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- <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
- <meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" />
- <meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 27." />
- <meta name="GENERATOR" content="vi and eight fingers" />
- <title>libstdc++-v3 HOWTO: Chapter 27</title>
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-<body>
-
-<h1 class="centered"><a name="top">Chapter 27: Input/Output</a></h1>
-
-<p>Chapter 27 deals with iostreams and all their subcomponents
- and extensions. All <em>kinds</em> of fun stuff.
-</p>
-
-
-<!-- ####################################################### -->
-<hr />
-<h1>Contents</h1>
-<ul>
- <li><a href="#1">Copying a file</a></li>
- <li><a href="#2">The buffering is screwing up my program!</a></li>
- <li><a href="#3">Binary I/O</a></li>
- <li><a href="#5">What is this <sstream>/stringstreams thing?</a></li>
- <li><a href="#6">Deriving a stream buffer</a></li>
- <li><a href="#7">More on binary I/O</a></li>
- <li><a href="#8">Pathetic performance? Ditch C.</a></li>
- <li><a href="#9">Threads and I/O</a></li>
- <li><a href="#10">Which header?</a></li>
-</ul>
-
-<hr />
-
-<!-- ####################################################### -->
-
-<h2><a name="1">Copying a file</a></h2>
- <p>So you want to copy a file quickly and easily, and most important,
- completely portably. And since this is C++, you have an open
- ifstream (call it IN) and an open ofstream (call it OUT):
- </p>
- <pre>
- #include <fstream>
-
- std::ifstream IN ("input_file");
- std::ofstream OUT ("output_file"); </pre>
- <p>Here's the easiest way to get it completely wrong:
- </p>
- <pre>
- OUT << IN;</pre>
- <p>For those of you who don't already know why this doesn't work
- (probably from having done it before), I invite you to quickly
- create a simple text file called "input_file" containing
- the sentence
- </p>
- <pre>
- The quick brown fox jumped over the lazy dog.</pre>
- <p>surrounded by blank lines. Code it up and try it. The contents
- of "output_file" may surprise you.
- </p>
- <p>Seriously, go do it. Get surprised, then come back. It's worth it.
- </p>
- <hr width="60%" />
- <p>The thing to remember is that the <code>basic_[io]stream</code> classes
- handle formatting, nothing else. In particular, they break up on
- whitespace. The actual reading, writing, and storing of data is
- handled by the <code>basic_streambuf</code> family. Fortunately, the
- <code>operator<<</code> is overloaded to take an ostream and
- a pointer-to-streambuf, in order to help with just this kind of
- "dump the data verbatim" situation.
- </p>
- <p>Why a <em>pointer</em> to streambuf and not just a streambuf? Well,
- the [io]streams hold pointers (or references, depending on the
- implementation) to their buffers, not the actual
- buffers. This allows polymorphic behavior on the part of the buffers
- as well as the streams themselves. The pointer is easily retrieved
- using the <code>rdbuf()</code> member function. Therefore, the easiest
- way to copy the file is:
- </p>
- <pre>
- OUT << IN.rdbuf();</pre>
- <p>So what <em>was</em> 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 hexidecimal
- address (quite a big surprise). Others don't compile at all.
- </p>
- <p>Also note that none of this is specific to o<b>*f*</b>streams.
- The operators shown above are all defined in the parent
- basic_ostream class and are therefore available with all possible
- descendents.
- </p>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-<hr />
-<h2><a name="2">The buffering is screwing up my program!</a></h2>
-<!--
- This is not written very well. I need to redo this section.
--->
- <p>First, are you sure that you understand buffering? Particularly
- the fact that C++ may not, in fact, have anything to do with it?
- </p>
- <p>The rules for buffering can be a little odd, but they aren't any
- different from those of C. (Maybe that's why they can be a bit
- odd.) Many people think that writing a newline to an output
- stream automatically flushes the output buffer. This is true only
- when the output stream is, in fact, a terminal and not a file
- or some other device -- and <em>that</em> may not even be true
- since C++ says nothing about files nor terminals. All of that is
- system-dependent. (The "newline-buffer-flushing only occurring
- on terminals" thing is mostly true on Unix systems, though.)
- </p>
- <p>Some people also believe that sending <code>endl</code> down an
- output stream only writes a newline. This is incorrect; after a
- newline is written, the buffer is also flushed. Perhaps this
- is the effect you want when writing to a screen -- get the text
- out as soon as possible, etc -- but the buffering is largely
- wasted when doing this to a file:
- </p>
- <pre>
- output << "a line of text" << endl;
- output << some_data_variable << endl;
- output << "another line of text" << endl; </pre>
- <p>The proper thing to do in this case to just write the data out
- and let the libraries and the system worry about the buffering.
- If you need a newline, just write a newline:
- </p>
- <pre>
- output << "a line of text\n"
- << some_data_variable << '\n'
- << "another line of text\n"; </pre>
- <p>I have also joined the output statements into a single statement.
- You could make the code prettier by moving the single newline to
- the start of the quoted text on the thing line, for example.
- </p>
- <p>If you do need to flush the buffer above, you can send an
- <code>endl</code> if you also need a newline, or just flush the buffer
- yourself:
- </p>
- <pre>
- output << ...... << flush; // can use std::flush manipulator
- output.flush(); // or call a member fn </pre>
- <p>On the other hand, there are times when writing to a file should
- be like writing to standard error; no buffering should be done
- because the data needs to appear quickly (a prime example is a
- log file for security-related information). The way to do this is
- just to turn off the buffering <em>before any I/O operations at
- all</em> have been done, i.e., as soon as possible after opening:
- </p>
- <pre>
- std::ofstream os ("/foo/bar/baz");
- std::ifstream is ("/qux/quux/quuux");
- int i;
-
- os.rdbuf()->pubsetbuf(0,0);
- is.rdbuf()->pubsetbuf(0,0);
- ...
- os << "this data is written immediately\n";
- is >> i; // and this will probably cause a disk read </pre>
- <p>Since all aspects of buffering are handled by a streambuf-derived
- member, it is necessary to get at that member with <code>rdbuf()</code>.
- Then the public version of <code>setbuf</code> can be called. The
- arguments are the same as those for the Standard C I/O Library
- function (a buffer area followed by its size).
- </p>
- <p>A great deal of this is implementation-dependent. For example,
- <code>streambuf</code> does not specify any actions for its own
- <code>setbuf()</code>-ish functions; the classes derived from
- <code>streambuf</code> each define behavior that "makes
- sense" for that class: an argument of (0,0) turns off buffering
- for <code>filebuf</code> but has undefined behavior for its sibling
- <code>stringbuf</code>, and specifying anything other than (0,0) has
- varying effects. Other user-defined class derived from streambuf can
- do whatever they want. (For <code>filebuf</code> and arguments for
- <code>(p,s)</code> other than zeros, libstdc++ does what you'd expect:
- the first <code>s</code> bytes of <code>p</code> are used as a buffer,
- which you must allocate and deallocate.)
- </p>
- <p>A last reminder: there are usually more buffers involved than
- just those at the language/library level. Kernel buffers, disk
- buffers, and the like will also have an effect. Inspecting and
- changing those are system-dependent.
- </p>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-<hr />
-<h2><a name="3">Binary I/O</a></h2>
- <p>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
- <em>not</em>, 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.
- </p>
- <p>Sorry. Them's the breaks.
- </p>
- <p>This isn't going to try and be a complete tutorial on reading and
- writing binary files (because "binary"
- <a href="#7">covers a lot of ground)</a>, but we will try and clear
- up a couple of misconceptions and common errors.
- </p>
- <p>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 part of the C++ implementation,
- possibly the runtime system).
- </p>
- <p>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 <em>formatting</em>,
- 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.
- </p>
- <p>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.)
- </p>
- <p>Notice how all the problems here are due to the inappropriate use
- of <em>formatting</em> functions and classes to perform something
- which <em>requires</em> that formatting not be done? There are a
- seemingly infinite number of solutions, and a few are listed here:
- </p>
- <ul>
- <li>"Derive your own fstream-type classes and write your own
- <</>> operators to do binary I/O on whatever data
- types you're using." 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.
- </li>
- <li>"Build the file structure in memory, then <code>mmap()</code>
- the file and copy the structure." 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...
- </li>
- <li>"Use streambufs, that's what they're there for."
- 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.
- </li>
- </ul>
- <p>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.
- </p>
- <p>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.
- </p>
- <p>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>.)
- </p>
- <p>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.
- An instructive thread from comp.lang.c++.moderated delved off into
- this topic starting more or less at
- <a href="http://www.deja.com/getdoc.xp?AN=436187505">this</a>
- article and continuing to the end of the thread. (You'll have to
- sort through some flames every couple of paragraphs, but the points
- made are good ones.)
- </p>
-
-<hr />
-<h2><a name="5">What is this <sstream>/stringstreams thing?</a></h2>
- <p>Stringstreams (defined in the header <code><sstream></code>)
- are in this author's opinion one of the coolest things since
- sliced time. An example of their use is in the Received Wisdom
- section for Chapter 21 (Strings),
- <a href="../21_strings/howto.html#1.1internal"> describing how to
- format strings</a>.
- </p>
- <p>The quick definition is: they are siblings of ifstream and ofstream,
- and they do for <code>std::string</code> what their siblings do for
- files. All that work you put into writing <code><<</code> and
- <code>>></code> functions for your classes now pays off
- <em>again!</em> Need to format a string before passing the string
- to a function? Send your stuff via <code><<</code> to an
- ostringstream. You've read a string as input and need to parse it?
- Initialize an istringstream with that string, and then pull pieces
- out of it with <code>>></code>. Have a stringstream and need to
- get a copy of the string inside? Just call the <code>str()</code>
- member function.
- </p>
- <p>This only works if you've written your
- <code><<</code>/<code>>></code> functions correctly, though,
- and correctly means that they take istreams and ostreams as
- parameters, not i<b>f</b>streams and o<b>f</b>streams. If they
- take the latter, then your I/O operators will work fine with
- file streams, but with nothing else -- including stringstreams.
- </p>
- <p>If you are a user of the strstream classes, you need to update
- your code. You don't have to explicitly append <code>ends</code> to
- terminate the C-style character array, you don't have to mess with
- "freezing" functions, and you don't have to manage the
- memory yourself. The strstreams have been officially deprecated,
- which means that 1) future revisions of the C++ Standard won't
- support them, and 2) if you use them, people will laugh at you.
- </p>
-
-<hr />
-<h2><a name="6">Deriving a stream buffer</a></h2>
- <p>Creating your own stream buffers for I/O can be remarkably easy.
- If you are interested in doing so, we highly recommend two very
- excellent books:
- <a href="http://home.camelot.de/langer/iostreams.htm">Standard C++
- IOStreams and Locales</a> by Langer and Kreft, ISBN 0-201-18395-1, and
- <a href="http://www.josuttis.com/libbook/">The C++ Standard Library</a>
- by Nicolai Josuttis, ISBN 0-201-37926-0. Both are published by
- Addison-Wesley, who isn't paying us a cent for saying that, honest.
- </p>
- <p>Here is a simple example, io/outbuf1, from the Josuttis text. It
- transforms everything sent through it to uppercase. This version
- assumes many things about the nature of the character type being
- used (for more information, read the books or the newsgroups):
- </p>
- <pre>
- #include <iostream>
- #include <streambuf>
- #include <locale>
- #include <cstdio>
-
- class outbuf : public std::streambuf
- {
- protected:
- /* central output function
- * - print characters in uppercase mode
- */
- virtual int_type overflow (int_type c) {
- if (c != EOF) {
- // convert lowercase to uppercase
- c = std::toupper(static_cast<char>(c),getloc());
-
- // and write the character to the standard output
- if (putchar(c) == EOF) {
- return EOF;
- }
- }
- return c;
- }
- };
-
- int main()
- {
- // create special output buffer
- outbuf ob;
- // initialize output stream with that output buffer
- std::ostream out(&ob);
-
- out << "31 hexadecimal: "
- << std::hex << 31 << std::endl;
- return 0;
- }
- </pre>
- <p>Try it yourself! More examples can be found in 3.1.x code, in
- <code>include/ext/*_filebuf.h</code>.
- </p>
-
-<hr />
-<h2><a name="7">More on binary I/O</a></h2>
- <p>Towards the beginning of February 2001, the subject of
- "binary" I/O was brought up in a couple of places at the
- same time. One notable place was Usenet, where James Kanze and
- Dietmar Kühl separately posted articles on why attempting
- generic binary I/O was not a good idea. (Here are copies of
- <a href="binary_iostreams_kanze.txt">Kanze's article</a> and
- <a href="binary_iostreams_kuehl.txt">Kühl's article</a>.)
- </p>
- <p>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.
- </p>
- <p>The entire Usenet thread is instructive, and took place under the
- subject heading "binary iostreams" on both comp.std.c++
- and comp.lang.c++.moderated in parallel. Also in that thread,
- Dietmar Kühl mentioned that he had written a pair of stream
- classes that would read and write XDR, which is a good step towards
- a portable binary format.
- </p>
-
-<hr />
-<h2><a name="8">Pathetic performance? Ditch C.</a></h2>
- <p>It sounds like a flame on C, but it isn't. Really. Calm down.
- I'm just saying it to get your attention.
- </p>
- <p>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:
- </p>
- <pre>
- #include <iostream>
- #include <cstdio>
-
- std::cout << "Hel";
- std::printf ("lo, worl");
- std::cout << "d!\n";
- </pre>
- <p>This must do what you think it does.
- </p>
- <p>Alert members of the audience will immediately notice that buffering
- is going to make a hash of the output unless special steps are taken.
- </p>
- <p>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 <a href="../explanations.html#cstdio">tricky to get right</a>.)
- 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.
- </p>
- <p>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
- </p>
- <pre>
- #include <em>any of the I/O headers such as ios, iostream, etc</em>
-
- std::ios::sync_with_stdio(false);
- </pre>
- <p>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.
- </p>
- <p>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.
- </p>
-
-<hr />
-<h2><a name="9">Threads and I/O</a></h2>
- <p>I'll assume that you have already read the
- <a href="../17_intro/howto.html#3">general notes on library threads</a>,
- and the
- <a href="../23_containers/howto.html#3">notes on threaded container
- access</a> (you might not think of an I/O stream as a container, but
- the points made there also hold here). If you have not read them,
- please do so first.
- </p>
- <p>This gets a bit tricky. Please read carefully, and bear with me.
- </p>
- <h3>Structure</h3>
- <p>As described <a href="../explanations.html#cstdio">here</a>, a wrapper
- type called <code>__basic_file</code> provides our abstraction layer
- for the <code>std::filebuf</code> classes. Nearly all decisions dealing
- with actual input and output must be made in <code>__basic_file</code>.
- </p>
- <p>A generic locking mechanism is somewhat in place at the filebuf layer,
- but is not used in the current code. Providing locking at any higher
- level is akin to providing locking within containers, and is not done
- for the same reasons (see the links above).
- </p>
- <h3>The defaults for 3.0.x</h3>
- <p>The __basic_file type is simply a collection of small wrappers around
- the C stdio layer (again, see the link under Structure). We do no
- locking ourselves, but simply pass through to calls to <code>fopen</code>,
- <code>fwrite</code>, and so forth.
- </p>
- <p>So, for 3.0, the question of "is multithreading safe for I/O"
- must be answered with, "is your platform's C library threadsafe
- for I/O?" Some are by default, some are not; many offer multiple
- implementations of the C library with varying tradeoffs of threadsafety
- and efficiency. You, the programmer, are always required to take care
- with multiple threads.
- </p>
- <p>(As an example, the POSIX standard requires that C stdio FILE*
- operations are atomic. POSIX-conforming C libraries (e.g, on Solaris
- and GNU/Linux) have an internal mutex to serialize operations on
- FILE*s. However, you still need to not do stupid things like calling
- <code>fclose(fs)</code> in one thread followed by an access of
- <code>fs</code> in another.)
- </p>
- <p>So, if your platform's C library is threadsafe, then your
- <code>fstream</code> I/O operations will be threadsafe at the lowest
- level. For higher-level operations, such as manipulating the data
- contained in the stream formatting classes (e.g., setting up callbacks
- inside an <code>std::ofstream</code>), you need to guard such accesses
- like any other critical shared resource.
- </p>
- <h3>The future</h3>
- <p>As already mentioned <a href="../explanations.html#cstdio">here</a>, a
- second choice is available for I/O implementations: libio. This is
- disabled by default, and in fact will not currently work due to other
- issues. It will be revisited, however.
- </p>
- <p>The libio code is a subset of the guts of the GNU libc (glibc) I/O
- implementation. When libio is in use, the <code>__basic_file</code>
- type is basically derived from FILE. (The real situation is more
- complex than that... it's derived from an internal type used to
- implement FILE. See libio/libioP.h to see scary things done with
- vtbls.) The result is that there is no "layer" of C stdio
- to go through; the filebuf makes calls directly into the same
- functions used to implement <code>fread</code>, <code>fwrite</code>,
- and so forth, using internal data structures. (And when I say
- "makes calls directly," I mean the function is literally
- replaced by a jump into an internal function. Fast but frightening.
- *grin*)
- </p>
- <p>Also, the libio internal locks are used. This requires pulling in
- large chunks of glibc, such as a pthreads implementation, and is one
- of the issues preventing widespread use of libio as the libstdc++
- cstdio implementation.
- </p>
- <p>But we plan to make this work, at least as an option if not a future
- default. Platforms running a copy of glibc with a recent-enough
- version will see calls from libstdc++ directly into the glibc already
- installed. For other platforms, a copy of the libio subsection will
- be built and included in libstdc++.
- </p>
- <h3>Alternatives</h3>
- <p>Don't forget that other cstdio implemenations are possible. You could
- easily write one to perform your own forms of locking, to solve your
- "interesting" problems.
- </p>
-
-<hr />
-<h2><a name="10">Which header?</a></h2>
- <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 <em>penalize
- your runtime as well.</em> Here are some tips on which header to use
- for which situations, starting with the simplest.
- </p>
- <p><strong><iosfwd></strong> should be included whenever you simply
- need the <em>name</em> 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>
- #include <iosfwd>
-
- class MyClass
- {
- ....
- std::ifstream input_file;
- };
-
- extern std::ostream& operator<< (std::ostream&, MyClass&);
- </pre>
- <p><strong><ios></strong> 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><strong><streambuf></strong> 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><strong><istream></strong>/<strong><ostream></strong> are
- the headers to include when you are using the >>/<<
- interface, or any of the other abstract stream formatting functions.
- For example,
- </p>
- <pre>
- #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><strong><iomanip></strong> provides "extractors and inserters
- that alter information maintained by class ios_base and its dervied
- classes," such as std::setprecision and std::setw. If you need
- to write expressions like <code>os << setw(3);</code> or
- <code>is >> setbase(8);</code>, you must include <iomanip>.
- </p>
- <p><strong><sstream></strong>/<strong><fstream></strong>
- 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, <strong><iostream></strong> 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>
- #include <ostream>
- #include <istream>
-
- namespace std
- {
- extern istream cin;
- extern ostream cout;
- ....
-
- // this is explained below
- <strong>static ios_base::Init __foo;</strong> // 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 <strong>__foo</strong> 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>static</code> keyword means that each object file compiled
- from a source file containing <iostream> will have its own
- private copy of <strong>__foo</strong>. 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
- <strong>__foo</strong> 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>
-
-
-<!-- ####################################################### -->
-
-<hr />
-<p class="fineprint"><em>
-See <a href="../17_intro/license.html">license.html</a> for copying conditions.
-Comments and suggestions are welcome, and may be sent to
-<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
-</em></p>
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