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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 13. Input and Output</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="std_contents.html" title="Part II. Standard Contents" /><link rel="prev" href="numerics_and_c.html" title="Interacting with C" /><link rel="next" href="streambufs.html" title="Stream Buffers" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 13.
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Input and Output
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</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="numerics_and_c.html">Prev</a> </td><th width="60%" align="center">Part II.
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Standard Contents
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</th><td width="20%" align="right"> <a accesskey="n" href="streambufs.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="std.io"></a>Chapter 13.
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Input and Output
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<a id="id-1.3.4.11.1.1.1" class="indexterm"></a>
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</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
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only include the headers you really need. Many people simply include
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<code class="filename"><iostream></code> when they don't
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need to -- and that can <span class="emphasis"><em>penalize your runtime as well.</em></span>
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Here are some tips on which header to use
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for which situations, starting with the simplest.
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</p><p><span class="emphasis"><em><code class="filename"><iosfwd></code></em></span>
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should be included whenever you simply need the <span class="emphasis"><em>name</em></span>
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of an I/O-related class, such as "<code class="classname">ofstream</code>" or
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"<code class="classname">basic_streambuf</code>".
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Like the name implies, these are forward declarations.
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(A word to all you fellow old school programmers:
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trying to forward declare classes like "<code class="code">class istream;</code>"
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won't work.
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Look in the <code class="filename"><iosfwd></code> header
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if you'd like to know why.) For example,
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</p><pre class="programlisting">
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#include <iosfwd>
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class MyClass
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{
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....
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std::ifstream& input_file;
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};
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extern std::ostream& operator<< (std::ostream&, MyClass&);
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</pre><p><span class="emphasis"><em><code class="filename"><ios></code></em></span>
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declares the base classes for the entire I/O stream hierarchy,
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<code class="classname">std::ios_base</code> and <code class="classname">std::basic_ios<charT></code>,
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the counting types <span class="type">std::streamoff</span> and <span class="type">std::streamsize</span>,
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the file positioning type <span class="type">std::fpos</span>,
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and the various manipulators like <code class="function">std::hex</code>,
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<code class="function">std::fixed</code>, <code class="function">std::noshowbase</code>,
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and so forth.
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</p><p>The <code class="classname">ios_base</code> class is what holds the format
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flags, the state flags, and the functions which change them
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(<code class="function">setf()</code>, <code class="function">width()</code>,
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<code class="function">precision()</code>, etc).
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You can also store extra data and register callback functions
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through <code class="classname">ios_base</code>, but that has been historically
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underused. Anything
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which doesn't depend on the type of characters stored is consolidated
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here.
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</p><p>The class template <code class="classname">basic_ios</code> is the highest
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class template in the
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hierarchy; it is the first one depending on the character type, and
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holds all general state associated with that type: the pointer to the
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polymorphic stream buffer, the facet information, etc.
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</p><p><span class="emphasis"><em><code class="filename"><streambuf></code></em></span>
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declares the class template <code class="classname">basic_streambuf</code>, and
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two standard instantiations, <span class="type">streambuf</span> and
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<span class="type">wstreambuf</span>. If you need to work with the vastly useful and
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capable stream buffer classes, e.g., to create a new form of storage
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transport, this header is the one to include.
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</p><p><span class="emphasis"><em><code class="filename"><istream></code></em></span>
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and <span class="emphasis"><em><code class="filename"><ostream></code></em></span>
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are the headers to include when you are using the overloaded
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<code class="code">>></code> and <code class="code"><<</code> operators,
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or any of the other abstract stream formatting functions.
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For example,
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</p><pre class="programlisting">
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#include <istream>
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std::ostream& operator<< (std::ostream& os, MyClass& c)
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{
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return os << c.data1() << c.data2();
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}
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</pre><p>The <span class="type">std::istream</span> and <span class="type">std::ostream</span> classes
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are the abstract parents of
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the various concrete implementations. If you are only using the
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interfaces, then you only need to use the appropriate interface header.
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</p><p><span class="emphasis"><em><code class="filename"><iomanip></code></em></span>
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provides "extractors and inserters that alter information maintained by
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class <code class="classname">ios_base</code> and its derived classes,"
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such as <code class="function">std::setprecision</code> and
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<code class="function">std::setw</code>. If you need
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to write expressions like <code class="code">os << setw(3);</code> or
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<code class="code">is >> setbase(8);</code>, you must include
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<code class="filename"><iomanip></code>.
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</p><p><span class="emphasis"><em><code class="filename"><sstream></code></em></span>
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and <span class="emphasis"><em><code class="filename"><fstream></code></em></span>
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declare the six stringstream and fstream classes. As they are the
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standard concrete descendants of <span class="type">istream</span> and <span class="type">ostream</span>,
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you will already know about them.
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</p><p>Finally, <span class="emphasis"><em><code class="filename"><iostream></code></em></span>
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provides the eight standard global objects
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(<code class="code">cin</code>, <code class="code">cout</code>, etc). To do this correctly, this
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header also provides the contents of the
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<code class="filename"><istream></code> and
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<code class="filename"><ostream></code>
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headers, but nothing else. The contents of this header look like:
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</p><pre class="programlisting">
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#include <ostream>
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#include <istream>
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namespace std
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{
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extern istream cin;
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extern ostream cout;
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....
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// this is explained below
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<span class="emphasis"><em>static ios_base::Init __foo;</em></span> // not its real name
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}
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</pre><p>Now, the runtime penalty mentioned previously: the global objects
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must be initialized before any of your own code uses them; this is
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guaranteed by the standard. Like any other global object, they must
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be initialized once and only once. This is typically done with a
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construct like the one above, and the nested class
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<code class="classname">ios_base::Init</code> is
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specified in the standard for just this reason.
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</p><p>How does it work? Because the header is included before any of your
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code, the <span class="emphasis"><em>__foo</em></span> object is constructed before any of
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your objects. (Global objects are built in the order in which they
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are declared, and destroyed in reverse order.) The first time the
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constructor runs, the eight stream objects are set up.
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</p><p>The <code class="code">static</code> keyword means that each object file compiled
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from a source file containing
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<code class="filename"><iostream></code> will have its own
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private copy of <span class="emphasis"><em>__foo</em></span>. There is no specified order
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of construction across object files (it's one of those pesky NP complete
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problems that make life so interesting), so one copy in each object
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file means that the stream objects are guaranteed to be set up before
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any of your code which uses them could run, thereby meeting the
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requirements of the standard.
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</p><p>The penalty, of course, is that after the first copy of
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<span class="emphasis"><em>__foo</em></span> is constructed, all the others are just wasted
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processor time. The time spent is merely for an increment-and-test
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inside a function call, but over several dozen or hundreds of object
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files, that time can add up. (It's not in a tight loop, either.)
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</p><p>The lesson? Only include
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<code class="filename"><iostream></code> when you need
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to use one of
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the standard objects in that source file; you'll pay less startup
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time. Only include the header files you need to in general; your
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compile times will go down when there's less parsing work to do.
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</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="numerics_and_c.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="std_contents.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">Interacting with C </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Stream Buffers</td></tr></table></div></body></html> |