blob: 8380683561a666c1076c050fabb2d6cb884bd8a9 [file] [log] [blame]
<chapter xmlns="" version="5.0"
xml:id="" xreflabel="Input and Output">
<?dbhtml filename="io.html"?>
Input and Output
<indexterm><primary>Input and Output</primary></indexterm>
<keyword>ISO C++</keyword>
<!-- Sect1 01 : Iostream Objects -->
<section xml:id="" xreflabel="IO Objects"><info><title>Iostream Objects</title></info>
<?dbhtml filename="iostream_objects.html"?>
<para>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
<filename class="headerfile">&lt;iostream&gt;</filename> when they don't
need to -- and that can <emphasis>penalize your runtime as well.</emphasis>
Here are some tips on which header to use
for which situations, starting with the simplest.
<para><emphasis><filename class="headerfile">&lt;iosfwd&gt;</filename></emphasis>
should be included whenever you simply need the <emphasis>name</emphasis>
of an I/O-related class, such as "<classname>ofstream</classname>" or
Like the name implies, these are forward declarations.
(A word to all you fellow old school programmers:
trying to forward declare classes like "<code>class istream;</code>"
won't work.
Look in the <filename class="headerfile">&lt;iosfwd&gt;</filename> header
if you'd like to know why.) For example,
#include &lt;iosfwd&gt;
class MyClass
std::ifstream&amp; input_file;
extern std::ostream&amp; operator&lt;&lt; (std::ostream&amp;, MyClass&amp;);
<para><emphasis><filename class="headerfile">&lt;ios&gt;</filename></emphasis>
declares the base classes for the entire I/O stream hierarchy,
<classname>std::ios_base</classname> and <classname>std::basic_ios&lt;charT&gt;</classname>,
the counting types <type>std::streamoff</type> and <type>std::streamsize</type>,
the file positioning type <type>std::fpos</type>,
and the various manipulators like <function>std::hex</function>,
<function>std::fixed</function>, <function>std::noshowbase</function>,
and so forth.
<para>The <classname>ios_base</classname> class is what holds the format
flags, the state flags, and the functions which change them
(<function>setf()</function>, <function>width()</function>,
<function>precision()</function>, etc).
You can also store extra data and register callback functions
through <classname>ios_base</classname>, but that has been historically
underused. Anything
which doesn't depend on the type of characters stored is consolidated
<para>The class template <classname>basic_ios</classname> is the highest
class template in the
hierarchy; it is the first one depending on the character type, and
holds all general state associated with that type: the pointer to the
polymorphic stream buffer, the facet information, etc.
<para><emphasis><filename class="headerfile">&lt;streambuf&gt;</filename></emphasis>
declares the class template <classname>basic_streambuf</classname>, and
two standard instantiations, <type>streambuf</type> and
<type>wstreambuf</type>. 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.
<para><emphasis><filename class="headerfile">&lt;istream&gt;</filename></emphasis>
and <emphasis><filename class="headerfile">&lt;ostream&gt;</filename></emphasis>
are the headers to include when you are using the overloaded
<code>&gt;&gt;</code> and <code>&lt;&lt;</code> operators,
or any of the other abstract stream formatting functions.
For example,
#include &lt;istream&gt;
std::ostream&amp; operator&lt;&lt; (std::ostream&amp; os, MyClass&amp; c)
return os &lt;&lt; c.data1() &lt;&lt; c.data2();
<para>The <type>std::istream</type> and <type>std::ostream</type> 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.
<para><emphasis><filename class="headerfile">&lt;iomanip&gt;</filename></emphasis>
provides "extractors and inserters that alter information maintained by
class <classname>ios_base</classname> and its derived classes,"
such as <function>std::setprecision</function> and
<function>std::setw</function>. If you need
to write expressions like <code>os &lt;&lt; setw(3);</code> or
<code>is &gt;&gt; setbase(8);</code>, you must include
<filename class="headerfile">&lt;iomanip&gt;</filename>.
<para><emphasis><filename class="headerfile">&lt;sstream&gt;</filename></emphasis>
and <emphasis><filename class="headerfile">&lt;fstream&gt;</filename></emphasis>
declare the six stringstream and fstream classes. As they are the
standard concrete descendants of <type>istream</type> and <type>ostream</type>,
you will already know about them.
<para>Finally, <emphasis><filename class="headerfile">&lt;iostream&gt;</filename></emphasis>
provides the eight standard global objects
(<code>cin</code>, <code>cout</code>, etc). To do this correctly, this
header also provides the contents of the
<filename class="headerfile">&lt;istream&gt;</filename> and
<filename class="headerfile">&lt;ostream&gt;</filename>
headers, but nothing else. The contents of this header look like:
#include &lt;ostream&gt;
#include &lt;istream&gt;
namespace std
extern istream cin;
extern ostream cout;
// this is explained below
<emphasis>static ios_base::Init __foo;</emphasis> // not its real name
<para>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
<classname>ios_base::Init</classname> is
specified in the standard for just this reason.
<para>How does it work? Because the header is included before any of your
code, the <emphasis>__foo</emphasis> 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.
<para>The <code>static</code> keyword means that each object file compiled
from a source file containing
<filename class="headerfile">&lt;iostream&gt;</filename> will have its own
private copy of <emphasis>__foo</emphasis>. There is no specified order
of construction across object files (it's one of those pesky NP complete
problems that make life so interesting), so one copy in each object
file means that the stream objects are guaranteed to be set up before
any of your code which uses them could run, thereby meeting the
requirements of the standard.
<para>The penalty, of course, is that after the first copy of
<emphasis>__foo</emphasis> 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.)
<para>The lesson? Only include
<filename class="headerfile">&lt;iostream&gt;</filename> 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.
<!-- Sect1 02 : Stream Buffers -->
<section xml:id="" xreflabel="Stream Buffers"><info><title>Stream Buffers</title></info>
<?dbhtml filename="streambufs.html"?>
<section xml:id="io.streambuf.derived" xreflabel="Derived streambuf Classes"><info><title>Derived streambuf Classes</title></info>
<para>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:
<link xmlns:xlink="" xlink:href="">Standard C++
IOStreams and Locales</link> by Langer and Kreft, ISBN 0-201-18395-1, and
<link xmlns:xlink="" xlink:href="">The C++ Standard Library</link>
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.
<para>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):
#include &lt;iostream&gt;
#include &lt;streambuf&gt;
#include &lt;locale&gt;
#include &lt;cstdio&gt;
class outbuf : public std::streambuf
/* 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&lt;char&gt;(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(&amp;ob);
out &lt;&lt; "31 hexadecimal: "
&lt;&lt; std::hex &lt;&lt; 31 &lt;&lt; std::endl;
return 0;
<para>Try it yourself! More examples can be found in 3.1.x code, in
<filename>include/ext/*_filebuf.h</filename>, and in the article
<link xmlns:xlink="" xlink:href="">Filtering
by James Kanze.
<section xml:id="io.streambuf.buffering" xreflabel="Buffering"><info><title>Buffering</title></info>
<para>First, are you sure that you understand buffering? Particularly
the fact that C++ may not, in fact, have anything to do with it?
<para>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 <emphasis>that</emphasis> 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.)
<para>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:
output &lt;&lt; "a line of text" &lt;&lt; endl;
output &lt;&lt; some_data_variable &lt;&lt; endl;
output &lt;&lt; "another line of text" &lt;&lt; endl; </programlisting>
<para>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:
output &lt;&lt; "a line of text\n"
&lt;&lt; some_data_variable &lt;&lt; '\n'
&lt;&lt; "another line of text\n"; </programlisting>
<para>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.
<para>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
output &lt;&lt; ...... &lt;&lt; flush; // can use std::flush manipulator
output.flush(); // or call a member fn </programlisting>
<para>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 <emphasis>before any I/O operations at
all</emphasis> have been done (note that opening counts as an I/O operation):
std::ofstream os;
std::ifstream is;
int i;
os &lt;&lt; "this data is written immediately\n";
is &gt;&gt; i; // and this will probably cause a disk read </programlisting>
<para>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).
<para>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.)
<para>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.
<!-- Sect1 03 : Memory-based Streams -->
<section xml:id="" xreflabel="Memory Streams"><info><title>Memory Based Streams</title></info>
<?dbhtml filename="stringstreams.html"?>
<section xml:id="" xreflabel="Compatibility strstream"><info><title>Compatibility With strstream</title></info>
<para>Stringstreams (defined in the header <code>&lt;sstream&gt;</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 Sect1 21 (Strings),
<link linkend="strings.string.Cstring"> describing how to
format strings</link>.
<para>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>&lt;&lt;</code> and
<code>&gt;&gt;</code> functions for your classes now pays off
<emphasis>again!</emphasis> Need to format a string before passing the string
to a function? Send your stuff via <code>&lt;&lt;</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>&gt;&gt;</code>. Have a stringstream and need to
get a copy of the string inside? Just call the <code>str()</code>
member function.
<para>This only works if you've written your
<code>&lt;&lt;</code>/<code>&gt;&gt;</code> functions correctly, though,
and correctly means that they take istreams and ostreams as
parameters, not i<emphasis>f</emphasis>streams and o<emphasis>f</emphasis>streams. If they
take the latter, then your I/O operators will work fine with
file streams, but with nothing else -- including stringstreams.
<para>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.
<!-- Sect1 04 : File-based Streams -->
<section xml:id="" xreflabel="File Streams"><info><title>File Based Streams</title></info>
<?dbhtml filename="fstreams.html"?>
<section xml:id="" xreflabel="Copying a File"><info><title>Copying a File</title></info>
<para>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):
#include &lt;fstream&gt;
std::ifstream IN ("input_file");
std::ofstream OUT ("output_file"); </programlisting>
<para>Here's the easiest way to get it completely wrong:
OUT &lt;&lt; IN;</programlisting>
<para>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
The quick brown fox jumped over the lazy dog.</programlisting>
<para>surrounded by blank lines. Code it up and try it. The contents
of "output_file" may surprise you.
<para>Seriously, go do it. Get surprised, then come back. It's worth it.
<para>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&lt;&lt;</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.
<para>Why a <emphasis>pointer</emphasis> 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 chapter 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:
OUT &lt;&lt; IN.rdbuf();</programlisting>
<para>So what <emphasis>was</emphasis> happening with OUT&lt;&lt;IN? Undefined
behavior, since that particular &lt;&lt; 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>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
<section xml:id="" xreflabel="Binary Input and Output"><info><title>Binary Input and Output</title></info>
<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>&lt;&lt;/&gt;&gt;</code>
operators of the normal fstreams to do binary I/O.
<para>Sorry. Them's the breaks.
<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>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>Second, using <code>&lt;&lt;</code> to write and <code>&gt;&gt;</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>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>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><quote>Derive your own fstream-type classes and write your own
&lt;&lt;/&gt;&gt; operators to do binary I/O on whatever data
types you're using.</quote>
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.
<quote>Build the file structure in memory, then
<code>mmap()</code> the file and copy the
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...
<quote>Use streambufs, that's what they're there for.</quote>
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>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>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>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>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
<link xmlns:xlink="" xlink:href="!topic/comp.std.c++/D4e0q9eVSoc">this post</link>
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>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.
<!-- Sect1 03 : Interacting with C -->
<section xml:id="" xreflabel="Interacting with C"><info><title>Interacting with C</title></info>
<?dbhtml filename="io_and_c.html"?>
<section xml:id="" xreflabel="Using FILE* and file descriptors"><info><title>Using FILE* and file descriptors</title></info>
See the <link linkend="">extensions</link> for using
<type>FILE</type> and <type>file descriptors</type> with
<classname>ofstream</classname> and
<section xml:id="" xreflabel="Performance Issues"><info><title>Performance</title></info>
Pathetic Performance? Ditch C.
<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>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:
#include &lt;iostream&gt;
#include &lt;cstdio&gt;
std::cout &lt;&lt; "Hel";
std::printf ("lo, worl");
std::cout &lt;&lt; "d!\n";
<para>This must do what you think it does.
<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>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>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
#include <emphasis>any of the I/O headers such as ios, iostream, etc</emphasis>
<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>Note, by the way, that the synchronization requirement only applies to
the standard streams (<code>cin</code>, <code>cout</code>,
<code>clog</code>, and their wide-character counterparts). File stream
objects that you declare yourself have no such requirement and are fully