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