libstdc++-v3/doc/html/manual/iostream_objects.html

   1 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
   2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
   3 <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 24. Iostream Objects</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="io.html" title="Part XI.  Input and Output" /><link rel="prev" href="io.html" title="Part XI.  Input and Output" /><link rel="next" href="streambufs.html" title="Chapter 25. Stream Buffers" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 24. Iostream Objects</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="io.html">Prev</a> </td><th width="60%" align="center">Part XI.
   4   Input and Output
   5
   6 </th><td width="20%" align="right"> <a accesskey="n" href="streambufs.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 24. Iostream Objects"><div class="titlepage"><div><div><h2 class="title"><a id="manual.io.objects"></a>Chapter 24. Iostream Objects</h2></div></div></div><p>To minimize the time you have to wait on the compiler, it's good to
   7       only include the headers you really need.  Many people simply include
   8       &lt;iostream&gt; when they don't need to -- and that can <span class="emphasis"><em>penalize
   9       your runtime as well.</em></span>  Here are some tips on which header to use
  10       for which situations, starting with the simplest.
  11    </p><p><span class="emphasis"><em>&lt;iosfwd&gt;</em></span> should be included whenever you simply
  12       need the <span class="emphasis"><em>name</em></span> of an I/O-related class, such as
  13       "ofstream" or "basic_streambuf".  Like the name
  14       implies, these are forward declarations.  (A word to all you fellow
  15       old school programmers:  trying to forward declare classes like
  16       "class istream;" won't work.  Look in the iosfwd header if
  17       you'd like to know why.)  For example,
  18    </p><pre class="programlisting">
  19     #include &lt;iosfwd&gt;
  20
  21     class MyClass
  22     {
  23         ....
  24         std::ifstream&amp;   input_file;
  25     };
  26
  27     extern std::ostream&amp; operator&lt;&lt; (std::ostream&amp;, MyClass&amp;);
  28    </pre><p><span class="emphasis"><em>&lt;ios&gt;</em></span> declares the base classes for the entire
  29       I/O stream hierarchy, std::ios_base and std::basic_ios&lt;charT&gt;, the
  30       counting types std::streamoff and std::streamsize, the file
  31       positioning type std::fpos, and the various manipulators like
  32       std::hex, std::fixed, std::noshowbase, and so forth.
  33    </p><p>The ios_base class is what holds the format flags, the state flags,
  34       and the functions which change them (setf(), width(), precision(),
  35       etc).  You can also store extra data and register callback functions
  36       through ios_base, but that has been historically underused.  Anything
  37       which doesn't depend on the type of characters stored is consolidated
  38       here.
  39    </p><p>The template class basic_ios is the highest template class in the
  40       hierarchy; it is the first one depending on the character type, and
  41       holds all general state associated with that type:  the pointer to the
  42       polymorphic stream buffer, the facet information, etc.
  43    </p><p><span class="emphasis"><em>&lt;streambuf&gt;</em></span> declares the template class
  44       basic_streambuf, and two standard instantiations, streambuf and
  45       wstreambuf.  If you need to work with the vastly useful and capable
  46       stream buffer classes, e.g., to create a new form of storage
  47       transport, this header is the one to include.
  48    </p><p><span class="emphasis"><em>&lt;istream&gt;</em></span>/<span class="emphasis"><em>&lt;ostream&gt;</em></span> are
  49       the headers to include when you are using the &gt;&gt;/&lt;&lt;
  50       interface, or any of the other abstract stream formatting functions.
  51       For example,
  52    </p><pre class="programlisting">
  53     #include &lt;istream&gt;
  54
  55     std::ostream&amp; operator&lt;&lt; (std::ostream&amp; os, MyClass&amp; c)
  56     {
  57        return os &lt;&lt; c.data1() &lt;&lt; c.data2();
  58     }
  59    </pre><p>The std::istream and std::ostream classes are the abstract parents of
  60       the various concrete implementations.  If you are only using the
  61       interfaces, then you only need to use the appropriate interface header.
  62    </p><p><span class="emphasis"><em>&lt;iomanip&gt;</em></span> provides "extractors and inserters
  63       that alter information maintained by class ios_base and its derived
  64       classes," such as std::setprecision and std::setw.  If you need
  65       to write expressions like <code class="code">os &lt;&lt; setw(3);</code> or
  66       <code class="code">is &gt;&gt; setbase(8);</code>, you must include &lt;iomanip&gt;.
  67    </p><p><span class="emphasis"><em>&lt;sstream&gt;</em></span>/<span class="emphasis"><em>&lt;fstream&gt;</em></span>
  68       declare the six stringstream and fstream classes.  As they are the
  69       standard concrete descendants of istream and ostream, you will already
  70       know about them.
  71    </p><p>Finally, <span class="emphasis"><em>&lt;iostream&gt;</em></span> provides the eight standard
  72       global objects (cin, cout, etc).  To do this correctly, this header
  73       also provides the contents of the &lt;istream&gt; and &lt;ostream&gt;
  74       headers, but nothing else.  The contents of this header look like
  75    </p><pre class="programlisting">
  76     #include &lt;ostream&gt;
  77     #include &lt;istream&gt;
  78
  79     namespace std
  80     {
  81         extern istream cin;
  82         extern ostream cout;
  83         ....
  84
  85         // this is explained below
  86         <span class="emphasis"><em>static ios_base::Init __foo;</em></span>    // not its real name
  87     }
  88    </pre><p>Now, the runtime penalty mentioned previously:  the global objects
  89       must be initialized before any of your own code uses them; this is
  90       guaranteed by the standard.  Like any other global object, they must
  91       be initialized once and only once.  This is typically done with a
  92       construct like the one above, and the nested class ios_base::Init is
  93       specified in the standard for just this reason.
  94    </p><p>How does it work?  Because the header is included before any of your
  95       code, the <span class="emphasis"><em>__foo</em></span> object is constructed before any of
  96       your objects.  (Global objects are built in the order in which they
  97       are declared, and destroyed in reverse order.)  The first time the
  98       constructor runs, the eight stream objects are set up.
  99    </p><p>The <code class="code">static</code> keyword means that each object file compiled
 100       from a source file containing &lt;iostream&gt; will have its own
 101       private copy of <span class="emphasis"><em>__foo</em></span>.  There is no specified order
 102       of construction across object files (it's one of those pesky NP
 103       problems that make life so interesting), so one copy in each object
 104       file means that the stream objects are guaranteed to be set up before
 105       any of your code which uses them could run, thereby meeting the
 106       requirements of the standard.
 107    </p><p>The penalty, of course, is that after the first copy of
 108       <span class="emphasis"><em>__foo</em></span> is constructed, all the others are just wasted
 109       processor time.  The time spent is merely for an increment-and-test
 110       inside a function call, but over several dozen or hundreds of object
 111       files, that time can add up.  (It's not in a tight loop, either.)
 112    </p><p>The lesson?  Only include &lt;iostream&gt; when you need to use one of
 113       the standard objects in that source file; you'll pay less startup
 114       time.  Only include the header files you need to in general; your
 115       compile times will go down when there's less parsing work to do.
 116    </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="io.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="io.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="streambufs.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Part XI.
 117   Input and Output
 118
 119  </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 25. Stream Buffers</td></tr></table></div></body></html>