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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. |
| Input and Output |
| |
| </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. |
| Standard Contents |
| </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. |
| Input and Output |
| <a id="id-1.3.4.11.1.1.1" class="indexterm"></a> |
| </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 |
| only include the headers you really need. Many people simply include |
| <code class="filename"><iostream></code> 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><code class="filename"><iosfwd></code></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 "<code class="classname">ofstream</code>" or |
| "<code class="classname">basic_streambuf</code>". |
| Like the name implies, these are forward declarations. |
| (A word to all you fellow old school programmers: |
| trying to forward declare classes like "<code class="code">class istream;</code>" |
| won't work. |
| Look in the <code class="filename"><iosfwd></code> 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><code class="filename"><ios></code></em></span> |
| declares the base classes for the entire I/O stream hierarchy, |
| <code class="classname">std::ios_base</code> and <code class="classname">std::basic_ios<charT></code>, |
| the counting types <span class="type">std::streamoff</span> and <span class="type">std::streamsize</span>, |
| the file positioning type <span class="type">std::fpos</span>, |
| and the various manipulators like <code class="function">std::hex</code>, |
| <code class="function">std::fixed</code>, <code class="function">std::noshowbase</code>, |
| and so forth. |
| </p><p>The <code class="classname">ios_base</code> class is what holds the format |
| flags, the state flags, and the functions which change them |
| (<code class="function">setf()</code>, <code class="function">width()</code>, |
| <code class="function">precision()</code>, etc). |
| You can also store extra data and register callback functions |
| through <code class="classname">ios_base</code>, but that has been historically |
| underused. Anything |
| which doesn't depend on the type of characters stored is consolidated |
| here. |
| </p><p>The class template <code class="classname">basic_ios</code> is the highest |
| class template 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><code class="filename"><streambuf></code></em></span> |
| declares the class template <code class="classname">basic_streambuf</code>, and |
| two standard instantiations, <span class="type">streambuf</span> and |
| <span class="type">wstreambuf</span>. 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><code class="filename"><istream></code></em></span> |
| and <span class="emphasis"><em><code class="filename"><ostream></code></em></span> |
| are the headers to include when you are using the overloaded |
| <code class="code">>></code> and <code class="code"><<</code> operators, |
| 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 <span class="type">std::istream</span> and <span class="type">std::ostream</span> 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><code class="filename"><iomanip></code></em></span> |
| provides "extractors and inserters that alter information maintained by |
| class <code class="classname">ios_base</code> and its derived classes," |
| such as <code class="function">std::setprecision</code> and |
| <code class="function">std::setw</code>. 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 |
| <code class="filename"><iomanip></code>. |
| </p><p><span class="emphasis"><em><code class="filename"><sstream></code></em></span> |
| and <span class="emphasis"><em><code class="filename"><fstream></code></em></span> |
| declare the six stringstream and fstream classes. As they are the |
| standard concrete descendants of <span class="type">istream</span> and <span class="type">ostream</span>, |
| you will already know about them. |
| </p><p>Finally, <span class="emphasis"><em><code class="filename"><iostream></code></em></span> |
| provides the eight standard global objects |
| (<code class="code">cin</code>, <code class="code">cout</code>, etc). To do this correctly, this |
| header also provides the contents of the |
| <code class="filename"><istream></code> and |
| <code class="filename"><ostream></code> |
| 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 |
| <code class="classname">ios_base::Init</code> 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 |
| <code class="filename"><iostream></code> 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 complete |
| 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 |
| <code class="filename"><iostream></code> 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. |
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