mirror of git://gcc.gnu.org/git/gcc.git
672 lines
31 KiB
XML
672 lines
31 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
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xml:id="std.io" xreflabel="Input and Output">
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<?dbhtml filename="io.html"?>
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<info><title>
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Input and Output
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<indexterm><primary>Input and Output</primary></indexterm>
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</title>
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<keywordset>
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<keyword>ISO C++</keyword>
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<keyword>library</keyword>
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</keywordset>
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</info>
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<!-- Sect1 01 : Iostream Objects -->
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<section xml:id="std.io.objects" xreflabel="IO Objects"><info><title>Iostream Objects</title></info>
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<?dbhtml filename="iostream_objects.html"?>
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<para>To minimize the time you have to wait on the compiler, it's good to
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only include the headers you really need. Many people simply include
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<filename class="headerfile"><iostream></filename> when they don't
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need to -- and that can <emphasis>penalize your runtime as well.</emphasis>
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Here are some tips on which header to use
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for which situations, starting with the simplest.
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</para>
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<para><emphasis><filename class="headerfile"><iosfwd></filename></emphasis>
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should be included whenever you simply need the <emphasis>name</emphasis>
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of an I/O-related class, such as "<classname>ofstream</classname>" or
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"<classname>basic_streambuf</classname>".
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Like the name implies, these are forward declarations.
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(A word to all you fellow old school programmers:
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trying to forward declare classes like "<code>class istream;</code>"
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won't work.
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Look in the <filename class="headerfile"><iosfwd></filename> header
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if you'd like to know why.) For example,
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</para>
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<programlisting>
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#include <iosfwd>
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class MyClass
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{
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....
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std::ifstream& input_file;
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};
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extern std::ostream& operator<< (std::ostream&, MyClass&);
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</programlisting>
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<para><emphasis><filename class="headerfile"><ios></filename></emphasis>
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declares the base classes for the entire I/O stream hierarchy,
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<classname>std::ios_base</classname> and <classname>std::basic_ios<charT></classname>,
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the counting types <type>std::streamoff</type> and <type>std::streamsize</type>,
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the file positioning type <type>std::fpos</type>,
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and the various manipulators like <function>std::hex</function>,
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<function>std::fixed</function>, <function>std::noshowbase</function>,
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and so forth.
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</para>
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<para>The <classname>ios_base</classname> class is what holds the format
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flags, the state flags, and the functions which change them
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(<function>setf()</function>, <function>width()</function>,
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<function>precision()</function>, etc).
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You can also store extra data and register callback functions
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through <classname>ios_base</classname>, but that has been historically
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underused. Anything
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which doesn't depend on the type of characters stored is consolidated
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here.
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</para>
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<para>The class template <classname>basic_ios</classname> is the highest
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class template in the
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hierarchy; it is the first one depending on the character type, and
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holds all general state associated with that type: the pointer to the
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polymorphic stream buffer, the facet information, etc.
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</para>
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<para><emphasis><filename class="headerfile"><streambuf></filename></emphasis>
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declares the class template <classname>basic_streambuf</classname>, and
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two standard instantiations, <type>streambuf</type> and
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<type>wstreambuf</type>. If you need to work with the vastly useful and
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capable stream buffer classes, e.g., to create a new form of storage
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transport, this header is the one to include.
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</para>
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<para><emphasis><filename class="headerfile"><istream></filename></emphasis>
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and <emphasis><filename class="headerfile"><ostream></filename></emphasis>
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are the headers to include when you are using the overloaded
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<code>>></code> and <code><<</code> operators,
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or any of the other abstract stream formatting functions.
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For example,
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</para>
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<programlisting>
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#include <istream>
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std::ostream& operator<< (std::ostream& os, MyClass& c)
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{
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return os << c.data1() << c.data2();
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}
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</programlisting>
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<para>The <type>std::istream</type> and <type>std::ostream</type> classes
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are the abstract parents of
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the various concrete implementations. If you are only using the
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interfaces, then you only need to use the appropriate interface header.
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</para>
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<para><emphasis><filename class="headerfile"><iomanip></filename></emphasis>
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provides "extractors and inserters that alter information maintained by
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class <classname>ios_base</classname> and its derived classes,"
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such as <function>std::setprecision</function> and
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<function>std::setw</function>. If you need
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to write expressions like <code>os << setw(3);</code> or
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<code>is >> setbase(8);</code>, you must include
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<filename class="headerfile"><iomanip></filename>.
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</para>
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<para><emphasis><filename class="headerfile"><sstream></filename></emphasis>
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and <emphasis><filename class="headerfile"><fstream></filename></emphasis>
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declare the six stringstream and fstream classes. As they are the
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standard concrete descendants of <type>istream</type> and <type>ostream</type>,
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you will already know about them.
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</para>
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<para>Finally, <emphasis><filename class="headerfile"><iostream></filename></emphasis>
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provides the eight standard global objects
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(<code>cin</code>, <code>cout</code>, etc). To do this correctly, this
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header also provides the contents of the
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<filename class="headerfile"><istream></filename> and
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<filename class="headerfile"><ostream></filename>
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headers, but nothing else. The contents of this header look like:
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</para>
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<programlisting>
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#include <ostream>
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#include <istream>
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namespace std
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{
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extern istream cin;
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extern ostream cout;
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....
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// this is explained below
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<emphasis>static ios_base::Init __foo;</emphasis> // not its real name
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}
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</programlisting>
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<para>Now, the runtime penalty mentioned previously: the global objects
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must be initialized before any of your own code uses them; this is
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guaranteed by the standard. Like any other global object, they must
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be initialized once and only once. This is typically done with a
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construct like the one above, and the nested class
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<classname>ios_base::Init</classname> is
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specified in the standard for just this reason.
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</para>
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<para>How does it work? Because the header is included before any of your
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code, the <emphasis>__foo</emphasis> object is constructed before any of
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your objects. (Global objects are built in the order in which they
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are declared, and destroyed in reverse order.) The first time the
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constructor runs, the eight stream objects are set up.
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</para>
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<para>The <code>static</code> keyword means that each object file compiled
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from a source file containing
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<filename class="headerfile"><iostream></filename> will have its own
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private copy of <emphasis>__foo</emphasis>. There is no specified order
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of construction across object files (it's one of those pesky NP complete
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problems that make life so interesting), so one copy in each object
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file means that the stream objects are guaranteed to be set up before
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any of your code which uses them could run, thereby meeting the
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requirements of the standard.
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</para>
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<para>The penalty, of course, is that after the first copy of
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<emphasis>__foo</emphasis> is constructed, all the others are just wasted
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processor time. The time spent is merely for an increment-and-test
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inside a function call, but over several dozen or hundreds of object
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files, that time can add up. (It's not in a tight loop, either.)
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</para>
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<para>The lesson? Only include
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<filename class="headerfile"><iostream></filename> when you need
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to use one of
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the standard objects in that source file; you'll pay less startup
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time. Only include the header files you need to in general; your
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compile times will go down when there's less parsing work to do.
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</para>
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</section>
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<!-- Sect1 02 : Stream Buffers -->
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<section xml:id="std.io.streambufs" xreflabel="Stream Buffers"><info><title>Stream Buffers</title></info>
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<?dbhtml filename="streambufs.html"?>
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<section xml:id="io.streambuf.derived" xreflabel="Derived streambuf Classes"><info><title>Derived streambuf Classes</title></info>
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<para>
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</para>
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<para>Creating your own stream buffers for I/O can be remarkably easy.
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If you are interested in doing so, we highly recommend two very
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excellent books:
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.angelikalanger.com/iostreams.html">Standard C++
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IOStreams and Locales</link> by Langer and Kreft, ISBN 0-201-18395-1, and
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.josuttis.com/libbook/">The C++ Standard Library</link>
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by Nicolai Josuttis, ISBN 0-201-37926-0. Both are published by
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Addison-Wesley, who isn't paying us a cent for saying that, honest.
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</para>
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<para>Here is a simple example, io/outbuf1, from the Josuttis text. It
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transforms everything sent through it to uppercase. This version
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assumes many things about the nature of the character type being
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used (for more information, read the books or the newsgroups):
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</para>
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<programlisting>
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#include <iostream>
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#include <streambuf>
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#include <locale>
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#include <cstdio>
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class outbuf : public std::streambuf
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{
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protected:
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/* central output function
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* - print characters in uppercase mode
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*/
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virtual int_type overflow (int_type c) {
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if (c != EOF) {
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// convert lowercase to uppercase
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c = std::toupper(static_cast<char>(c),getloc());
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// and write the character to the standard output
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if (putchar(c) == EOF) {
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return EOF;
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}
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}
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return c;
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}
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};
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int main()
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{
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// create special output buffer
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outbuf ob;
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// initialize output stream with that output buffer
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std::ostream out(&ob);
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out << "31 hexadecimal: "
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<< std::hex << 31 << std::endl;
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return 0;
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}
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</programlisting>
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<para>Try it yourself! More examples can be found in 3.1.x code, in
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<filename>include/ext/*_filebuf.h</filename>, and in this article by James Kanze:
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://kanze.james.neuf.fr/articles/fltrsbf1.html">Filtering
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Streambufs</link>.
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</para>
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</section>
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<section xml:id="io.streambuf.buffering" xreflabel="Buffering"><info><title>Buffering</title></info>
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<para>First, are you sure that you understand buffering? Particularly
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the fact that C++ may not, in fact, have anything to do with it?
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</para>
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<para>The rules for buffering can be a little odd, but they aren't any
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different from those of C. (Maybe that's why they can be a bit
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odd.) Many people think that writing a newline to an output
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stream automatically flushes the output buffer. This is true only
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when the output stream is, in fact, a terminal and not a file
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or some other device -- and <emphasis>that</emphasis> may not even be true
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since C++ says nothing about files nor terminals. All of that is
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system-dependent. (The "newline-buffer-flushing only occurring
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on terminals" thing is mostly true on Unix systems, though.)
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</para>
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<para>Some people also believe that sending <code>endl</code> down an
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output stream only writes a newline. This is incorrect; after a
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newline is written, the buffer is also flushed. Perhaps this
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is the effect you want when writing to a screen -- get the text
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out as soon as possible, etc -- but the buffering is largely
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wasted when doing this to a file:
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</para>
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<programlisting>
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output << "a line of text" << endl;
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output << some_data_variable << endl;
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output << "another line of text" << endl; </programlisting>
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<para>The proper thing to do in this case to just write the data out
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and let the libraries and the system worry about the buffering.
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If you need a newline, just write a newline:
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</para>
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<programlisting>
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output << "a line of text\n"
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<< some_data_variable << '\n'
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<< "another line of text\n"; </programlisting>
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<para>I have also joined the output statements into a single statement.
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You could make the code prettier by moving the single newline to
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the start of the quoted text on the last line, for example.
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</para>
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<para>If you do need to flush the buffer above, you can send an
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<code>endl</code> if you also need a newline, or just flush the buffer
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yourself:
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</para>
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<programlisting>
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output << ...... << flush; // can use std::flush manipulator
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output.flush(); // or call a member fn </programlisting>
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<para>On the other hand, there are times when writing to a file should
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be like writing to standard error; no buffering should be done
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because the data needs to appear quickly (a prime example is a
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log file for security-related information). The way to do this is
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just to turn off the buffering <emphasis>before any I/O operations at
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all</emphasis> have been done (note that opening counts as an I/O operation):
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</para>
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<programlisting>
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std::ofstream os;
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std::ifstream is;
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int i;
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os.rdbuf()->pubsetbuf(0,0);
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is.rdbuf()->pubsetbuf(0,0);
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os.open("/foo/bar/baz");
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is.open("/qux/quux/quuux");
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...
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os << "this data is written immediately\n";
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is >> i; // and this will probably cause a disk read </programlisting>
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<para>Since all aspects of buffering are handled by a streambuf-derived
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member, it is necessary to get at that member with <code>rdbuf()</code>.
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Then the public version of <code>setbuf</code> can be called. The
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arguments are the same as those for the Standard C I/O Library
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function (a buffer area followed by its size).
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</para>
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<para>A great deal of this is implementation-dependent. For example,
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<code>streambuf</code> does not specify any actions for its own
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<code>setbuf()</code>-ish functions; the classes derived from
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<code>streambuf</code> each define behavior that "makes
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sense" for that class: an argument of (0,0) turns off buffering
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for <code>filebuf</code> but does nothing at all for its siblings
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<code>stringbuf</code> and <code>strstreambuf</code>, and specifying
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anything other than (0,0) has varying effects.
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User-defined classes derived from <code>streambuf</code> can
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do whatever they want. (For <code>filebuf</code> and arguments for
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<code>(p,s)</code> other than zeros, libstdc++ does what you'd expect:
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the first <code>s</code> bytes of <code>p</code> are used as a buffer,
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which you must allocate and deallocate.)
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</para>
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<para>A last reminder: there are usually more buffers involved than
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just those at the language/library level. Kernel buffers, disk
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buffers, and the like will also have an effect. Inspecting and
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changing those are system-dependent.
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</para>
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</section>
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</section>
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<!-- Sect1 03 : Memory-based Streams -->
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<section xml:id="std.io.memstreams" xreflabel="Memory Streams"><info><title>Memory Based Streams</title></info>
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<?dbhtml filename="stringstreams.html"?>
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<section xml:id="std.io.memstreams.compat" xreflabel="Compatibility strstream"><info><title>Compatibility With strstream</title></info>
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<para>
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</para>
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<para>Stringstreams (defined in the header <code><sstream></code>)
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are in this author's opinion one of the coolest things since
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sliced time. An example of their use is in the Received Wisdom
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section for Sect1 21 (Strings),
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<link linkend="strings.string.Cstring"> describing how to
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format strings</link>.
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</para>
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<para>The quick definition is: they are siblings of ifstream and ofstream,
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and they do for <code>std::string</code> what their siblings do for
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files. All that work you put into writing <code><<</code> and
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<code>>></code> functions for your classes now pays off
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<emphasis>again!</emphasis> Need to format a string before passing the string
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to a function? Send your stuff via <code><<</code> to an
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ostringstream. You've read a string as input and need to parse it?
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Initialize an istringstream with that string, and then pull pieces
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out of it with <code>>></code>. Have a stringstream and need to
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get a copy of the string inside? Just call the <code>str()</code>
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member function.
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</para>
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<para>This only works if you've written your
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<code><<</code>/<code>>></code> functions correctly, though,
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and correctly means that they take istreams and ostreams as
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parameters, not i<emphasis>f</emphasis>streams and o<emphasis>f</emphasis>streams. If they
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take the latter, then your I/O operators will work fine with
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file streams, but with nothing else -- including stringstreams.
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</para>
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<para>If you are a user of the strstream classes, you need to update
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your code. You don't have to explicitly append <code>ends</code> to
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terminate the C-style character array, you don't have to mess with
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"freezing" functions, and you don't have to manage the
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memory yourself. The strstreams have been officially deprecated,
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which means that 1) future revisions of the C++ Standard won't
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support them, and 2) if you use them, people will laugh at you.
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</para>
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</section>
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</section>
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<!-- Sect1 04 : File-based Streams -->
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<section xml:id="std.io.filestreams" xreflabel="File Streams"><info><title>File Based Streams</title></info>
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<?dbhtml filename="fstreams.html"?>
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<section xml:id="std.io.filestreams.copying_a_file" xreflabel="Copying a File"><info><title>Copying a File</title></info>
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<para>
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</para>
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<para>So you want to copy a file quickly and easily, and most important,
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completely portably. And since this is C++, you have an open
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ifstream (call it IN) and an open ofstream (call it OUT):
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</para>
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<programlisting>
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#include <fstream>
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std::ifstream IN ("input_file");
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std::ofstream OUT ("output_file"); </programlisting>
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<para>Here's the easiest way to get it completely wrong:
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</para>
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<programlisting>
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OUT << IN;</programlisting>
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<para>For those of you who don't already know why this doesn't work
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(probably from having done it before), I invite you to quickly
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create a simple text file called "input_file" containing
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the sentence
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</para>
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<programlisting>
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The quick brown fox jumped over the lazy dog.</programlisting>
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<para>surrounded by blank lines. Code it up and try it. The contents
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of "output_file" may surprise you.
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</para>
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<para>Seriously, go do it. Get surprised, then come back. It's worth it.
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</para>
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<para>The thing to remember is that the <code>basic_[io]stream</code> classes
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handle formatting, nothing else. In particular, they break up on
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whitespace. The actual reading, writing, and storing of data is
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handled by the <code>basic_streambuf</code> family. Fortunately, the
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<code>operator<<</code> is overloaded to take an ostream and
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a pointer-to-streambuf, in order to help with just this kind of
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"dump the data verbatim" situation.
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</para>
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<para>Why a <emphasis>pointer</emphasis> to streambuf and not just a streambuf? Well,
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the [io]streams hold pointers (or references, depending on the
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implementation) to their buffers, not the actual
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buffers. This allows polymorphic behavior on the chapter of the buffers
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as well as the streams themselves. The pointer is easily retrieved
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|
using the <code>rdbuf()</code> member function. Therefore, the easiest
|
|
way to copy the file is:
|
|
</para>
|
|
<programlisting>
|
|
OUT << IN.rdbuf();</programlisting>
|
|
<para>So what <emphasis>was</emphasis> happening with OUT<<IN? Undefined
|
|
behavior, since that particular << isn't defined by the Standard.
|
|
I have seen instances where it is implemented, but the character
|
|
extraction process removes all the whitespace, leaving you with no
|
|
blank lines and only "Thequickbrownfox...". With
|
|
libraries that do not define that operator, IN (or one of IN's
|
|
member pointers) sometimes gets converted to a void*, and the output
|
|
file then contains a perfect text representation of a hexadecimal
|
|
address (quite a big surprise). Others don't compile at all.
|
|
</para>
|
|
<para>Also note that none of this is specific to o<emphasis>*f*</emphasis>streams.
|
|
The operators shown above are all defined in the parent
|
|
basic_ostream class and are therefore available with all possible
|
|
descendants.
|
|
</para>
|
|
|
|
</section>
|
|
|
|
<section xml:id="std.io.filestreams.binary" xreflabel="Binary Input and Output"><info><title>Binary Input and Output</title></info>
|
|
|
|
<para>
|
|
</para>
|
|
<para>The first and most important thing to remember about binary I/O is
|
|
that opening a file with <code>ios::binary</code> is not, repeat
|
|
<emphasis>not</emphasis>, the only thing you have to do. It is not a silver
|
|
bullet, and will not allow you to use the <code><</>></code>
|
|
operators of the normal fstreams to do binary I/O.
|
|
</para>
|
|
<para>Sorry. Them's the breaks.
|
|
</para>
|
|
<para>This isn't going to try and be a complete tutorial on reading and
|
|
writing binary files (because "binary"
|
|
covers a lot of ground), but we will try and clear
|
|
up a couple of misconceptions and common errors.
|
|
</para>
|
|
<para>First, <code>ios::binary</code> has exactly one defined effect, no more
|
|
and no less. Normal text mode has to be concerned with the newline
|
|
characters, and the runtime system will translate between (for
|
|
example) '\n' and the appropriate end-of-line sequence (LF on Unix,
|
|
CRLF on DOS, CR on Macintosh, etc). (There are other things that
|
|
normal mode does, but that's the most obvious.) Opening a file in
|
|
binary mode disables this conversion, so reading a CRLF sequence
|
|
under Windows won't accidentally get mapped to a '\n' character, etc.
|
|
Binary mode is not supposed to suddenly give you a bitstream, and
|
|
if it is doing so in your program then you've discovered a bug in
|
|
your vendor's compiler (or some other chapter of the C++ implementation,
|
|
possibly the runtime system).
|
|
</para>
|
|
<para>Second, using <code><<</code> to write and <code>>></code> to
|
|
read isn't going to work with the standard file stream classes, even
|
|
if you use <code>skipws</code> during reading. Why not? Because
|
|
ifstream and ofstream exist for the purpose of <emphasis>formatting</emphasis>,
|
|
not reading and writing. Their job is to interpret the data into
|
|
text characters, and that's exactly what you don't want to happen
|
|
during binary I/O.
|
|
</para>
|
|
<para>Third, using the <code>get()</code> and <code>put()/write()</code> member
|
|
functions still aren't guaranteed to help you. These are
|
|
"unformatted" I/O functions, but still character-based.
|
|
(This may or may not be what you want, see below.)
|
|
</para>
|
|
<para>Notice how all the problems here are due to the inappropriate use
|
|
of <emphasis>formatting</emphasis> functions and classes to perform something
|
|
which <emphasis>requires</emphasis> that formatting not be done? There are a
|
|
seemingly infinite number of solutions, and a few are listed here:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para><quote>Derive your own fstream-type classes and write your own
|
|
<</>> operators to do binary I/O on whatever data
|
|
types you're using.</quote>
|
|
</para>
|
|
<para>
|
|
This is a Bad Thing, because while
|
|
the compiler would probably be just fine with it, other humans
|
|
are going to be confused. The overloaded bitshift operators
|
|
have a well-defined meaning (formatting), and this breaks it.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<quote>Build the file structure in memory, then
|
|
<code>mmap()</code> the file and copy the
|
|
structure.
|
|
</quote>
|
|
</para>
|
|
<para>
|
|
Well, this is easy to make work, and easy to break, and is
|
|
pretty equivalent to using <code>::read()</code> and
|
|
<code>::write()</code> directly, and makes no use of the
|
|
iostream library at all...
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
<quote>Use streambufs, that's what they're there for.</quote>
|
|
</para>
|
|
<para>
|
|
While not trivial for the beginner, this is the best of all
|
|
solutions. The streambuf/filebuf layer is the layer that is
|
|
responsible for actual I/O. If you want to use the C++
|
|
library for binary I/O, this is where you start.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>How to go about using streambufs is a bit beyond the scope of this
|
|
document (at least for now), but while streambufs go a long way,
|
|
they still leave a couple of things up to you, the programmer.
|
|
As an example, byte ordering is completely between you and the
|
|
operating system, and you have to handle it yourself.
|
|
</para>
|
|
<para>Deriving a streambuf or filebuf
|
|
class from the standard ones, one that is specific to your data
|
|
types (or an abstraction thereof) is probably a good idea, and
|
|
lots of examples exist in journals and on Usenet. Using the
|
|
standard filebufs directly (either by declaring your own or by
|
|
using the pointer returned from an fstream's <code>rdbuf()</code>)
|
|
is certainly feasible as well.
|
|
</para>
|
|
<para>One area that causes problems is trying to do bit-by-bit operations
|
|
with filebufs. C++ is no different from C in this respect: I/O
|
|
must be done at the byte level. If you're trying to read or write
|
|
a few bits at a time, you're going about it the wrong way. You
|
|
must read/write an integral number of bytes and then process the
|
|
bytes. (For example, the streambuf functions take and return
|
|
variables of type <code>int_type</code>.)
|
|
</para>
|
|
<para>Another area of problems is opening text files in binary mode.
|
|
Generally, binary mode is intended for binary files, and opening
|
|
text files in binary mode means that you now have to deal with all of
|
|
those end-of-line and end-of-file problems that we mentioned before.
|
|
</para>
|
|
<para>
|
|
An instructive thread from comp.lang.c++.moderated delved off into
|
|
this topic starting more or less at
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://groups.google.com/group/comp.std.c++/browse_thread/thread/f87b4abd7954a87/946a3eb9921e382d?q=comp.std.c%2B%2B+binary+iostream#946a3eb9921e382d">this</link>
|
|
post and continuing to the end of the thread. (The subject heading is "binary iostreams" on both comp.std.c++
|
|
and comp.lang.c++.moderated.) Take special note of the replies by James Kanze and Dietmar Kühl.
|
|
</para>
|
|
<para>Briefly, the problems of byte ordering and type sizes mean that
|
|
the unformatted functions like <code>ostream::put()</code> and
|
|
<code>istream::get()</code> cannot safely be used to communicate
|
|
between arbitrary programs, or across a network, or from one
|
|
invocation of a program to another invocation of the same program
|
|
on a different platform, etc.
|
|
</para>
|
|
</section>
|
|
|
|
</section>
|
|
|
|
<!-- Sect1 03 : Interacting with C -->
|
|
<section xml:id="std.io.c" xreflabel="Interacting with C"><info><title>Interacting with C</title></info>
|
|
<?dbhtml filename="io_and_c.html"?>
|
|
|
|
|
|
|
|
<section xml:id="std.io.c.FILE" xreflabel="Using FILE* and file descriptors"><info><title>Using FILE* and file descriptors</title></info>
|
|
|
|
<para>
|
|
See the <link linkend="manual.ext.io">extensions</link> for using
|
|
<type>FILE</type> and <type>file descriptors</type> with
|
|
<classname>ofstream</classname> and
|
|
<classname>ifstream</classname>.
|
|
</para>
|
|
</section>
|
|
|
|
<section xml:id="std.io.c.sync" xreflabel="Performance Issues"><info><title>Performance</title></info>
|
|
|
|
<para>
|
|
Pathetic Performance? Ditch C.
|
|
</para>
|
|
<para>It sounds like a flame on C, but it isn't. Really. Calm down.
|
|
I'm just saying it to get your attention.
|
|
</para>
|
|
<para>Because the C++ library includes the C library, both C-style and
|
|
C++-style I/O have to work at the same time. For example:
|
|
</para>
|
|
<programlisting>
|
|
#include <iostream>
|
|
#include <cstdio>
|
|
|
|
std::cout << "Hel";
|
|
std::printf ("lo, worl");
|
|
std::cout << "d!\n";
|
|
</programlisting>
|
|
<para>This must do what you think it does.
|
|
</para>
|
|
<para>Alert members of the audience will immediately notice that buffering
|
|
is going to make a hash of the output unless special steps are taken.
|
|
</para>
|
|
<para>The special steps taken by libstdc++, at least for version 3.0,
|
|
involve doing very little buffering for the standard streams, leaving
|
|
most of the buffering to the underlying C library. (This kind of
|
|
thing is tricky to get right.)
|
|
The upside is that correctness is ensured. The downside is that
|
|
writing through <code>cout</code> can quite easily lead to awful
|
|
performance when the C++ I/O library is layered on top of the C I/O
|
|
library (as it is for 3.0 by default). Some patches have been applied
|
|
which improve the situation for 3.1.
|
|
</para>
|
|
<para>However, the C and C++ standard streams only need to be kept in sync
|
|
when both libraries' facilities are in use. If your program only uses
|
|
C++ I/O, then there's no need to sync with the C streams. The right
|
|
thing to do in this case is to call
|
|
</para>
|
|
<programlisting>
|
|
#include <emphasis>any of the I/O headers such as ios, iostream, etc</emphasis>
|
|
|
|
std::ios::sync_with_stdio(false);
|
|
</programlisting>
|
|
<para>You must do this before performing any I/O via the C++ stream objects.
|
|
Once you call this, the C++ streams will operate independently of the
|
|
(unused) C streams. For GCC 3.x, this means that <code>cout</code> and
|
|
company will become fully buffered on their own.
|
|
</para>
|
|
<para>Note, by the way, that the synchronization requirement only applies to
|
|
the standard streams (<code>cin</code>, <code>cout</code>,
|
|
<code>cerr</code>,
|
|
<code>clog</code>, and their wide-character counterparts). File stream
|
|
objects that you declare yourself have no such requirement and are fully
|
|
buffered.
|
|
</para>
|
|
|
|
|
|
</section>
|
|
</section>
|
|
|
|
</chapter>
|