gcc/libstdc++-v3/doc/html/manual/iostream_objects.html - Issue 3050029: [gcc] GCC 4.5.0=>4.5.1

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Issue 3050029: [gcc] GCC 4.5.0=>4.5.1 (Closed) Base URL: ssh://git@gitrw.chromium.org:9222/nacl-toolchain.git

Patch Set: Created 10 years, 5 months ago

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-<?xml version="1.0" encoding="UTF-8" standalone="no"?>

-<!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 24. Iostream Objects</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content="
 ISO C++
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 " /><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>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 penalize

- your runtime as well. Here are some tips on which header to use

- for which situations, starting with the simplest.

- <iosfwd> should be included whenever you simply

- need the name 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,

- <pre class="programlisting">

- #include <iosfwd>

- class MyClass

- {

- ....

- std::ifstream& input_file;

- };

- extern std::ostream& operator<< (std::ostream&, MyClass&);

- </pre><ios> 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.

- 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.

- 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.

- <streambuf> 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.

- <istream>/<ostream> are

- the headers to include when you are using the >>/<<

- interface, or any of the other abstract stream formatting functions.

- For example,

- <pre class="programlisting">

- #include <istream>

- std::ostream& operator<< (std::ostream& os, MyClass& c)

- {

- return os << c.data1() << c.data2();

- }

- </pre>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.

- <iomanip> 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>.

- <sstream>/<fstream>

- declare the six stringstream and fstream classes. As they are the

- standard concrete descendants of istream and ostream, you will already

- know about them.

- Finally, <iostream> 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

- <pre class="programlisting">

- #include <ostream>

- #include <istream>

- namespace std

- {

- extern istream cin;

- extern ostream cout;

- ....

- // this is explained below

- static ios_base::Init __foo; // not its real name

- }

- </pre>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.

- How does it work? Because the header is included before any of your

- code, the __foo 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.

- 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 __foo. 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.

- The penalty, of course, is that after the first copy of

- __foo 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.)

- 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.

- </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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