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='dictionary_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='multichar_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="unix2dos"></A> 28<H2>2.2.7. UNIX-to-DOS Filters</H2> 29 30<P>Suppose you want to write a Filter to convert <CODE>UNIX</CODE> line endings to <CODE>DOS</CODE> line-endings. The basic idea is simple: you process the characters in a sequence one at a time, and whenever you encounter the character 31<CODE>'\n'</CODE> you replace it with the two-character sequence <CODE>'\r'</CODE>, <CODE>'\n'</CODE>. In the following sections I'll implement this algorithm as a <CODE>stdio_filter</CODE>, an InputFilter and an OutputFilter. The source code can be found in the header <A HREF="../../example/unix2dos_filter.hpp"><<CODE>libs/iostreams/example/unix2dos_filter.hpp</CODE>></A></P> 32 33<A NAME="unix2dos_stdio_filter"></A> 34<H4><CODE>unix2dos_stdio_filter</CODE></H4> 35 36<P>You can express a <CODE>UNIX</CODE>-to-<CODE>DOS</CODE> Filter as a <CODE>stdio_filter</CODE> by deriving from <CODE>stdio_filter</CODE> and overriding the <CODE>private</CODE> <CODE>virtual</CODE> function do_filter as follows:</P> 37 38<PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS='literal'><cstdio></SPAN> <SPAN CLASS='comment'>// EOF</SPAN> 39<SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS='literal'><iostream></SPAN> <SPAN CLASS='comment'>// cin, cout</SPAN> 40<SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/filter/stdio.hpp"><SPAN CLASS='literal'><boost/iostreams/filter/stdio.hpp></SPAN></A> 41 42<SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example { 43 44<SPAN CLASS="keyword">class</SPAN> unix2dos_stdio_filter : <SPAN CLASS="keyword">public</SPAN> stdio_filter { 45<SPAN CLASS="keyword">private</SPAN>: 46 <SPAN CLASS="keyword">void</SPAN> do_filter() 47 { 48 <SPAN CLASS="keyword">int</SPAN> c; 49 <SPAN CLASS="keyword">while</SPAN> ((c = std::cin.get()) != <SPAN CLASS='numeric_literal'>EOF</SPAN>) { 50 <SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS='literal'>'\n'</SPAN>) 51 std::cout.put(<SPAN CLASS='literal'>'\r'</SPAN>); 52 std::cout.put(c); 53 } 54 } 55}; 56 57} } } <SPAN CLASS='comment'>// End namespace boost::iostreams:example</SPAN></PRE> 58 59<P>The function <CODE>do_filter</CODE> 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 <CODE>'\n'</CODE> it writes <CODE>'\r'</CODE>, <CODE>'\n'</CODE>. 60 61<A NAME="unix2dos_input_filter"></A> 62<H4><CODE>unix2dos_input_filter</CODE></H4> 63 64<P>Now, let's express a <CODE>UNIX</CODE>-to-<CODE>DOS</CODE> Filter as an <A HREF="../concepts/input_filter.html">InputFilter</A>. 65 66<PRE CLASS="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/categories.hpp"><SPAN CLASS='literal'><boost/iostreams/categories.hpp></SPAN></A> <SPAN CLASS='comment'>// input_filter_tag</SPAN> 67<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> 68 69<SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example { 70 71<SPAN CLASS="keyword">class</SPAN> unix2dos_input_filter { 72<SPAN CLASS="keyword">public</SPAN>: 73 <SPAN CLASS='keyword'>typedef</SPAN> <SPAN CLASS='keyword'>char</SPAN> char_type; 74 <SPAN CLASS='keyword'>typedef</SPAN> input_filter_tag category; 75 76 unix2dos_input_filter() : has_linefeed_(<SPAN CLASS='keyword'>false</SPAN>) { } 77 78 <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source> 79 <SPAN CLASS="keyword">int</SPAN> get(Source& src) 80 { 81 <SPAN CLASS='comment'>// Handle unfinished business</SPAN> 82 <SPAN CLASS="keyword">if</SPAN> (has_linefeed_) { 83 has_linefeed_ = <SPAN CLASS="keyword">false</SPAN>; 84 <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="literal">'\n'</SPAN>; 85 } 86 87 <SPAN CLASS='comment'>// Forward all characters except '\n'</SPAN> 88 <SPAN CLASS="keyword">int</SPAN> c; 89 if ((c = iostreams::get(src)) == <SPAN CLASS='literal'>'\n'</SPAN>) { 90 has_linefeed_ = true; 91 <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS='literal'>'\r'</SPAN>; 92 } 93 94 <SPAN CLASS="keyword">return</SPAN> c; 95 } 96 97 <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source> 98 <SPAN CLASS="keyword">void</SPAN> close(Source&); 99<SPAN CLASS="keyword">private</SPAN>: 100 <SPAN CLASS="keyword">bool</SPAN> has_linefeed_; 101}; 102 103} } } <SPAN CLASS='comment'>// End namespace boost::iostreams:example</SPAN></PRE> 104 105<P>The implementation of <CODE>get</CODE> can be described as follows. Most of the time, you simply read a character from <CODE>src</CODE> and return it. The special values <CODE>EOF</CODE> and <CODE>WOULD_BLOCK</CODE> are treated the same way: they are simply forwarded <I>as-is</I>. The exception is when <CODE>iostreams::get</CODE> returns <CODE>'\n'</CODE>. In this case, you return <CODE>'\r'</CODE> instead and make a note to return <CODE>'\n'</CODE> the next time <CODE>get</CODE> is called.</P> 106 107<P>As usual, the member function <CODE>close</CODE> reset's the Filter's state:</P> 108 109<PRE CLASS="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source> 110 <SPAN CLASS="keyword">void</SPAN> close(Source&) { skip_ = <SPAN CLASS="keyword">false</SPAN>; }</PRE> 111 112<A NAME="unix2dos_output_filter"></A> 113<H4><CODE>unix2dos_output_filter</CODE></H4> 114 115<P>You can express a <CODE>UNIX</CODE>-to-<CODE>DOS</CODE> Filter as an <A HREF="../concepts/output_filter.html">OutputFilter</A> as follows:</P> 116 117<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> 118<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> 119 120<SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example { 121 122<SPAN CLASS="keyword">class</SPAN> unix2dos_output_filter : <SPAN CLASS="keyword">public</SPAN> output_filter { 123<SPAN CLASS="keyword">public</SPAN>: 124 unix2dos_output_filter() : has_linefeed_(<SPAN CLASS="keyword">false</SPAN>) { } 125 126 <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink> 127 <SPAN CLASS="keyword">bool</SPAN> put(Sink& dest, <SPAN CLASS="keyword">int</SPAN> c); 128 129 <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink> 130 <SPAN CLASS="keyword">void</SPAN> close(Sink&) { has_linefeed_ = <SPAN CLASS="keyword">false</SPAN>; } 131<SPAN CLASS="keyword">private</SPAN>: 132 <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink> 133 <SPAN CLASS="keyword">bool</SPAN> put_char(Sink& dest, <SPAN CLASS="keyword">int</SPAN> c); 134 135 <SPAN CLASS="keyword">bool</SPAN> has_linefeed_; 136}; 137 138} } } <SPAN CLASS='comment'>// End namespace boost::iostreams:example</SPAN></PRE> 139 140<P> 141 Here I've derived from the helper class <A HREF="../classes/filter.html#synopsis"><CODE>output_filter</CODE></A>, which provides a member type <CODE>char_type</CODE> equal to <CODE>char</CODE> and a category tag convertible to <A HREF="../guide/traits.html#category_tags"><CODE>output_filter_tag</CODE></A> and to <A HREF="../guide/traits.html#category_tags"><CODE>closable_tag</CODE></A>. 142</P> 143 144<P>Let's look first at the helper function <CODE>put_char</CODE>:</P> 145 146<PRE CLASS='broken_ie'> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink> 147 <SPAN CLASS="keyword">bool</SPAN> put_char(Sink& dest, <SPAN CLASS="keyword">int</SPAN> c) 148 { 149 <SPAN CLASS="keyword">bool</SPAN> result; 150 <SPAN CLASS="keyword">if</SPAN> ((result = iostreams::put(dest, c)) == <SPAN CLASS="keyword">true</SPAN>) { 151 has_linefeed_ = 152 c == <SPAN CLASS="literal">'\r'</SPAN> ? 153 <SPAN CLASS="keyword">true</SPAN> : 154 c == <SPAN CLASS="literal">'\n'</SPAN> ? 155 <SPAN CLASS="keyword">false</SPAN> : 156 has_linefeed_; 157 } 158 <SPAN CLASS="keyword">return</SPAN> result; 159 }</PRE> 160 161<P> 162 This function attempts to write a single character to the Sink dest, returning <CODE>true</CODE> for success. If successful, it updates the flag <CODE>has_linefeed_</CODE>, which indicates that an attempt to write a <CODE>DOS</CODE> line ending sequence failed after the first character was written. 163</P> 164 165<P>Using <CODE>put_char</CODE> you can implement <CODE>put</CODE> as follows:</P> 166 167<PRE CLASS='broken_ie'> <SPAN CLASS="keyword">bool</SPAN> put(Sink& dest, <SPAN CLASS="keyword">int</SPAN> c) 168 { 169 <SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="literal">'\n'</SPAN>) 170 <SPAN CLASS="keyword">return</SPAN> has_linefeed_ ? 171 put_char(dest, <SPAN CLASS="literal">'\n'</SPAN>) : 172 put_char(dest, <SPAN CLASS="literal">'\r'</SPAN>) ? 173 this->put(dest, <SPAN CLASS="literal">'\n'</SPAN>) : 174 <SPAN CLASS="keyword">false</SPAN>; 175 <SPAN CLASS="keyword">return</SPAN> iostreams::put(dest, c); 176 }</PRE> 177 178<P>The implementation works like so:</P> 179 180<OL> 181<LI> 182 If you're at the beginning of a <CODE>DOS</CODE> line-ending sequence — that is, if <CODE>c</CODE> is <CODE>'n'</CODE> and <CODE>has_line_feed_</CODE> is <CODE>false</CODE> — you attempt to write <CODE>'\r'</CODE> and then <CODE>'\n'</CODE> to <CODE>dest</CODE>. 183</LI> 184<LI> 185 If you're in the middle of a <CODE>DOS</CODE> line-ending sequence — that is, if <CODE>c</CODE> is <CODE>'n'</CODE> and <CODE>has_line_feed_</CODE> is <CODE>true</CODE> — you attempt to complete it by writing <CODE>'\n'</CODE>. 186</LI> 187<LI> 188 Otherwise, you attempt to write <CODE>c</CODE> to <CODE>dest</CODE>. 189</LI> 190</OL> 191 192<P> 193 There are two subtle points. First, why does <CODE>c == 'n'</CODE> and <CODE>has_line_feed_ == true</CODE> mean that you're in the middle of a <CODE>DOS</CODE> line-ending sequence? Because when you attempt to write <CODE>'\r'</CODE>, <CODE>'\n'</CODE> but only the first character succeeds, you set <CODE>has_line_feed_</CODE> and return <CODE>false</CODE>. This causes the user of the Filter to <I>resend</I> the character <CODE>'\n'</CODE> which triggered the line-ending sequence. Second, note that to write the second character of a line-ending sequence you call <CODE>put</CODE> recursively instead of calling <CODE>put_char</CODE>. 194</P> 195 196<P> 197 Comparing the implementations of <CODE>unix2dos_input_filter</CODE> and <CODE>unix2dos_output_filter</CODE>, 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 <A HREF="../functions/invert.html#invert"><CODE>inverse</CODE></A> to construct an OutputFilter from <CODE>unix2dos_input_filter</CODE>: 198</P> 199 200<PRE CLASS="broken_ie"><SPAN CLASS='literal'>#include</SPAN></SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS='literal'><boost/iostreams/invert.hpp></SPAN></A> <SPAN CLASS='comment'>// inverse</SPAN> 201 202<SPAN CLASS='keyword'>namespace</SPAN> io = boost::iostreams; 203<SPAN CLASS='keyword'>namespace</SPAN> ex = boost::iostreams::example; 204 205<SPAN CLASS="keyword">typedef</SPAN> io::inverse<ex::unix2dos_input_filter> unix2dos_output_filter;</PRE> 206 207<P>Even this is more work than necessary, however, since line-ending conversions can be handled easily with the built-in component <A HREF="../classes/newline_filter.html#newline_filter"><CODE>newline_filter</CODE></A>.</P> 208 209<!-- Begin Nav --> 210 211<DIV CLASS='nav'> 212 <A HREF='dictionary_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/prev.png'></A> 213 <A HREF='tutorial.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/up.png'></A> 214 <A HREF='multichar_filters.html'><IMG BORDER=0 WIDTH=19 HEIGHT=19 SRC='../../../../doc/src/images/next.png'></A> 215</DIV> 216 217<!-- End Nav --> 218 219<!-- Begin Footer --> 220 221<HR> 222 223 224<P CLASS="copyright">© Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>© Copyright 2004-2007 <a href="https://www.boost.org/users/people/jonathan_turkanis.html" target="_top">Jonathan Turkanis</a></P> 225<P CLASS="copyright"> 226 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>) 227</P> 228<!-- End Footer --> 229 230</BODY>
