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 <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& src);

     <SPAN CLASS="keyword">template</SPAN>&lt;<SPAN CLASS="keyword">typename</SPAN> Source&gt;
     <SPAN CLASS="keyword">void</SPAN> close(Source&);
 <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& 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&)
     {
         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& 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&);
 <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& 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& 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& 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&)
     {
         col_no_ = 0;
         spaces_ = 0;
     }</PRE>

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	<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><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>).
	</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"><cstdio></SPAN> <SPAN CLASS="comment">// EOF</SPAN>
	<SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><iostream></SPAN> <SPAN CLASS="comment">// cin, cout</SPAN>
	<SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/filter/stdio.hpp"><SPAN CLASS="literal"><boost/iostreams/filter/stdio.hpp></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 > 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 > 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"><boost/iostreams/char_traits.hpp></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"><boost/iostreams/concepts.hpp></SPAN></A> <SPAN CLASS="comment">// input_filter</SPAN>
	<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>

	<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 > 0);
	}

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

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
	<SPAN CLASS="keyword">void</SPAN> close(Source&);
	<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><<SPAN CLASS="keyword">typename</SPAN> Source>
	<SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source& src)
	{
	<SPAN CLASS="keyword">if</SPAN> (spaces_ > 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->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&)
	{
	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"><boost/iostreams/concepts.hpp></SPAN></A> <SPAN CLASS="comment">// output_filter</SPAN>
	<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>

	<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 > 0);
	}

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

	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
	<SPAN CLASS="keyword">void</SPAN> close(Sink&);
	<SPAN CLASS="keyword">private</SPAN>:
	<SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
	<SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put_char(Sink& 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><<SPAN CLASS="keyword">typename</SPAN> Sink>
	<SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put_char(Sink& 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><<SPAN CLASS="keyword">typename</SPAN> Sink>
	<SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink& dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
	{
	for (; spaces_ > 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->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&)
	{
	col_no_ = 0;
	spaces_ = 0;
	}</PRE>

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