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+<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 V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><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.
+ Input and Output
+
+</th><td width="20%" align="right"> <a accesskey="n" href="streambufs.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><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
+ only include the headers you really need. Many people simply include
+ <iostream> when they don't need to -- and that can <span class="emphasis"><em>penalize
+ your runtime as well.</em></span> Here are some tips on which header to use
+ for which situations, starting with the simplest.
+ </p><p><span class="emphasis"><em><iosfwd></em></span> should be included whenever you simply
+ need the <span class="emphasis"><em>name</em></span> of an I/O-related class, such as
+ "ofstream" or "basic_streambuf". Like the name
+ implies, these are forward declarations. (A word to all you fellow
+ old school programmers: trying to forward declare classes like
+ "class istream;" won't work. Look in the iosfwd header if
+ you'd like to know why.) For example,
+ </p><pre class="programlisting">
+ #include <iosfwd>
+
+ class MyClass
+ {
+ ....
+ std::ifstream& input_file;
+ };
+
+ extern std::ostream& operator<< (std::ostream&, MyClass&);
+ </pre><p><span class="emphasis"><em><ios></em></span> declares the base classes for the entire
+ I/O stream hierarchy, std::ios_base and std::basic_ios<charT>, the
+ counting types std::streamoff and std::streamsize, the file
+ positioning type std::fpos, and the various manipulators like
+ std::hex, std::fixed, std::noshowbase, and so forth.
+ </p><p>The ios_base class is what holds the format flags, the state flags,
+ and the functions which change them (setf(), width(), precision(),
+ etc). You can also store extra data and register callback functions
+ through ios_base, but that has been historically underused. Anything
+ which doesn't depend on the type of characters stored is consolidated
+ here.
+ </p><p>The template class basic_ios is the highest template class in the
+ hierarchy; it is the first one depending on the character type, and
+ holds all general state associated with that type: the pointer to the
+ polymorphic stream buffer, the facet information, etc.
+ </p><p><span class="emphasis"><em><streambuf></em></span> declares the template class
+ basic_streambuf, and two standard instantiations, streambuf and
+ wstreambuf. If you need to work with the vastly useful and capable
+ stream buffer classes, e.g., to create a new form of storage
+ transport, this header is the one to include.
+ </p><p><span class="emphasis"><em><istream></em></span>/<span class="emphasis"><em><ostream></em></span> are
+ the headers to include when you are using the >>/<<
+ interface, or any of the other abstract stream formatting functions.
+ For example,
+ </p><pre class="programlisting">
+ #include <istream>
+
+ std::ostream& operator<< (std::ostream& os, MyClass& c)
+ {
+ return os << c.data1() << c.data2();
+ }
+ </pre><p>The std::istream and std::ostream classes are the abstract parents of
+ the various concrete implementations. If you are only using the
+ interfaces, then you only need to use the appropriate interface header.
+ </p><p><span class="emphasis"><em><iomanip></em></span> provides "extractors and inserters
+ that alter information maintained by class ios_base and its derived
+ classes," such as std::setprecision and std::setw. If you need
+ to write expressions like <code class="code">os << setw(3);</code> or
+ <code class="code">is >> setbase(8);</code>, you must include <iomanip>.
+ </p><p><span class="emphasis"><em><sstream></em></span>/<span class="emphasis"><em><fstream></em></span>
+ declare the six stringstream and fstream classes. As they are the
+ standard concrete descendants of istream and ostream, you will already
+ know about them.
+ </p><p>Finally, <span class="emphasis"><em><iostream></em></span> provides the eight standard
+ global objects (cin, cout, etc). To do this correctly, this header
+ also provides the contents of the <istream> and <ostream>
+ headers, but nothing else. The contents of this header look like
+ </p><pre class="programlisting">
+ #include <ostream>
+ #include <istream>
+
+ namespace std
+ {
+ extern istream cin;
+ extern ostream cout;
+ ....
+
+ // this is explained below
+ <span class="emphasis"><em>static ios_base::Init __foo;</em></span> // not its real name
+ }
+ </pre><p>Now, the runtime penalty mentioned previously: the global objects
+ must be initialized before any of your own code uses them; this is
+ guaranteed by the standard. Like any other global object, they must
+ be initialized once and only once. This is typically done with a
+ construct like the one above, and the nested class ios_base::Init is
+ specified in the standard for just this reason.
+ </p><p>How does it work? Because the header is included before any of your
+ code, the <span class="emphasis"><em>__foo</em></span> object is constructed before any of
+ your objects. (Global objects are built in the order in which they
+ are declared, and destroyed in reverse order.) The first time the
+ constructor runs, the eight stream objects are set up.
+ </p><p>The <code class="code">static</code> keyword means that each object file compiled
+ from a source file containing <iostream> will have its own
+ private copy of <span class="emphasis"><em>__foo</em></span>. There is no specified order
+ of construction across object files (it's one of those pesky NP
+ problems that make life so interesting), so one copy in each object
+ file means that the stream objects are guaranteed to be set up before
+ any of your code which uses them could run, thereby meeting the
+ requirements of the standard.
+ </p><p>The penalty, of course, is that after the first copy of
+ <span class="emphasis"><em>__foo</em></span> is constructed, all the others are just wasted
+ processor time. The time spent is merely for an increment-and-test
+ inside a function call, but over several dozen or hundreds of object
+ files, that time can add up. (It's not in a tight loop, either.)
+ </p><p>The lesson? Only include <iostream> when you need to use one of
+ the standard objects in that source file; you'll pay less startup
+ time. Only include the header files you need to in general; your
+ compile times will go down when there's less parsing work to do.
+ </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.
+ Input and Output
+
+ </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>
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