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diff --git a/gcc-4.4.3/libstdc++-v3/doc/html/manual/iostream_objects.html b/gcc-4.4.3/libstdc++-v3/doc/html/manual/iostream_objects.html new file mode 100644 index 000000000..5b6de7060 --- /dev/null +++ b/gcc-4.4.3/libstdc++-v3/doc/html/manual/iostream_objects.html @@ -0,0 +1,119 @@ +<?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++ , 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> |