gcc/libstdc++-v3/doc/xml/faq.xml

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<book>

<article id="faq" xreflabel="Frequently Asked Questions">
<?dbhtml filename="faq.html"?>
 
<articleinfo>
  <title>Frequently Asked Questions</title>
  <copyright>
    <year>
      2008
    </year>
    <holder>
      <ulink url="http://fsf.org">FSF</ulink>
    </holder>
  </copyright>
</articleinfo>

<!-- FAQ starts here -->
<qandaset>

<!-- General Information -->
<qandadiv id="faq.info" xreflabel="General Information">
<title>General Information</title>

<qandaentry id="faq.what">
  <question id="faq.what.q">
    <para>
      What is libstdc++?
    </para>
  </question>
  <answer id="faq.what.a">
    <para>
     The GNU Standard C++ Library v3 is an ongoing project to
     implement the ISO 14882 Standard C++ library as described in
     chapters 17 through 27 and annex D.  For those who want to see
     exactly how far the project has come, or just want the latest
     bleeding-edge code, the up-to-date source is available over
     anonymous SVN, and can even be browsed over
     the <ulink url="http://gcc.gnu.org/svn.html">web</ulink>.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.why">
  <question id="q-why">
    <para>
      Why should I use libstdc++?
    </para>
  </question>
  <answer id="a-why">
    <para>
    The completion of the ISO C++ standardization gave the C++
    community a powerful set of reuseable tools in the form of the C++
    Standard Library.  However, all existing C++ implementations are
    (as the Draft Standard used to say) <quote>incomplet and
    incorrekt</quote>, and many suffer from limitations of the compilers
    that use them.
    </para> 
    <para>
    The GNU compiler collection
    (<command>gcc</command>, <command>g++</command>, etc) is widely
    considered to be one of the leading compilers in the world.  Its
    development is overseen by the
    <ulink url="http://gcc.gnu.org/">GCC team</ulink>.  All of
    the rapid development and near-legendary
    <ulink url="http://gcc.gnu.org/buildstat.html">portability</ulink>
    that are the hallmarks of an open-source project are being
    applied to libstdc++.
    </para> 
    <para>
    That means that all of the Standard classes and functions will be
    freely available and fully compliant. (Such as
    <classname>string</classname>,
    <classname>vector&lt;&gt;</classname>, iostreams, and algorithms.)
    Programmers will no longer need to <quote>roll their own</quote>
    nor be worried about platform-specific incompatibilities.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.who">
  <question id="q-who">
    <para>
      Who's in charge of it?
    </para>
  </question>
  <answer id="a-who">
    <para>
     The libstdc++ project is contributed to by several developers
     all over the world, in the same way as GCC or Linux.
     Benjamin Kosnik, Gabriel Dos Reis, Phil Edwards, Ulrich Drepper,
     Loren James Rittle, and Paolo Carlini are the lead maintainers of
     the SVN archive.
    </para>
    <para>
    Development and discussion is held on the libstdc++ mailing
    list.  Subscribing to the list, or searching the list
    archives, is open to everyone.  You can read instructions for
    doing so on the <ulink url="http://gcc.gnu.org/libstdc++/">homepage</ulink>.
    If you have questions, ideas, code, or are just curious, sign up!
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.when">
  <question id="q-when">
    <para>
      When is libstdc++ going to be finished?
    </para>
  </question>
  <answer id="a-when">
    <para>
    Nathan Myers gave the best of all possible answers, responding to
    a Usenet article asking this question: <emphasis>Sooner, if you
    help.</emphasis>
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.how">
  <question id="q-how">
    <para>
      How do I contribute to the effort?
    </para>
  </question>
  <answer id="a-how">
    <para>
    Here is <link linkend="appendix.contrib">a page devoted to
    this topic</link>. Subscribing to the mailing list (see above, or
    the homepage) is a very good idea if you have something to
    contribute, or if you have spare time and want to
    help. Contributions don't have to be in the form of source code;
    anybody who is willing to help write documentation, for example,
    or has found a bug in code that we all thought was working and is
    willing to provide details, is more than welcome!
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.whereis_old">
  <question id="q-whereis_old">
    <para>
      What happened to the older libg++? I need that!
    </para>
  </question>
  <answer id="a-whereis_old">
    <para>
    The most recent libg++ README states that libg++ is no longer
    being actively maintained.  It should not be used for new
    projects, and is only being kicked along to support older code.
    </para> 
    <para>
    More information in the <link linkend="manual.appendix.porting.backwards">backwards compatibility documentation</link>
    </para>
  </answer>
</qandaentry>

<qandaentry id="faq.more_questions">
  <question id="q-more_questions">
    <para>
      What if I have more questions?
    </para>
  </question>
  <answer id="a-more_questions">
    <para>
    If you have read the README file, and your question remains
    unanswered, then just ask the mailing list. At present, you do not
    need to be subscribed to the list to send a message to it.  More
    information is available on the homepage (including how to browse
    the list archives); to send a message to the list,
    use <email>libstdc++@gcc.gnu.org</email>.
    </para> 

    <para> 
    If you have a question that you think should be included
    here, or if you have a question <emphasis>about</emphasis> a question/answer
    here, please send email to the libstdc++ mailing list, as above.
    </para> 
  </answer>
</qandaentry>

</qandadiv>

<!-- License -->
<qandadiv id="faq.license" xreflabel="License QA">
<title>License</title>

<qandaentry id="faq.license.what">
  <question id="q-license.what">
    <para>
      What are the license terms for libstdc++?
    </para>
  </question>
  <answer id="a-license.what">
    <para>
    See <link linkend="manual.intro.status.license">our license description</link>
    for these and related questions.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.license.any_program">
  <question id="q-license.any_program">
    <para>
      So any program which uses libstdc++ falls under the GPL?
    </para>
  </question>
  <answer id="a-license.any_program">
    <para>
     No. The special exception permits use of the library in
     proprietary applications.
    </para> 
  </answer>
</qandaentry>


<qandaentry id="faq.license.lgpl">
  <question id="q-license.lgpl">
    <para>
      How is that different from the GNU {Lesser,Library} GPL?
    </para>
  </question>
  <answer id="a-license.lgpl">
    <para>
      The LGPL requires that users be able to replace the LGPL code with a
     modified version; this is trivial if the library in question is a C
     shared library.  But there's no way to make that work with C++, where
     much of the library consists of inline functions and templates, which
     are expanded inside the code that uses the library.  So to allow people
     to replace the library code, someone using the library would have to
     distribute their own source, rendering the LGPL equivalent to the GPL.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.license.what_restrictions">
  <question id="q-license.what_restrictions">
    <para>
      I see. So, what restrictions are there on programs that use the library?
    </para>
  </question>
  <answer id="a-license.what_restrictions">
    <para>
      None.  We encourage such programs to be released as open source,
     but we won't punish you or sue you if you choose otherwise.
    </para> 
  </answer>
</qandaentry>

</qandadiv>

<!-- Installation -->
<qandadiv id="faq.installation" xreflabel="Installation">
<title>Installation</title>

<qandaentry id="faq.how_to_install">
  <question id="q-how_to_install">
    <para>How do I install libstdc++?
    </para>
  </question>
  <answer id="a-how_to_install">
    <para>
    Often libstdc++ comes pre-installed as an integral part of many
    existing Linux and Unix systems, as well as many embedded
    development tools. It may be necessary to install extra
    development packages to get the headers, or the documentation, or
    the source: please consult your vendor for details.
    </para> 
    <para> 
    To build and install from the GNU GCC sources, please consult the 
    <ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/install.html">install
    documentation</ulink> for detailed
    instructions. You may wish to browse those files ahead
    of time to get a feel for what's required.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.how_to_get_sources">
  <question id="q-how_to_get_sources">
    <para>How does one get current libstdc++ sources?
    </para>
  </question>
  <answer id="a-how_to_get_sources">
    <para>
    Libstdc++ sources for all official releases can be obtained as
    part of the GCC sources, available from various sites and
    mirrors. A full <ulink url="http://gcc.gnu.org/mirrors.html">list of 
    download sites</ulink> is provided on the main GCC site.
    </para>
    <para>
    Current libstdc++ sources can always be checked out of the main
    GCC source repository using the appropriate version control
    tool. At this time, that tool
    is <application>Subversion</application>.
    </para>
    <para>
    <application>Subversion</application>, or <acronym>SVN</acronym>, is
    one of several revision control packages.  It was selected for GNU
    projects because it's free (speech), free (beer), and very high
    quality.  The <ulink url="http://subversion.tigris.org"> Subversion
    home page</ulink> has a better description.
    </para>
    <para>
    The <quote>anonymous client checkout</quote> feature of SVN is
    similar to anonymous FTP in that it allows anyone to retrieve
    the latest libstdc++ sources.
    </para> 
    <para>
    For more information
    see <ulink url="http://gcc.gnu.org/svn.html"><acronym>SVN</acronym>
    details</ulink>.
    </para>
  </answer>
</qandaentry>

<qandaentry id="faq.how_to_test">
  <question id="q-how_to_test">
    <para>How do I know if it works?
    </para>
  </question>
  <answer id="a-how_to_test">
    <para>
    Libstdc++ comes with its own validation testsuite, which includes
    conformance testing, regression testing, ABI testing, and
    performance testing. Please consult the 
    <ulink url="http://gcc.gnu.org/install/test.html">testing
    documentation</ulink> for more details.
    </para> 
    <para>
    If you find bugs in the testsuite programs themselves, or if you
    think of a new test program that should be added to the suite,
    <emphasis>please</emphasis> write up your idea and send it to the list!
    </para>
  </answer>
</qandaentry>

<qandaentry id="faq.how_to_set_paths">
  <question id="q-how_to_set_paths">
    <para>How do I insure that the dynamically linked library will be found?
    </para>
  </question>
  <answer id="a-how_to_set_paths">
    <para>
    Depending on your platform and library version, the error message might
    be similar to one of the following:
    </para> 

    <screen>
    ./a.out: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory

    /usr/libexec/ld-elf.so.1: Shared object "libstdc++.so.6" not found
    </screen>

    <para>
    This doesn't mean that the shared library isn't installed, only
    that the dynamic linker can't find it. When a dynamically-linked
    executable is run the linker finds and loads the required shared
    libraries by searching a pre-configured list of directories. If
    the directory where you've installed libstdc++ is not in this list
    then the libraries won't be found. The simplest way to fix this is
    to use the <literal>LD_LIBRARY_PATH</literal> environment variable,
    which is a colon-separated list of directories in which the linker
    will search for shared libraries:
    </para> 

    <screen>
    LD_LIBRARY_PATH=${prefix}/lib:$LD_LIBRARY_PATH
    export LD_LIBRARY_PATH
    </screen>

    <para>
    The exact environment variable to use will depend on your
    platform, e.g. DYLD_LIBRARY_PATH for Darwin,
    LD_LIBRARY_PATH_32/LD_LIBRARY_PATH_64 for Solaris 32-/64-bit,
    LD_LIBRARYN32_PATH/LD_LIBRARY64_PATH for Irix N32/64-bit ABIs and
    SHLIB_PATH for HP-UX.
    </para>
    <para>
    See the man pages for <command>ld</command>, <command>ldd</command>
    and <command>ldconfig</command> for more information. The dynamic
    linker has different names on different platforms but the man page
    is usually called something such as <filename>ld.so/rtld/dld.so</filename>.
    </para>
  </answer>
</qandaentry>

<qandaentry id="faq.what_is_libsupcxx">
  <question id="q-what_is_libsupcxx">
    <para>
      What's libsupc++?
    </para>
  </question>
  <answer id="a-what_is_libsupcxx">
    <para>
      If the only functions from <filename>libstdc++.a</filename>
      which you need are language support functions (those listed in
      <link linkend="manual.support">clause 18</link> of the
      standard, e.g., <function>new</function> and
      <function>delete</function>), then try linking against
      <filename>libsupc++.a</filename>, which is a subset of
      <filename>libstdc++.a</filename>.  (Using <command>gcc</command>
      instead of <command>g++</command> and explicitly linking in
      <filename>libsupc++.a</filename> via <literal>-lsupc++</literal>
      for the final link step will do it).  This library contains only
      those support routines, one per object file.  But if you are
      using anything from the rest of the library, such as IOStreams
      or vectors, then you'll still need pieces from
      <filename>libstdc++.a</filename>.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.size">
  <question id="q-size">
    <para>
      This library is HUGE!
    </para>
  </question>
  <answer id="a-size">
    <para>
    Usually the size of libraries on disk isn't noticeable.  When a
    link editor (or simply <quote>linker</quote>) pulls things from a
    static archive library, only the necessary object files are copied
    into your executable, not the entire library.  Unfortunately, even
    if you only need a single function or variable from an object file,
    the entire object file is extracted.  (There's nothing unique to C++
    or libstdc++ about this; it's just common behavior, given here
    for background reasons.)
    </para>
    <para>
    Some of the object files which make up libstdc++.a are rather large.
    If you create a statically-linked executable with
    <literal>-static</literal>, those large object files are suddenly part
    of your executable.  Historically the best way around this was to
    only place a very few functions (often only a single one) in each
    source/object file; then extracting a single function is the same
    as extracting a single .o file.  For libstdc++ this is only
    possible to a certain extent; the object files in question contain
    template classes and template functions, pre-instantiated, and
    splitting those up causes severe maintenance headaches.
    </para> 
    <para>
    On supported platforms, libstdc++ takes advantage of garbage
    collection in the GNU linker to get a result similar to separating
    each symbol into a separate source and object files. On these platforms,
    GNU ld can place each function and variable into its own
    section in a .o file.  The GNU linker can then perform garbage
    collection on unused sections; this reduces the situation to only
    copying needed functions into the executable, as before, but all
    happens automatically.
    </para>
  </answer>
</qandaentry>

</qandadiv>


<!-- Platform-Specific Issues -->
<qandadiv id="faq.platform-specific" xreflabel="Platform-Specific Issues">
<title>Platform-Specific Issues</title>

<qandaentry id="faq.other_compilers">
  <question id="q-other_compilers">
    <para>
      Can libstdc++ be used with non-GNU compilers?
    </para>
  </question>
  <answer id="a-other_compilers">
    <para>
    Perhaps.
    </para> 
    <para>
    Since the goal of ISO Standardization is for all C++
    implementations to be able to share code, libstdc++ should be
    usable under any ISO-compliant compiler, at least in theory.
    </para> 
    <para>
    However, the reality is that libstdc++ is targeted and optimized
    for GCC/g++. This means that often libstdc++ uses specific,
    non-standard features of g++ that are not present in older
    versions of proprietary compilers. It may take as much as a year or two
    after an official release of GCC that contains these features for
    proprietary tools support these constructs.
    </para>
    <para>
    In the near past, specific released versions of libstdc++ have
    been known to work with versions of the EDG C++ compiler, and
    vendor-specific proprietary C++ compilers such as the Intel ICC
    C++ compiler.
    </para> 

  </answer>
</qandaentry>

<qandaentry id="faq.solaris_long_long">
  <question id="q-solaris_long_long">
    <para>
      No 'long long' type on Solaris?
    </para>
  </question>
  <answer id="a-solaris_long_long">
    <para>
    By default we try to support the C99 <type>long long</type> type.
    This requires that certain functions from your C library be present.
    </para> 
    <para> 
    Up through release 3.0.2 the platform-specific tests performed by
    libstdc++ were too general, resulting in a conservative approach
    to enabling the <type>long long</type> code paths. The most
    commonly reported platform affected was Solaris.
    </para> 
    <para> 
    This has been fixed for libstdc++ releases greater than 3.0.3.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.predefined">
  <question id="q-predefined">
    <para>
      <constant>_XOPEN_SOURCE</constant> and <constant>_GNU_SOURCE</constant> are always defined?
    </para>
  </question>
  <answer id="a-predefined">
      <para>On Solaris, g++ (but not gcc) always defines the preprocessor
         macro <constant>_XOPEN_SOURCE</constant>.  On GNU/Linux, the same happens
         with <constant>_GNU_SOURCE</constant>.  (This is not an exhaustive list;
         other macros and other platforms are also affected.)
      </para>
      <para>These macros are typically used in C library headers, guarding new
         versions of functions from their older versions.  The C++ standard
         library includes the C standard library, but it requires the C90
         version, which for backwards-compatibility reasons is often not the
         default for many vendors.
      </para>
      <para>More to the point, the C++ standard requires behavior which is only
         available on certain platforms after certain symbols are defined.
         Usually the issue involves I/O-related typedefs.  In order to
         ensure correctness, the compiler simply predefines those symbols.
      </para>
      <para>Note that it's not enough to #define them only when the library is
         being built (during installation).  Since we don't have an 'export'
         keyword, much of the library exists as headers, which means that
         the symbols must also be defined as your programs are parsed and
         compiled.
      </para>
      <para>To see which symbols are defined, look for CPLUSPLUS_CPP_SPEC in
         the gcc config headers for your target (and try changing them to
         see what happens when building complicated code).  You can also run
         <command>g++ -E -dM - &lt; /dev/null&quot;</command> to display
         a list of predefined macros for any particular installation.
      </para>
      <para>This has been discussed on the mailing lists
         <ulink url="http://gcc.gnu.org/cgi-bin/htsearch?method=and&amp;format=builtin-long&amp;sort=score&amp;words=_XOPEN_SOURCE+Solaris">quite a bit</ulink>.
      </para>
      <para>This method is something of a wart.  We'd like to find a cleaner
         solution, but nobody yet has contributed the time.
      </para>

  </answer>
</qandaentry>

<qandaentry id="faq.darwin_ctype">
  <question id="q-darwin_ctype">
    <para>
      Mac OS X <filename class="headerfile">ctype.h</filename> is broken! How can I fix it?
    </para>
  </question>
  <answer id="a-darwin_ctype">
      <para>This is a long-standing bug in the OS X support.  Fortunately,
         the patch is quite simple, and well-known.
         <ulink url="http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html"> Here's a
         link to the solution</ulink>.
      </para>

  </answer>
</qandaentry>

<qandaentry id="faq.threads_i386">
  <question id="q-threads_i386">
    <para>
      Threading is broken on i386?
    </para>
  </question>
  <answer id="a-threads_i386">
    <para>
    </para> 
      <para>Support for atomic integer operations is/was broken on i386
         platforms.  The assembly code accidentally used opcodes that are
         only available on the i486 and later.  So if you configured GCC
         to target, for example, i386-linux, but actually used the programs
         on an i686, then you would encounter no problems.  Only when
         actually running the code on a i386 will the problem appear.
      </para>
      <para>This is fixed in 3.2.2.
      </para>

  </answer>
</qandaentry>

<qandaentry id="faq.atomic_mips">
  <question id="q-atomic_mips">
    <para>
      MIPS atomic operations
    </para>
  </question>
  <answer id="a-atomic_mips">
    <para>
    The atomic locking routines for MIPS targets requires MIPS II
    and later.  A patch went in just after the 3.3 release to
    make mips* use the generic implementation instead.  You can also
    configure for mipsel-elf as a workaround.
    </para>
    <para>    
    The mips*-*-linux* port continues to use the MIPS II routines, and more
    work in this area is expected.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.linux_glibc">
  <question id="q-linux_glibc">
    <para>
      Recent GNU/Linux glibc required?
    </para>
  </question>
  <answer id="a-linux_glibc">
      <para>When running on GNU/Linux, libstdc++ 3.2.1 (shared library version
         5.0.1) and later uses localization and formatting code from the system
         C library (glibc) version 2.2.5.  That version of glibc is over a
         year old and contains necessary bugfixes.  Many GNU/Linux distros make
         glibc version 2.3.x available now.
      </para>
      <para>The guideline is simple:  the more recent the C++ library, the
         more recent the C library.  (This is also documented in the main
         GCC installation instructions.)
      </para>

  </answer>
</qandaentry>

<qandaentry id="faq.freebsd_wchar">
  <question id="q-freebsd_wchar">
    <para>
      Can't use wchar_t/wstring on FreeBSD
    </para>
  </question>
  <answer id="a-freebsd_wchar">
    <para>
    Older versions of FreeBSD's C library do not have sufficient
    support for wide character functions, and as a result the
    libstdc++ configury decides that wchar_t support should be
    disabled. In addition, the libstdc++ platform checks that
    enabled <type>wchar_t</type> were quite strict, and not granular
    enough to detect when the minimal support to
    enable <type>wchar_t</type> and C++ library structures
    like <classname>wstring</classname> were present. This impacted Solaris,
    Darwin, and BSD variants, and is fixed in libstdc++ versions post 4.1.0.
    </para> 
    <para> 
    </para> 
  </answer>
</qandaentry>

</qandadiv>


<!-- Known Bugs -->
<qandadiv id="faq.known_bugs" xreflabel="Known Bugs">
<title>Known Bugs</title>

<qandaentry id="faq.what_works">
  <question id="q-what_works">
    <para>
      What works already?
    </para>
  </question>
  <answer id="a-what_works">
    <para>
    Short answer: Pretty much everything <emphasis>works</emphasis>
    except for some corner cases.  Support for localization
    in <classname>locale</classname> may be incomplete on non-GNU
    platforms. Also dependant on the underlying platform is support
    for <type>wchar_t</type> and <type>long
    long</type> specializations, and details of thread support.
    </para>
    <para>    
    Long answer: See the implementation status pages for 
    <ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/17_intro/c++1998_status.html">C++98</ulink>,
    <ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/17_intro/tr1_status.html">TR1</ulink>, and <ulink url="http://gcc.gnu.org/onlinedocs/libstdc++/17_intro/c++0x_status.html">C++0x</ulink>.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.standard_bugs">
  <question id="q-standard_bugs">
    <para>
      Bugs in the ISO C++ language or library specification
    </para>
  </question>
  <answer id="a-standard_bugs">
    <para>
    Unfortunately, there are some. 
    </para>
    <para>
    For those people who are not part of the ISO Library Group
    (i.e., nearly all of us needing to read this page in the first
    place), a public list of the library defects is occasionally
    published <ulink url="http://anubis.dkuug.dk/jtc1/sc22/wg21/">here</ulink>.
    Some of these issues have resulted in code changes in libstdc++.
    </para> 
    <para>
    If you think you've discovered a new bug that is not listed,
    please post a message describing your problem
    to <email>libstdc++@gcc.gnu.org</email> or the Usenet group
    comp.lang.c++.moderated.
    </para>
  </answer>
</qandaentry>

<qandaentry id="faq.compiler_bugs">
  <question id="q-compiler_bugs">
    <para>
      Bugs in the compiler (gcc/g++) and not libstdc++
    </para>
  </question>
  <answer id="a-compiler_bugs">
    <para>
    On occasion, the compiler is wrong. Please be advised that this
    happens much less often than one would think, and avoid jumping to
    conclusions.
    </para>
    <para>
    First, examine the ISO C++ standard. Second, try another compiler
    or an older version of the GNU compilers. Third, you can find more
    information on the libstdc++ and the GCC mailing lists: search
    these lists with terms describing your issue.
    </para> 
    <para> 
    Before reporting a bug, please examine the
    <ulink url="http://gcc.gnu.org/bugs.html">bugs database</ulink> with the
    category set to <quote>g++</quote>. 
    </para> 
  </answer>
</qandaentry>

</qandadiv>

<!-- Known Non-Bugs -->
<qandadiv id="faq.known_non-bugs" xreflabel="Known Non-Bugs">
<title>Known Non-Bugs</title>

<qandaentry id="faq.stream_reopening_fails">
  <question id="q-stream_reopening_fails">
    <para>
      Reopening a stream fails
    </para>
  </question>
  <answer id="a-stream_reopening_fails">
    <para>
    One of the most-reported non-bug reports. Executing a sequence like:
    </para>

    <literallayout>
    #include &lt;fstream&gt;
    ...
    std::fstream  fs(<quote>a_file</quote>);
    // .
    // . do things with fs...
    // .
    fs.close();
    fs.open(<quote>a_new_file</quote>);
    </literallayout>
    
    <para>
    All operations on the re-opened <varname>fs</varname> will fail, or at
    least act very strangely.  Yes, they often will, especially if
    <varname>fs</varname> reached the EOF state on the previous file.  The
    reason is that the state flags are <emphasis>not</emphasis> cleared
    on a successful call to open().  The standard unfortunately did
    not specify behavior in this case, and to everybody's great sorrow,
    the <link linkend="manual.intro.status.bugs">proposed LWG resolution in
      DR #22</link> is to leave the flags unchanged.  You must insert a call
    to <function>fs.clear()</function> between the calls to close() and open(),
    and then everything will work like we all expect it to work.
    <emphasis>Update:</emphasis> for GCC 4.0 we implemented the resolution
    of <link linkend="manual.intro.status.bugs">DR #409</link> and open() 
    now calls <function>clear()</function> on success!
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.wefcxx_verbose">
  <question id="q-wefcxx_verbose">
    <para>
      -Weffc++ complains too much
    </para>
  </question>
  <answer id="a-wefcxx_verbose">
    <para>
    Many warnings are emitted when <literal>-Weffc++</literal> is used.  Making
    libstdc++ <literal>-Weffc++</literal>-clean is not a goal of the project,
    for a few reasons.  Mainly, that option tries to enforce
    object-oriented programming, while the Standard Library isn't
    necessarily trying to be OO.
    </para> 
    <para>
    We do, however, try to have libstdc++ sources as clean as possible. If
    you see some simple changes that pacify <literal>-Weffc++</literal>
    without other drawbacks, send us a patch.
    </para>
  </answer>
</qandaentry>

<qandaentry id="faq.ambiguous_overloads">
  <question id="q-ambiguous_overloads">
    <para>
      Ambiguous overloads after including an old-style header
    </para>
  </question>
  <answer id="a-ambiguous_overloads">
    <para>
    Another problem is the <literal>rel_ops</literal> namespace and the template
    comparison operator functions contained therein.  If they become
    visible in the same namespace as other comparison functions
    (e.g., <quote>using</quote> them and the &lt;iterator&gt; header),
    then you will suddenly be faced with huge numbers of ambiguity
    errors.  This was discussed on the -v3 list; Nathan Myers
    <ulink url="http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html">sums
      things up here</ulink>.  The collisions with vector/string iterator
    types have been fixed for 3.1.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.v2_headers">
  <question id="q-v2_headers">
    <para>
      The g++-3 headers are <emphasis>not ours</emphasis>
    </para>
  </question>
  <answer id="a-v2_headers">
      <para>
	If you have found an extremely broken header file which is
	causing problems for you, look carefully before submitting a
	&quot;high&quot; priority bug report (which you probably
	shouldn't do anyhow; see the last paragraph of the page
	describing <ulink url="http://gcc.gnu.org/bugs.html">the GCC
	bug database</ulink>).
      </para>
      <para>
	If the headers are in <filename>${prefix}/include/g++-3</filename>, or
	if the installed library's name looks like
	<filename>libstdc++-2.10.a</filename> or
	<filename>libstdc++-libc6-2.10.so</filename>, then you are using the
	old libstdc++-v2 library, which is nonstandard and
	unmaintained.  Do not report problems with -v2 to the -v3
	mailing list.
      </para>
      <para>
	For GCC versions 3.0 and 3.1 the libstdc++ header files are
	installed in <filename>${prefix}/include/g++-v3</filename> (see the
	'v'?).  Starting with version 3.2 the headers are installed in
	<filename>${prefix}/include/c++/${version}</filename> as this prevents
	headers from previous versions being found by mistake.
      </para>

  </answer>
</qandaentry>

<qandaentry id="faq.boost_concept_checks">
  <question id="q-boost_concept_checks">
    <para>
      Errors about <emphasis>*Concept</emphasis> and
      <emphasis>constraints</emphasis> in the STL
    </para>
  </question>
  <answer id="a-boost_concept_checks">
    <para>
    If you see compilation errors containing messages about
    <errortext>foo Concept </errortext>and something to do with a
    <errortext>constraints</errortext> member function, then most
    likely you have violated one of the requirements for types used
    during instantiation of template containers and functions.  For
    example, EqualityComparableConcept appears if your types must be
    comparable with == and you have not provided this capability (a
    typo, or wrong visibility, or you just plain forgot, etc).
    </para>
    <para>
    More information, including how to optionally enable/disable the
    checks, is available
    <link linkend="manual.diagnostics.concept_checking">here</link>.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.dlopen_crash">
  <question id="q-dlopen_crash">
    <para>
      Program crashes when using library code in a
      dynamically-loaded library
    </para>
  </question>
  <answer id="a-dlopen_crash">
    <para>
    If you are using the C++ library across dynamically-loaded
    objects, make certain that you are passing the correct options
    when compiling and linking:
    </para>

    <literallayout>
    // compile your library components
    g++ -fPIC -c a.cc
    g++ -fPIC -c b.cc
    ...
    g++ -fPIC -c z.cc

    // create your library
    g++ -fPIC -shared -rdynamic -o libfoo.so a.o b.o ... z.o

    // link the executable
    g++ -fPIC -rdynamic -o foo ... -L. -lfoo -ldl
    </literallayout>
  </answer>
</qandaentry>

<qandaentry id="faq.memory_leaks">
  <question id="q-memory_leaks">
    <para>
      <quote>Memory leaks</quote> in containers
    </para>
  </question>
  <answer id="a-memory_leaks">
    <para>
    A few people have reported that the standard containers appear
    to leak memory when tested with memory checkers such as
    <ulink url="http://valgrind.org/">valgrind</ulink>.
    The library's default allocators keep free memory in a pool
    for later reuse, rather than returning it to the OS.  Although
    this memory is always reachable by the library and is never
    lost, memory debugging tools can report it as a leak.  If you
    want to test the library for memory leaks please read
    <link linkend="debug.memory">Tips for memory leak hunting</link>
    first.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.list_size_on">
  <question id="q-list_size_on">
    <para>
      list::size() is O(n)!
    </para>
  </question>
  <answer id="a-list_size_on">
    <para>
    See
    the <link linkend="manual.containers">Containers</link>
    chapter.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.easy_to_fix">
  <question id="q-easy_to_fix">
    <para>
      Aw, that's easy to fix!
    </para>
  </question>
  <answer id="a-easy_to_fix">
    <para>
    If you have found a bug in the library and you think you have
    a working fix, then send it in!  The main GCC site has a page
    on <ulink url="http://gcc.gnu.org/contribute.html">submitting
    patches</ulink> that covers the procedure, but for libstdc++ you
    should also send the patch to our mailing list in addition to
    the GCC patches mailing list.  The libstdc++
    <link linkend="appendix.contrib">contributors' page</link>
    also talks about how to submit patches.
    </para>
    <para>
    In addition to the description, the patch, and the ChangeLog
    entry, it is a Good Thing if you can additionally create a small
    test program to test for the presence of the bug that your
    patch fixes.  Bugs have a way of being reintroduced; if an old
    bug creeps back in, it will be caught immediately by the
    <ulink url="#2_4">testsuite</ulink> -- but only if such a test exists.
    </para> 
  </answer>
</qandaentry>

</qandadiv>


<!-- Miscellaneous -->
<qandadiv id="faq.misc" xreflabel="Miscellaneous">
<title>Miscellaneous</title>

<qandaentry id="faq.iterator_as_pod">
  <question id="faq.iterator_as_pod_q">
    <para>
      string::iterator is not char*; vector&lt;T&gt;::iterator is not T*
    </para>
  </question>
  <answer id="faq.iterator_as_pod_a">
    <para>
    If you have code that depends on container&lt;T&gt; iterators
    being implemented as pointer-to-T, your code is broken. It's
    considered a feature, not a bug, that libstdc++ points this out.
    </para>
    <para>
    While there are arguments for iterators to be implemented in
    that manner, A) they aren't very good ones in the long term,
    and B) they were never guaranteed by the Standard anyway.  The
    type-safety achieved by making iterators a real class rather
    than a typedef for <type>T*</type> outweighs nearly all opposing
    arguments.
    </para>
    <para>
    Code which does assume that a vector iterator <varname>i</varname>
    is a pointer can often be fixed by changing <varname>i</varname> in
    certain expressions to <varname>&amp;*i</varname>.  Future revisions
    of the Standard are expected to bless this usage for
    vector&lt;&gt; (but not for basic_string&lt;&gt;).
    </para>
  </answer>
</qandaentry>

<qandaentry id="faq.what_is_next">
  <question id="q-what_is_next">
    <para>
      What's next after libstdc++?
    </para>
  </question>
  <answer id="a-what_is_next">
      <para>
	Hopefully, not much.  The goal of libstdc++ is to produce a
	fully-compliant, fully-portable Standard Library.  After that,
	we're mostly done: there won't <emphasis>be</emphasis> any
	more compliance work to do.
      </para>
      <para>
	There is an effort underway to add significant extensions to
	the standard library specification.  The latest version of
	this effort is described in
         <ulink url="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">
         The C++ Library Technical Report 1</ulink>.
      </para>
  </answer>
</qandaentry>

<qandaentry id="faq.sgi_stl">
  <question id="q-sgi_stl">
    <para>
      What about the STL from SGI?
    </para>
  </question>
  <answer id="a-sgi_stl">
    <para>
      The <ulink url="http://www.sgi.com/tech/stl/">STL from SGI</ulink>,
    version 3.3, was the final merge of the STL codebase.  The
    code in libstdc++ contains many fixes and changes, and
    the SGI code is no longer under active
    development.  We expect that no future merges will take place.
    </para>
    <para>
    In particular, <classname>string</classname> is not from SGI and makes no
    use of their &quot;rope&quot; class (which is included as an
    optional extension), nor is <classname>valarray</classname> and some others.
    Classes like <classname>vector&lt;&gt;</classname> are, but have been
    extensively modified.
    </para>
    <para>
    More information on the evolution of libstdc++ can be found at the
    <link linkend="appendix.porting.api">API
    evolution</link>
    and <link linkend="manual.appendix.porting.backwards">backwards
    compatibility</link> documentation.
    </para>
    <para>
    The FAQ for SGI's STL (one jump off of their main page) is
    still recommended reading.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.extensions_and_backwards_compat">
  <question id="q-extensions_and_backwards_compat">
    <para>
      Extensions and Backward Compatibility
    </para>
  </question>
  <answer id="a-extensions_and_backwards_compat">
    <para>
      See the <link linkend="manual.appendix.porting.backwards">link</link> on backwards compatibility and <link linkend="appendix.porting.api">link</link> on evolution.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.tr1_support">
  <question id="q-tr1_support">
    <para>
      Does libstdc++ support TR1?
    </para>
  </question>
  <answer id="a-tr1_support">
    <para>
    Yes.
    </para>
    <para>
    The C++ Standard Library Technical Report adds many new features to 
    the library.  The latest version of this effort is described in
    <ulink url=
	 "http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">
         Technical Report 1</ulink>.
    </para>
    <para>
    The implementation status of TR1 in libstdc++ can be tracked <link
    linkend="manual.intro.status.standard.tr1">on the TR1 status
    page</link>.
    </para>
  </answer>
</qandaentry>

<qandaentry id="faq.get_iso_cxx">
  <question id="q-get_iso_cxx">
    <para>How do I get a copy of the ISO C++ Standard?
    </para>
  </question>
  <answer id="a-get_iso_cxx">
    <para>
    Copies of the full ISO 14882 standard are available on line via
    the ISO mirror site for committee members.  Non-members, or those
    who have not paid for the privilege of sitting on the committee
    and sustained their two-meeting commitment for voting rights, may
    get a copy of the standard from their respective national
    standards organization.  In the USA, this national standards
    organization is ANSI and their website is
    right <ulink url="http://www.ansi.org">here</ulink>.  (And if
    you've already registered with them, clicking this link will take
    you to directly to the place where you can
    <ulink url="http://webstore.ansi.org/ansidocstore/product.asp?sku=ISO%2FIEC+14882%3A2003">buy the standard on-line</ulink>.
    </para>
    <para>
    Who is your country's member body?  Visit the
    <ulink url="http://www.iso.ch/">ISO homepage</ulink> and find out!
    </para>
    <para>
    The 2003 version of the standard (the 1998 version plus TC1) is
    available in print, ISBN 0-470-84674-7.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.what_is_abi">
  <question id="q-what_is_abi">
    <para>
      What's an ABI and why is it so messy?
    </para>
  </question>
  <answer id="a-what_is_abi">
    <para>
    <acronym>ABI</acronym> stands for <quote>Application Binary
     Interface</quote>.  Conventionally, it refers to a great
    mass of details about how arguments are arranged on the call
    stack and/or in registers, and how various types are arranged
    and padded in structs.  A single CPU design may suffer
    multiple ABIs designed by different development tool vendors
    who made different choices, or even by the same vendor for
    different target applications or compiler versions.  In ideal
    circumstances the CPU designer presents one ABI and all the
    OSes and compilers use it.  In practice every ABI omits
    details that compiler implementers (consciously or
    accidentally) must choose for themselves.
    </para>
    <para>
    That ABI definition suffices for compilers to generate code so a
    program can interact safely with an OS and its lowest-level libraries.
    Users usually want an ABI to encompass more detail, allowing libraries
    built with different compilers (or different releases of the same
    compiler!) to be linked together.  For C++, this includes many more
    details than for C, and CPU designers (for good reasons elaborated
    below) have not stepped up to publish C++ ABIs.  The details include
    virtual function implementation, struct inheritance layout, name
    mangling, and exception handling.  Such an ABI has been defined for
    GNU C++, and is immediately useful for embedded work relying only on
    a <quote>free-standing implementation</quote> that doesn't include (much
    of) the standard library.  It is a good basis for the work to come.
    </para>
    <para>
    A useful C++ ABI must also incorporate many details of the standard
    library implementation.  For a C ABI, the layouts of a few structs
    (such as FILE, stat, jmpbuf, and the like) and a few macros suffice.
    For C++, the details include the complete set of names of functions
    and types used, the offsets of class members and virtual functions,
    and the actual definitions of all inlines.  C++ exposes many more
    library details to the caller than C does.  It makes defining
    a complete ABI a much bigger undertaking, and requires not just
    documenting library implementation details, but carefully designing
    those details so that future bug fixes and optimizations don't
    force breaking the ABI.
    </para>
    <para>
    There are ways to help isolate library implementation details from the
    ABI, but they trade off against speed.  Library details used in
    inner loops (e.g., getchar) must be exposed and frozen for all
    time, but many others may reasonably be kept hidden from user code,
    so they may later be changed.  Deciding which, and implementing
    the decisions, must happen before you can reasonably document a
    candidate C++ ABI that encompasses the standard library.
    </para> 
  </answer>
</qandaentry>

<qandaentry id="faq.size_equals_capacity">
  <question id="q-size_equals_capacity">
    <para>
      How do I make std::vector&lt;T&gt;::capacity() == std::vector&lt;T&gt;::size?
    </para>
  </question>
  <answer id="a-size_equals_capacity">
    <para>
    The standard idiom for deallocating a <classname>vector&lt;T&gt;</classname>'s
    unused memory is to create a temporary copy of the vector and swap their
    contents, e.g. for <classname>vector&lt;T&gt; v</classname>
    </para>
    <literallayout>
     std::vector&lt;T&gt;(v).swap(v);
    </literallayout>
    <para>
    The copy will take O(n) time and the swap is constant time.
    </para>
    <para>
    See <link linkend="strings.string.shrink">Shrink-to-fit
    strings</link> for a similar solution for strings.
    </para> 
  </answer>
</qandaentry>

</qandadiv>


<!-- FAQ ends here -->
</qandaset>

</article>

</book>