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diff --git a/libstdc++-v3/doc/html/27_io/howto.html b/libstdc++-v3/doc/html/27_io/howto.html new file mode 100644 index 00000000000..46d03b34630 --- /dev/null +++ b/libstdc++-v3/doc/html/27_io/howto.html @@ -0,0 +1,779 @@ +<?xml version="1.0" encoding="ISO-8859-1"?> +<!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" xml:lang="en" lang="en"> +<head> + <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> + <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++ HOWTO: Chapter 27: Input/Output</title> +<link rel="StyleSheet" href="../lib3styles.css" type="text/css" /> +<link rel="Start" href="../documentation.html" type="text/html" + title="GNU C++ Standard Library" /> +<link rel="Prev" href="../26_numerics/howto.html" type="text/html" + title="Numerics" /> +<link rel="Next" href="../ext/howto.html" type="text/html" + title="Extensions" /> +<link rel="Copyright" href="../17_intro/license.html" type="text/html" /> +<link rel="Help" href="../faq/index.html" type="text/html" title="F.A.Q." /> +</head> +<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> + <li><a href="#11">Using FILE*s and file descriptors with IOStreams</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 hexadecimal + 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 + descendants. + </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 last 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 (note that opening counts as an I/O operation): + </p> + <pre> + std::ofstream os; + std::ifstream is; + int i; + + os.rdbuf()->pubsetbuf(0,0); + is.rdbuf()->pubsetbuf(0,0); + + os.open("/foo/bar/baz"); + is.open("/qux/quux/quuux"); + ... + 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 does nothing at all for its siblings + <code>stringbuf</code> and <code>strstreambuf</code>, and specifying + anything other than (0,0) has varying effects. + User-defined classes derived from <code>streambuf</code> 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://groups.google.com/groups?oi=djq&selm=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://www.langer.camelot.de/iostreams.html">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>, and on + <a href="http://www.informatik.uni-konstanz.de/~kuehl/c++/iostream/">Dietmar + Kühl's IOStreams page</a>. + </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 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. + </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>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> A + second choice may be 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 implementations 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 derived + 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 /> +<h2><a name="11">Using FILE*s and file descriptors with IOStreams</a></h2> + <!-- referenced by ext/howto.html#2, update link if numbering changes --> + <p>The v2 library included non-standard extensions to construct + <code>std::filebuf</code>s from C stdio types such as + <code>FILE*</code>s and POSIX file descriptors. + Today the recommended way to use stdio types with libstdc++ + IOStreams is via the <code>stdio_filebuf</code> class (see below), + but earlier releases provided slightly different mechanisms. + </p> + <ul> + <li>3.0.x <code>filebuf</code>s have another ctor with this signature: + <br /> + <code>basic_filebuf(__c_file_type*, ios_base::openmode, int_type);</code> + <br />This comes in very handy in a number of places, such as + attaching Unix sockets, pipes, and anything else which uses file + descriptors, into the IOStream buffering classes. The three + arguments are as follows: + <ul> + <li><code>__c_file_type* F </code> + // the __c_file_type typedef usually boils down to stdio's FILE + </li> + <li><code>ios_base::openmode M </code> + // same as all the other uses of openmode + </li> + <li><code>int_type B </code> + // buffer size, defaults to BUFSIZ if not specified + </li> + </ul> + For those wanting to use file descriptors instead of FILE*'s, I + invite you to contemplate the mysteries of C's <code>fdopen()</code>. + </li> + <li>In library snapshot 3.0.95 and later, <code>filebuf</code>s bring + back an old extension: the <code>fd()</code> member function. The + integer returned from this function can be used for whatever file + descriptors can be used for on your platform. Naturally, the + library cannot track what you do on your own with a file descriptor, + so if you perform any I/O directly, don't expect the library to be + aware of it. + </li> + <li>Beginning with 3.1, the extra <code>filebuf</code> constructor and + the <code>fd()</code> function were removed from the standard + filebuf. Instead, <code><ext/stdio_filebuf.h></code> contains + a derived class called + <a href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/class____gnu__cxx_1_1stdio__filebuf.html"><code>__gnu_cxx::stdio_filebuf</code></a>. + This class can be constructed from a C <code>FILE*</code> or a file + descriptor, and provides the <code>fd()</code> function. + </li> + </ul> + <p>If you want to access a <code>filebuf</code>'s file descriptor to + implement file locking (e.g. using the <code>fcntl()</code> system + call) then you might be interested in Henry Suter's + <a href="http://suter.home.cern.ch/suter/RWLock.html">RWLock</a> + class. + </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> + + +</body> +</html> + + |
