[ceph.git] / ceph / src / boost / libs / iostreams / doc / tutorial / tab_expanding_filters.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML>
<HEAD>
    <TITLE>Tutorial</TITLE>
    <LINK REL="stylesheet" HREF="../../../../boost.css">
    <LINK REL="stylesheet" HREF="../theme/iostreams.css">
</HEAD>
<BODY>

<!-- Begin Banner -->

    <H1 CLASS="title">Tutorial</H1>
    <HR CLASS="banner">

<!-- End Banner -->

<!-- Begin Nav -->

<DIV CLASS='nav'>
    <A HREF='line_wrapping_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A>
    <A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A>
    <A HREF='dictionary_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A>
</DIV>

<!-- End Nav -->

<A NAME="tab_expanding"></A>
<H2>2.2.5. Tab-Expanding Filters</H2>

<P>
    Suppose you want to write a filter which replaces each tab character with one or more space characters in such a way that the document appears unchanged when displayed. The basic algorithm is as follows: You examine characters one at a time, forwarding them <I>as-is</I> and keeping track of the current column number. When you encounter a tab character, you replace it with a sequence of space characters whose length depends on the current column count. When you encounter a newline character, you forward it and reset the column count.
</P>

<P>
    In the next three sections, I'll express this algorithm as a <A HREF="../classes/stdio_filter.html"><CODE>stdio_filter</CODE></A>, an <A HREF="../concepts/input_filter.html">InputFilter</A> and an <A HREF="../concepts/output_filter.html">OutputFilter</A>. The source code can be found in the header <A HREF="../../example/tab_expanding_filter.hpp"><CODE>&lt;libs/iostreams/example/tab_expanding_filter.hpp&gt;</CODE></A>. These examples were inspired by James Kanze's <CODE>ExpandTabsInserter.hh</CODE> (<I>see</I> <A CLASS="bib_ref" HREF="../bibliography.html#kanze">[Kanze]</A>).
</P>

<A NAME="tab_expanding_stdio_filter"></A>
<H4><CODE>tab_expanding_stdio_filter</CODE></H4>

<P>You can express a tab-expanding Filter as a <A HREF="../classes/stdio_filter.html"><CODE>stdio_filter</CODE></A> as follows:</P>

<PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal">&lt;cstdio&gt;</SPAN>    <SPAN CLASS="comment">// EOF</SPAN>
<SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal">&lt;iostream&gt;</SPAN>  <SPAN CLASS="comment">// cin, cout</SPAN>
<SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/filter/stdio.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/filter/stdio.hpp&gt;</SPAN></A>

<SPAN CLASS="keyword">class</SPAN> tab_expanding_stdio_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> stdio_filter {
<SPAN CLASS="keyword">public</SPAN>:
    explicit tab_expanding_stdio_filter(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size = 8)
        : tab_size_(tab_size), col_no_(0)
    {
        assert(tab_size &gt; 0);
    }
<SPAN CLASS="keyword">private</SPAN>:
    <SPAN CLASS="keyword">void</SPAN> do_filter();
    <SPAN CLASS="keyword">void</SPAN> do_close();
    <SPAN CLASS="keyword">void</SPAN> put_char(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN>  tab_size_;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN>  col_no_;
};

} } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>

<P>The helper function <CODE>put_char</CODE> is identical to <A HREF="line_wrapping_filters.html#line_wrapping_stdio_filter"><CODE>line_wrapping_stdio_filter::put_char</CODE></A>. It writes a character to <CODE>std::cout</CODE> and updates the column count:</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword">void</SPAN> put_char(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
    {
        std::cout.put(c);
        <SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="literal">'\n'</SPAN>) {
            col_no_ = 0;
        } <SPAN CLASS="keyword">else</SPAN> {
            ++col_no_;
        }
    }</PRE>

<P>Using <CODE>put_char</CODE> you can implement <CODE>do_filter</CODE> as follows:</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword">void</SPAN> do_filter()
    {
        <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c;
        <SPAN CLASS="keyword">while</SPAN> ((c = std::cin.get()) != <SPAN CLASS="numeric_literal">EOF</SPAN>) {
            <SPAN CLASS="keyword">if</SPAN> (c == '\t') {
                <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> spaces = tab_size_ - (col_no_ % tab_size_);
                for (; spaces &gt; 0; --spaces)
                    put_char(' ');
            } <SPAN CLASS="keyword">else</SPAN> {
                put_char(c);
            }
        }
    }</PRE>

<P>The <CODE>while</CODE> loop reads a character from <CODE>std::cin</CODE> and writes it to <CODE>std::cout</CODE>, unless it is a tab character, in which case it writes an appropriate number of space characters to <CODE>std::cout</CODE>.</P>

<P>As with <A HREF="line_wrapping_filters.html#line_wrapping_stdio_filter"><CODE>line_wrapping_stdio_filter</CODE></A>, the <CODE>virtual</CODE> function <CODE>do_close</CODE> resets the Filter's state:</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword">void</SPAN> do_close() { col_no_ = 0; }</PRE>

<A NAME="tab_expanding_input_filter"></A>
<H4><CODE>tab_expanding_input_filter</CODE></H4>

<P>You can express a tab-expanding Filter as an <A HREF="../concepts/input_filter.html">InputFilter</A> as follows:</P>

<PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/char_traits.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/char_traits.hpp&gt;</SPAN></A> <SPAN CLASS="comment">// EOF, WOULD_BLOCK</SPAN>
<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/concepts.hpp&gt;</SPAN></A>    <SPAN CLASS="comment">// input_filter</SPAN>
<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/operations.hpp&gt;</SPAN></A>  <SPAN CLASS="comment">// get</SPAN>

<SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {

<SPAN CLASS="keyword">class</SPAN> tab_expanding_input_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> input_filter {
<SPAN CLASS="keyword">public</SPAN>:
    explicit tab_expanding_input_filter(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size = 8)
        : tab_size_(tab_size), col_no_(0), spaces_(0)
    { 
        assert(tab_size &gt; 0); 
    }

    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source&amp; src);

    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
    <SPAN CLASS="keyword">void</SPAN> close(Source&amp;);
<SPAN CLASS="keyword">private</SPAN>:
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get_char(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN>   tab_size_;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN>   col_no_;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN>   spaces_;
};

} } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>

<P>Let's look first at the helper function <CODE>get_char</CODE>:</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get_char(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
    {
        <SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="literal">'\n'</SPAN>) {
            col_no_ = 0;
        } <SPAN CLASS="keyword">else</SPAN> {
            ++col_no_;
        }
        <SPAN CLASS="keyword">return</SPAN> c;
    }</PRE>

<P>This function updates the column count based on the given character <CODE>c</CODE> and returns <CODE>c</CODE>. Using <CODE>get_char</CODE> you can implement <CODE>get</CODE> as follows:</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source&amp; src)
    {
        <SPAN CLASS="keyword">if</SPAN> (spaces_ &gt; 0) {
            --spaces_;
            <SPAN CLASS="keyword">return</SPAN> get_char(' ');
        }

        <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c;
        <SPAN CLASS="keyword">if</SPAN> ((c = iostreams::get(src)) == <SPAN CLASS="numeric_literal">EOF</SPAN> || c == WOULD_BLOCK)
            <SPAN CLASS="keyword">return</SPAN> c;

        <SPAN CLASS="keyword">if</SPAN> (c != '\t')
            <SPAN CLASS="keyword">return</SPAN> get_char(c);

        <SPAN CLASS='comment'>// Found a tab. Call this filter recursively.</SPAN>
        spaces_ = tab_size_ - (col_no_ % tab_size_);
        <SPAN CLASS="keyword">return</SPAN> this-&gt;get(src);
    }</PRE>

<P>
    The implementation is similar to that of <A HREF="line_wrapping_filters.html#line_wrapping_input_filter"><CODE>line_wrapping_input_filter::get</CODE></A>. Since <CODE>get</CODE> can only return a single character at a time, whenever a tab character must be replaced by a sequence of space character, only the first space character can be returned. The rest must be returned by subsequent invocations of <CODE>get</CODE>. The member variable <CODE>spaces_</CODE> is used to store the number of such space characters.
</P>
<P>
    The implementation begins by checking whether any space characters remain to be returned. If so, it decrements <CODE>spaces_</CODE> and returns a space. Otherwise, a character is read from <CODE>src</CODE>. Ordinary characters, as well as the special values <CODE>EOF</CODE> and <CODE>WOULD_BLOCK</CODE>, are returned <I>as-is</I>. When a tab character is encountered, the number of spaces which must be returned by future invocations of get is recorded, and a space character is returned.
</P>

<P>As usual, the function <CODE>close</CODE> resets the Filter's state:</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword">void</SPAN> close(Source&amp;)
    {
        col_no_ = 0;
        spaces_ = 0;
    }</PRE>

<A NAME="tab_expanding_output_filter"></A>
<H4><CODE>tab_expanding_output_filter</CODE></H4>

<P>You can express a tab-expanding Filter as an <A HREF="../concepts/output_filter.html">OutputFilter</A> as follows:</P>

<PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/concepts.hpp&gt;</SPAN></A>    <SPAN CLASS="comment">// output_filter</SPAN>
<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal">&lt;boost/iostreams/operations.hpp&gt;</SPAN></A>  <SPAN CLASS="comment">// put</SPAN>

<SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {

<SPAN CLASS="keyword">class</SPAN> tab_expanding_output_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> output_filter {
<SPAN CLASS="keyword">public</SPAN>:
    explicit tab_expanding_output_filter(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size = 8)
        : tab_size_(tab_size), col_no_(0), spaces_(0)
    { 
        assert(tab_size &gt; 0); 
    }

    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink&amp; dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);

    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
    <SPAN CLASS="keyword">void</SPAN> close(Sink&amp;);
<SPAN CLASS="keyword">private</SPAN>:
    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put_char(Sink&amp; dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN>  tab_size_;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN>  col_no_;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN>  spaces_;
};

} } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>

<P>The implemenation helper function <CODE>put_char</CODE> is the same as that of <A HREF="line_wrapping_filters.html#line_wrapping_output_filter"><CODE>line_wrapping_output_filter::put_char</CODE></A>: it writes the given character to <CODE>std::cout</CODE> and increments the column number, unless the character is a newline, in which case the column number is reset.</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put_char(Sink&amp; dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
    {
        <SPAN CLASS="keyword">if</SPAN> (!iostreams::put(dest, c))
            <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">false</SPAN>;
        <SPAN CLASS="keyword">if</SPAN> (c != <SPAN CLASS="literal">'\n'</SPAN>)
            ++col_no_;
        <SPAN CLASS="keyword">else</SPAN>
            col_no_ = 0;
        <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">true</SPAN>;
    }</PRE>

<P>Using <CODE>put_char</CODE> you can implement <CODE>put</CODE> as follows:</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Sink&gt;
    <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink&amp; dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
    {
        for (; spaces_ &gt; 0; --spaces_)
            <SPAN CLASS="keyword">if</SPAN> (!put_char(dest, ' '))
                <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">false</SPAN>;

        <SPAN CLASS="keyword">if</SPAN> (c == '\t') {
            spaces_ = tab_size_ - (col_no_ % tab_size_) - 1;
            <SPAN CLASS="keyword">return</SPAN> this-&gt;put(dest, ' ');
        } 

        <SPAN CLASS="keyword">return</SPAN> put_char(dest, c);
    }</PRE>

<P>
    The implementation begins by attempting to write any space characters left over from previously encountered tabs. If successful, it examine the given character <CODE>c</CODE>. If <CODE>c</CODE> is not a tab character, it attempts to write it to <CODE>dest</CODE>. Otherwise, it calculates the number of spaces which must be inserted and calls itself recursively. Using recursion here saves us from having to decrement the member variable <CODE>spaces_</CODE> at two different points in the code.
</P>
<P>
    Note that after a tab character is encountered, get will return false until all the associated space characters have been written.
</P>

<P>As usual, the function <CODE>close</CODE> resets the Filter's state:</P>

<PRE class="broken_ie">    <SPAN CLASS="keyword">void</SPAN> close(Source&amp;)
    {
        col_no_ = 0;
        spaces_ = 0;
    }</PRE>

<!-- Begin Nav -->

<DIV CLASS='nav'>
    <A HREF='line_wrapping_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A>
    <A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A>
    <A HREF='dictionary_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A>
</DIV>

<!-- End Nav -->

<!-- Begin Footer -->

<HR>


<P CLASS="copyright">&copy; Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>&copy; Copyright 2004-2007 <a href="http://www.coderage.com/turkanis/" target="_top">Jonathan Turkanis</a></P>
<P CLASS="copyright"> 
    Use, modification, and distribution are subject to the Boost Software License, Version 2.0. (See accompanying file <A HREF="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A> or copy at <A HREF="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>)
</P>
<!-- End Footer -->

</BODY>
Commit	Line	Data
7c673cae FG	1	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
	2	<HTML>
	3	<HEAD>
	4	<TITLE>Tutorial</TITLE>
	5	<LINK REL="stylesheet" HREF="../../../../boost.css">
	6	<LINK REL="stylesheet" HREF="../theme/iostreams.css">
	7	</HEAD>
	8	<BODY>
	9
	10	<!-- Begin Banner -->
	11
	12	<H1 CLASS="title">Tutorial</H1>
	13	<HR CLASS="banner">
	14
	15	<!-- End Banner -->
	16
	17	<!-- Begin Nav -->
	18
	19	<DIV CLASS='nav'>
	20	<A HREF='line_wrapping_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A>
	21	<A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A>
	22	<A HREF='dictionary_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A>
	23	</DIV>
	24
	25	<!-- End Nav -->
	26
	27	<A NAME="tab_expanding"></A>
	28	<H2>2.2.5. Tab-Expanding Filters</H2>
	29
	30	<P>
	31	Suppose you want to write a filter which replaces each tab character with one or more space characters in such a way that the document appears unchanged when displayed. The basic algorithm is as follows: You examine characters one at a time, forwarding them <I>as-is</I> and keeping track of the current column number. When you encounter a tab character, you replace it with a sequence of space characters whose length depends on the current column count. When you encounter a newline character, you forward it and reset the column count.
	32	</P>
	33
	34	<P>
	35	In the next three sections, I'll express this algorithm as a <A HREF="../classes/stdio_filter.html"><CODE>stdio_filter</CODE></A>, an <A HREF="../concepts/input_filter.html">InputFilter</A> and an <A HREF="../concepts/output_filter.html">OutputFilter</A>. The source code can be found in the header <A HREF="../../example/tab_expanding_filter.hpp"><CODE><libs/iostreams/example/tab_expanding_filter.hpp></CODE></A>. These examples were inspired by James Kanze's <CODE>ExpandTabsInserter.hh</CODE> (<I>see</I> <A CLASS="bib_ref" HREF="../bibliography.html#kanze">[Kanze]</A>).
	36	</P>
	37
	38	<A NAME="tab_expanding_stdio_filter"></A>
	39	<H4><CODE>tab_expanding_stdio_filter</CODE></H4>
	40
	41	<P>You can express a tab-expanding Filter as a <A HREF="../classes/stdio_filter.html"><CODE>stdio_filter</CODE></A> as follows:</P>
	42
	43	<PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal"><cstdio></SPAN> <SPAN CLASS="comment">// EOF</SPAN>
	44	<SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><iostream></SPAN> <SPAN CLASS="comment">// cin, cout</SPAN>
	45	<SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/filter/stdio.hpp"><SPAN CLASS="literal"><boost/iostreams/filter/stdio.hpp></SPAN></A>
	46
	47	<SPAN CLASS="keyword">class</SPAN> tab_expanding_stdio_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> stdio_filter {
	48	<SPAN CLASS="keyword">public</SPAN>:
	49	explicit tab_expanding_stdio_filter(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size = 8)
	50	: tab_size_(tab_size), col_no_(0)
	51	{
	52	assert(tab_size > 0);
	53	}
	54	<SPAN CLASS="keyword">private</SPAN>:
	55	<SPAN CLASS="keyword">void</SPAN> do_filter();
	56	<SPAN CLASS="keyword">void</SPAN> do_close();
	57	<SPAN CLASS="keyword">void</SPAN> put_char(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);
	58	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size_;
	59	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> col_no_;
	60	};
	61
	62	} } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
	63
	64	<P>The helper function <CODE>put_char</CODE> is identical to <A HREF="line_wrapping_filters.html#line_wrapping_stdio_filter"><CODE>line_wrapping_stdio_filter::put_char</CODE></A>. It writes a character to <CODE>std::cout</CODE> and updates the column count:</P>
65
66	<PRE class="broken_ie"> <SPAN CLASS="keyword">void</SPAN> put_char(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
67	{
68	std::cout.put(c);
69	<SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="literal">'\n'</SPAN>) {
70	col_no_ = 0;
71	} <SPAN CLASS="keyword">else</SPAN> {
72	++col_no_;
73	}
74	}</PRE>
75
76	<P>Using <CODE>put_char</CODE> you can implement <CODE>do_filter</CODE> as follows:</P>
77
78	<PRE class="broken_ie"> <SPAN CLASS="keyword">void</SPAN> do_filter()
79	{
80	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c;
81	<SPAN CLASS="keyword">while</SPAN> ((c = std::cin.get()) != <SPAN CLASS="numeric_literal">EOF</SPAN>) {
82	<SPAN CLASS="keyword">if</SPAN> (c == '\t') {
83	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> spaces = tab_size_ - (col_no_ % tab_size_);
84	for (; spaces > 0; --spaces)
85	put_char(' ');
86	} <SPAN CLASS="keyword">else</SPAN> {
87	put_char(c);
88	}
89	}
90	}</PRE>
91
92	<P>The <CODE>while</CODE> loop reads a character from <CODE>std::cin</CODE> and writes it to <CODE>std::cout</CODE>, unless it is a tab character, in which case it writes an appropriate number of space characters to <CODE>std::cout</CODE>.</P>
93
94	<P>As with <A HREF="line_wrapping_filters.html#line_wrapping_stdio_filter"><CODE>line_wrapping_stdio_filter</CODE></A>, the <CODE>virtual</CODE> function <CODE>do_close</CODE> resets the Filter's state:</P>
95
96	<PRE class="broken_ie"> <SPAN CLASS="keyword">void</SPAN> do_close() { col_no_ = 0; }</PRE>
97
98	<A NAME="tab_expanding_input_filter"></A>
99	<H4><CODE>tab_expanding_input_filter</CODE></H4>
100
101	<P>You can express a tab-expanding Filter as an <A HREF="../concepts/input_filter.html">InputFilter</A> as follows:</P>
102
103	<PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/char_traits.hpp"><SPAN CLASS="literal"><boost/iostreams/char_traits.hpp></SPAN></A> <SPAN CLASS="comment">// EOF, WOULD_BLOCK</SPAN>
104	<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal"><boost/iostreams/concepts.hpp></SPAN></A> <SPAN CLASS="comment">// input_filter</SPAN>
105	<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal"><boost/iostreams/operations.hpp></SPAN></A> <SPAN CLASS="comment">// get</SPAN>
106
107	<SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {
108
109	<SPAN CLASS="keyword">class</SPAN> tab_expanding_input_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> input_filter {
110	<SPAN CLASS="keyword">public</SPAN>:
111	explicit tab_expanding_input_filter(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size = 8)
112	: tab_size_(tab_size), col_no_(0), spaces_(0)
113	{
114	assert(tab_size > 0);
115	}
116
117	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
118	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source& src);
119
120	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
121	<SPAN CLASS="keyword">void</SPAN> close(Source&);
122	<SPAN CLASS="keyword">private</SPAN>:
123	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get_char(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);
124	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size_;
125	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> col_no_;
126	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> spaces_;
127	};
128
129	} } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
130
131	<P>Let's look first at the helper function <CODE>get_char</CODE>:</P>
132
133	<PRE class="broken_ie"> <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get_char(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
134	{
135	<SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="literal">'\n'</SPAN>) {
136	col_no_ = 0;
137	} <SPAN CLASS="keyword">else</SPAN> {
138	++col_no_;
139	}
140	<SPAN CLASS="keyword">return</SPAN> c;
141	}</PRE>
142
143	<P>This function updates the column count based on the given character <CODE>c</CODE> and returns <CODE>c</CODE>. Using <CODE>get_char</CODE> you can implement <CODE>get</CODE> as follows:</P>
144
145	<PRE class="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
146	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source& src)
147	{
148	<SPAN CLASS="keyword">if</SPAN> (spaces_ > 0) {
149	--spaces_;
150	<SPAN CLASS="keyword">return</SPAN> get_char(' ');
151	}
152
153	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c;
154	<SPAN CLASS="keyword">if</SPAN> ((c = iostreams::get(src)) == <SPAN CLASS="numeric_literal">EOF</SPAN> \|\| c == WOULD_BLOCK)
155	<SPAN CLASS="keyword">return</SPAN> c;
156
157	<SPAN CLASS="keyword">if</SPAN> (c != '\t')
158	<SPAN CLASS="keyword">return</SPAN> get_char(c);
159
160	<SPAN CLASS='comment'>// Found a tab. Call this filter recursively.</SPAN>
161	spaces_ = tab_size_ - (col_no_ % tab_size_);
162	<SPAN CLASS="keyword">return</SPAN> this->get(src);
163	}</PRE>
164
165	<P>
166	The implementation is similar to that of <A HREF="line_wrapping_filters.html#line_wrapping_input_filter"><CODE>line_wrapping_input_filter::get</CODE></A>. Since <CODE>get</CODE> can only return a single character at a time, whenever a tab character must be replaced by a sequence of space character, only the first space character can be returned. The rest must be returned by subsequent invocations of <CODE>get</CODE>. The member variable <CODE>spaces_</CODE> is used to store the number of such space characters.
167	</P>
168	<P>
169	The implementation begins by checking whether any space characters remain to be returned. If so, it decrements <CODE>spaces_</CODE> and returns a space. Otherwise, a character is read from <CODE>src</CODE>. Ordinary characters, as well as the special values <CODE>EOF</CODE> and <CODE>WOULD_BLOCK</CODE>, are returned <I>as-is</I>. When a tab character is encountered, the number of spaces which must be returned by future invocations of get is recorded, and a space character is returned.
170	</P>
171
172	<P>As usual, the function <CODE>close</CODE> resets the Filter's state:</P>
173
174	<PRE class="broken_ie"> <SPAN CLASS="keyword">void</SPAN> close(Source&)
175	{
176	col_no_ = 0;
177	spaces_ = 0;
178	}</PRE>
179
180	<A NAME="tab_expanding_output_filter"></A>
181	<H4><CODE>tab_expanding_output_filter</CODE></H4>
182
183	<P>You can express a tab-expanding Filter as an <A HREF="../concepts/output_filter.html">OutputFilter</A> as follows:</P>
184
185	<PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal"><boost/iostreams/concepts.hpp></SPAN></A> <SPAN CLASS="comment">// output_filter</SPAN>
186	<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal"><boost/iostreams/operations.hpp></SPAN></A> <SPAN CLASS="comment">// put</SPAN>
187
188	<SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {
189
190	<SPAN CLASS="keyword">class</SPAN> tab_expanding_output_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> output_filter {
191	<SPAN CLASS="keyword">public</SPAN>:
192	explicit tab_expanding_output_filter(<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size = 8)
193	: tab_size_(tab_size), col_no_(0), spaces_(0)
194	{
195	assert(tab_size > 0);
196	}
197
198	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
199	<SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink& dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);
200
201	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
202	<SPAN CLASS="keyword">void</SPAN> close(Sink&);
203	<SPAN CLASS="keyword">private</SPAN>:
204	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
205	<SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put_char(Sink& dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);
206	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> tab_size_;
207	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> col_no_;
208	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> spaces_;
209	};
210
211	} } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
212
213	<P>The implemenation helper function <CODE>put_char</CODE> is the same as that of <A HREF="line_wrapping_filters.html#line_wrapping_output_filter"><CODE>line_wrapping_output_filter::put_char</CODE></A>: it writes the given character to <CODE>std::cout</CODE> and increments the column number, unless the character is a newline, in which case the column number is reset.</P>
214
215	<PRE class="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
216	<SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put_char(Sink& dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
217	{
218	<SPAN CLASS="keyword">if</SPAN> (!iostreams::put(dest, c))
219	<SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">false</SPAN>;
220	<SPAN CLASS="keyword">if</SPAN> (c != <SPAN CLASS="literal">'\n'</SPAN>)
221	++col_no_;
222	<SPAN CLASS="keyword">else</SPAN>
223	col_no_ = 0;
224	<SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">true</SPAN>;
225	}</PRE>
226
227	<P>Using <CODE>put_char</CODE> you can implement <CODE>put</CODE> as follows:</P>
228
229	<PRE class="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
230	<SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink& dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
231	{
232	for (; spaces_ > 0; --spaces_)
233	<SPAN CLASS="keyword">if</SPAN> (!put_char(dest, ' '))
234	<SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">false</SPAN>;
235
236	<SPAN CLASS="keyword">if</SPAN> (c == '\t') {
237	spaces_ = tab_size_ - (col_no_ % tab_size_) - 1;
238	<SPAN CLASS="keyword">return</SPAN> this->put(dest, ' ');
239	}
240
241	<SPAN CLASS="keyword">return</SPAN> put_char(dest, c);
242	}</PRE>
243
244	<P>
245	The implementation begins by attempting to write any space characters left over from previously encountered tabs. If successful, it examine the given character <CODE>c</CODE>. If <CODE>c</CODE> is not a tab character, it attempts to write it to <CODE>dest</CODE>. Otherwise, it calculates the number of spaces which must be inserted and calls itself recursively. Using recursion here saves us from having to decrement the member variable <CODE>spaces_</CODE> at two different points in the code.
246	</P>
247	<P>
248	Note that after a tab character is encountered, get will return false until all the associated space characters have been written.
249	</P>
250
251	<P>As usual, the function <CODE>close</CODE> resets the Filter's state:</P>
252
253	<PRE class="broken_ie"> <SPAN CLASS="keyword">void</SPAN> close(Source&)
254	{
255	col_no_ = 0;
256	spaces_ = 0;
257	}</PRE>
258
259	<!-- Begin Nav -->
260
261	<DIV CLASS='nav'>
262	<A HREF='line_wrapping_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A>
263	<A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A>
264	<A HREF='dictionary_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A>
265	</DIV>
266
267	<!-- End Nav -->
268
269	<!-- Begin Footer -->
270
271	<HR>
272
273
274	<P CLASS="copyright">© Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>© Copyright 2004-2007 <a href="http://www.coderage.com/turkanis/" target="_top">Jonathan Turkanis</a></P>
275	<P CLASS="copyright">
276	Use, modification, and distribution are subject to the Boost Software License, Version 2.0. (See accompanying file <A HREF="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A> or copy at <A HREF="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>)
277	</P>
278	<!-- End Footer -->
279
280	</BODY>