `iostreams::write` can consume fewer characters than requested. If this occurs, the member function `put` is allowed to return `false`, indicating that `c` could not be consumed. Otherwise, it must consume `c` and return `true`.

You could also write the above example as follows:

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

    class my_output_filter : public output_filter {
    public:
        template<typename Sink>
        bool put(Sink& dest, int c);

        /* Other members */
    };

Here `output_filter` is a convenience base class which provides the member types `char_type` and `category`, as well as no-op implementations of member functions `close` and `imbue`.

You're now ready to express a shell comments Filter as an OutputFilter:

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

    namespace boost { namespace iostreams { namespace example {

    class shell_comments_output_filter : public output_filter {
    public:
        explicit shell_comments_output_filter(char comment_char = '#')
            : comment_char_(comment_char), skip_(false)
            { }

        template<typename Sink>
        bool put(Sink& dest, int c)
        {
            skip_ = c == comment_char_ ?
                true :
                c == '\n' ?
                    false :
                    skip_;

            if (skip_)
                return true;

            return iostreams::put(dest, c);
        }

        template<typename Source>
        void close(Source&) { skip_ = false; }
    private:
        char comment_char_;
        bool skip_;
    };

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

The member function `put` first examines the given character `c` and updates the member variable `skip_`; next, unless `skip_` is `true`, it attempt to write c. The member function `close` simply clears the `skip_` flag so that the Filter will be ready to be used again.

© 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)