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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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