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   4   Input and Output
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   9   Input and Output
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  11 </h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="io.html#std.io.objects">Iostream Objects</a></span></dt><dt><span class="sect1"><a href="streambufs.html">Stream Buffers</a></span></dt><dd><dl><dt><span class="sect2"><a href="streambufs.html#io.streambuf.derived">Derived streambuf Classes</a></span></dt><dt><span class="sect2"><a href="streambufs.html#io.streambuf.buffering">Buffering</a></span></dt></dl></dd><dt><span class="sect1"><a href="stringstreams.html">Memory Based Streams</a></span></dt><dd><dl><dt><span class="sect2"><a href="stringstreams.html#std.io.memstreams.compat">Compatibility With strstream</a></span></dt></dl></dd><dt><span class="sect1"><a href="fstreams.html">File Based Streams</a></span></dt><dd><dl><dt><span class="sect2"><a href="fstreams.html#std.io.filestreams.copying_a_file">Copying a File</a></span></dt><dt><span class="sect2"><a href="fstreams.html#std.io.filestreams.binary">Binary Input and Output</a></span></dt></dl></dd><dt><span class="sect1"><a href="io_and_c.html">Interacting with C</a></span></dt><dd><dl><dt><span class="sect2"><a href="io_and_c.html#std.io.c.FILE">Using FILE* and file descriptors</a></span></dt><dt><span class="sect2"><a href="io_and_c.html#std.io.c.sync">Performance</a></span></dt></dl></dd></dl></div><div class="sect1" title="Iostream Objects"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.io.objects"></a>Iostream Objects</h2></div></div></div><p>To minimize the time you have to wait on the compiler, it's good to
  12       only include the headers you really need.  Many people simply include
  13       &lt;iostream&gt; when they don't need to -- and that can <span class="emphasis"><em>penalize
  14       your runtime as well.</em></span>  Here are some tips on which header to use
  15       for which situations, starting with the simplest.
  16    </p><p><span class="emphasis"><em>&lt;iosfwd&gt;</em></span> should be included whenever you simply
  17       need the <span class="emphasis"><em>name</em></span> of an I/O-related class, such as
  18       "ofstream" or "basic_streambuf".  Like the name
  19       implies, these are forward declarations.  (A word to all you fellow
  20       old school programmers:  trying to forward declare classes like
  21       "class istream;" won't work.  Look in the iosfwd header if
  22       you'd like to know why.)  For example,
  23    </p><pre class="programlisting">
  24     #include &lt;iosfwd&gt;
  25
  26     class MyClass
  27     {
  28         ....
  29         std::ifstream&amp;   input_file;
  30     };
  31
  32     extern std::ostream&amp; operator&lt;&lt; (std::ostream&amp;, MyClass&amp;);
  33    </pre><p><span class="emphasis"><em>&lt;ios&gt;</em></span> declares the base classes for the entire
  34       I/O stream hierarchy, std::ios_base and std::basic_ios&lt;charT&gt;, the
  35       counting types std::streamoff and std::streamsize, the file
  36       positioning type std::fpos, and the various manipulators like
  37       std::hex, std::fixed, std::noshowbase, and so forth.
  38    </p><p>The ios_base class is what holds the format flags, the state flags,
  39       and the functions which change them (setf(), width(), precision(),
  40       etc).  You can also store extra data and register callback functions
  41       through ios_base, but that has been historically underused.  Anything
  42       which doesn't depend on the type of characters stored is consolidated
  43       here.
  44    </p><p>The template class basic_ios is the highest template class in the
  45       hierarchy; it is the first one depending on the character type, and
  46       holds all general state associated with that type:  the pointer to the
  47       polymorphic stream buffer, the facet information, etc.
  48    </p><p><span class="emphasis"><em>&lt;streambuf&gt;</em></span> declares the template class
  49       basic_streambuf, and two standard instantiations, streambuf and
  50       wstreambuf.  If you need to work with the vastly useful and capable
  51       stream buffer classes, e.g., to create a new form of storage
  52       transport, this header is the one to include.
  53    </p><p><span class="emphasis"><em>&lt;istream&gt;</em></span>/<span class="emphasis"><em>&lt;ostream&gt;</em></span> are
  54       the headers to include when you are using the &gt;&gt;/&lt;&lt;
  55       interface, or any of the other abstract stream formatting functions.
  56       For example,
  57    </p><pre class="programlisting">
  58     #include &lt;istream&gt;
  59
  60     std::ostream&amp; operator&lt;&lt; (std::ostream&amp; os, MyClass&amp; c)
  61     {
  62        return os &lt;&lt; c.data1() &lt;&lt; c.data2();
  63     }
  64    </pre><p>The std::istream and std::ostream classes are the abstract parents of
  65       the various concrete implementations.  If you are only using the
  66       interfaces, then you only need to use the appropriate interface header.
  67    </p><p><span class="emphasis"><em>&lt;iomanip&gt;</em></span> provides "extractors and inserters
  68       that alter information maintained by class ios_base and its derived
  69       classes," such as std::setprecision and std::setw.  If you need
  70       to write expressions like <code class="code">os &lt;&lt; setw(3);</code> or
  71       <code class="code">is &gt;&gt; setbase(8);</code>, you must include &lt;iomanip&gt;.
  72    </p><p><span class="emphasis"><em>&lt;sstream&gt;</em></span>/<span class="emphasis"><em>&lt;fstream&gt;</em></span>
  73       declare the six stringstream and fstream classes.  As they are the
  74       standard concrete descendants of istream and ostream, you will already
  75       know about them.
  76    </p><p>Finally, <span class="emphasis"><em>&lt;iostream&gt;</em></span> provides the eight standard
  77       global objects (cin, cout, etc).  To do this correctly, this header
  78       also provides the contents of the &lt;istream&gt; and &lt;ostream&gt;
  79       headers, but nothing else.  The contents of this header look like
  80    </p><pre class="programlisting">
  81     #include &lt;ostream&gt;
  82     #include &lt;istream&gt;
  83
  84     namespace std
  85     {
  86         extern istream cin;
  87         extern ostream cout;
  88         ....
  89
  90         // this is explained below
  91         <span class="emphasis"><em>static ios_base::Init __foo;</em></span>    // not its real name
  92     }
  93    </pre><p>Now, the runtime penalty mentioned previously:  the global objects
  94       must be initialized before any of your own code uses them; this is
  95       guaranteed by the standard.  Like any other global object, they must
  96       be initialized once and only once.  This is typically done with a
  97       construct like the one above, and the nested class ios_base::Init is
  98       specified in the standard for just this reason.
  99    </p><p>How does it work?  Because the header is included before any of your
 100       code, the <span class="emphasis"><em>__foo</em></span> object is constructed before any of
 101       your objects.  (Global objects are built in the order in which they
 102       are declared, and destroyed in reverse order.)  The first time the
 103       constructor runs, the eight stream objects are set up.
 104    </p><p>The <code class="code">static</code> keyword means that each object file compiled
 105       from a source file containing &lt;iostream&gt; will have its own
 106       private copy of <span class="emphasis"><em>__foo</em></span>.  There is no specified order
 107       of construction across object files (it's one of those pesky NP
 108       problems that make life so interesting), so one copy in each object
 109       file means that the stream objects are guaranteed to be set up before
 110       any of your code which uses them could run, thereby meeting the
 111       requirements of the standard.
 112    </p><p>The penalty, of course, is that after the first copy of
 113       <span class="emphasis"><em>__foo</em></span> is constructed, all the others are just wasted
 114       processor time.  The time spent is merely for an increment-and-test
 115       inside a function call, but over several dozen or hundreds of object
 116       files, that time can add up.  (It's not in a tight loop, either.)
 117    </p><p>The lesson?  Only include &lt;iostream&gt; when you need to use one of
 118       the standard objects in that source file; you'll pay less startup
 119       time.  Only include the header files you need to in general; your
 120       compile times will go down when there's less parsing work to do.
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