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<book xmlns="http://docbook.org/ns/docbook" version="5.0">
<article xml:id="faq" xreflabel="Frequently Asked Questions">
<?dbhtml filename="faq.html"?>
<info><title>Frequently Asked Questions</title>
<copyright>
<year>
2008, 2010
</year>
<holder>
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.fsf.org">FSF</link>
</holder>
</copyright>
</info>
<!-- FAQ starts here -->
<qandaset>
<!-- General Information -->
<qandadiv xml:id="faq.info" xreflabel="General Information">
<qandaentry xml:id="faq.what">
<question xml:id="faq.what.q">
<para>
What is libstdc++?
</para>
</question>
<answer xml: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 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/svn.html">web</link>.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.why">
<question xml:id="q-why">
<para>
Why should I use libstdc++?
</para>
</question>
<answer xml: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
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/">GCC team</link>. All of
the rapid development and near-legendary
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/buildstat.html">portability</link>
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<></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 xml:id="faq.who">
<question xml:id="q-who">
<para>
Who's in charge of it?
</para>
</question>
<answer xml:id="a-who">
<para>
The libstdc++ project is contributed to by several developers
all over the world, in the same way as GCC or the Linux kernel.
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 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/libstdc++/">homepage</link>.
If you have questions, ideas, code, or are just curious, sign up!
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.when">
<question xml:id="q-when">
<para>
When is libstdc++ going to be finished?
</para>
</question>
<answer xml: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 xml:id="faq.how">
<question xml:id="q-how">
<para>
How do I contribute to the effort?
</para>
</question>
<answer xml: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 xml:id="faq.whereis_old">
<question xml:id="q-whereis_old">
<para>
What happened to the older libg++? I need that!
</para>
</question>
<answer xml: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 xml:id="faq.more_questions">
<question xml:id="q-more_questions">
<para>
What if I have more questions?
</para>
</question>
<answer xml: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 xml:id="faq.license" xreflabel="License QA">
<qandaentry xml:id="faq.license.what">
<question xml:id="q-license.what">
<para>
What are the license terms for libstdc++?
</para>
</question>
<answer xml: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 xml:id="faq.license.any_program">
<question xml:id="q-license.any_program">
<para>
So any program which uses libstdc++ falls under the GPL?
</para>
</question>
<answer xml:id="a-license.any_program">
<para>
No. The special exception permits use of the library in
proprietary applications.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.license.lgpl">
<question xml:id="q-license.lgpl">
<para>
How is that different from the GNU {Lesser,Library} GPL?
</para>
</question>
<answer xml: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 xml:id="faq.license.what_restrictions">
<question xml:id="q-license.what_restrictions">
<para>
I see. So, what restrictions are there on programs that use the library?
</para>
</question>
<answer xml: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 xml:id="faq.installation" xreflabel="Installation">
<qandaentry xml:id="faq.how_to_install">
<question xml:id="q-how_to_install">
<para>How do I install libstdc++?
</para>
</question>
<answer xml:id="a-how_to_install">
<para>
Often libstdc++ comes pre-installed as an integral part of many
existing GNU/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
<link linkend="manual.intro.setup">setup
documentation</link> 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 xml:id="faq.how_to_get_sources">
<question xml:id="q-how_to_get_sources">
<para>How does one get current libstdc++ sources?
</para>
</question>
<answer xml: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 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/mirrors.html">list of
download sites</link> 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 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://subversion.tigris.org"> Subversion
home page</link> 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 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/svn.html"><acronym>SVN</acronym>
details</link>.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.how_to_test">
<question xml:id="q-how_to_test">
<para>How do I know if it works?
</para>
</question>
<answer xml: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
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/install/test.html">testing
documentation</link> 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 xml:id="faq.how_to_set_paths">
<question xml:id="q-how_to_set_paths">
<para>How do I insure that the dynamically linked library will be found?
</para>
</question>
<answer xml: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>
<para>
Using LD_LIBRARY_PATH is not always the best solution, <link linkend="manual.intro.using.linkage.dynamic">Finding Dynamic or Shared
Libraries</link> in the manual gives some alternatives.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.what_is_libsupcxx">
<question xml:id="q-what_is_libsupcxx">
<para>
What's libsupc++?
</para>
</question>
<answer xml: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="std.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 xml:id="faq.size">
<question xml:id="q-size">
<para>
This library is HUGE!
</para>
</question>
<answer xml: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 xml:id="faq.platform-specific" xreflabel="Platform-Specific Issues">
<qandaentry xml:id="faq.other_compilers">
<question xml:id="q-other_compilers">
<para>
Can libstdc++ be used with non-GNU compilers?
</para>
</question>
<answer xml: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 to 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 xml:id="faq.solaris_long_long">
<question xml:id="q-solaris_long_long">
<para>
No 'long long' type on Solaris?
</para>
</question>
<answer xml: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 xml:id="faq.predefined">
<question xml:id="q-predefined">
<para>
<constant>_XOPEN_SOURCE</constant> and <constant>_GNU_SOURCE</constant> are always defined?
</para>
</question>
<answer xml: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 - < /dev/null"</command> to display
a list of predefined macros for any particular installation.
</para>
<para>This has been discussed on the mailing lists
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/cgi-bin/htsearch?method=and&format=builtin-long&sort=score&words=_XOPEN_SOURCE+Solaris">quite a bit</link>.
</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 xml:id="faq.darwin_ctype">
<question xml:id="q-darwin_ctype">
<para>
Mac OS X <filename class="headerfile">ctype.h</filename> is broken! How can I fix it?
</para>
</question>
<answer xml: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.
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html"> Here's a
link to the solution</link>.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.threads_i386">
<question xml:id="q-threads_i386">
<para>
Threading is broken on i386?
</para>
</question>
<answer xml: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 xml:id="faq.atomic_mips">
<question xml:id="q-atomic_mips">
<para>
MIPS atomic operations
</para>
</question>
<answer xml: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 xml:id="faq.linux_glibc">
<question xml:id="q-linux_glibc">
<para>
Recent GNU/Linux glibc required?
</para>
</question>
<answer xml: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 which contains necessary bugfixes.
Most GNU/Linux distros make more recent versions available now.
libstdc++ 4.6.0 and later require glibc 2.3 or later for this
localization and formatting code.
</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 xml:id="faq.freebsd_wchar">
<question xml:id="q-freebsd_wchar">
<para>
Can't use wchar_t/wstring on FreeBSD
</para>
</question>
<answer xml: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 xml:id="faq.known_bugs" xreflabel="Known Bugs">
<qandaentry xml:id="faq.what_works">
<question xml:id="q-what_works">
<para>
What works already?
</para>
</question>
<answer xml: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
<link linkend="status.iso.1998">C++98</link>,
<link linkend="status.iso.tr1">TR1</link>, and
<link linkend="status.iso.2011">C++11</link>.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.standard_bugs">
<question xml:id="q-standard_bugs">
<para>
Bugs in the ISO C++ language or library specification
</para>
</question>
<answer xml: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 on <link xmlns:xlink="http://www.w3.org/1999/xlink"
xlink:href="http://www.open-std.org/jtc1/sc22/wg21/">the WG21
website</link>.
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 the author of
the library issues list or the Usenet group comp.lang.c++.moderated.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.compiler_bugs">
<question xml:id="q-compiler_bugs">
<para>
Bugs in the compiler (gcc/g++) and not libstdc++
</para>
</question>
<answer xml: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
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/bugs/">bugs database</link> with the
category set to <quote>g++</quote>.
</para>
</answer>
</qandaentry>
</qandadiv>
<!-- Known Non-Bugs -->
<qandadiv xml:id="faq.known_non-bugs" xreflabel="Known Non-Bugs">
<qandaentry xml:id="faq.stream_reopening_fails">
<question xml:id="q-stream_reopening_fails">
<para>
Reopening a stream fails
</para>
</question>
<answer xml:id="a-stream_reopening_fails">
<para>
One of the most-reported non-bug reports. Executing a sequence like:
</para>
<literallayout class="normal">
#include <fstream>
...
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 xml:id="faq.wefcxx_verbose">
<question xml:id="q-wefcxx_verbose">
<para>
-Weffc++ complains too much
</para>
</question>
<answer xml: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 xml:id="faq.ambiguous_overloads">
<question xml:id="q-ambiguous_overloads">
<para>
Ambiguous overloads after including an old-style header
</para>
</question>
<answer xml: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 <iterator> header),
then you will suddenly be faced with huge numbers of ambiguity
errors. This was discussed on the -v3 list; Nathan Myers
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html">sums
things up here</link>. The collisions with vector/string iterator
types have been fixed for 3.1.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.v2_headers">
<question xml:id="q-v2_headers">
<para>
The g++-3 headers are <emphasis>not ours</emphasis>
</para>
</question>
<answer xml:id="a-v2_headers">
<para>
If you are using headers 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 xml:id="faq.boost_concept_checks">
<question xml:id="q-boost_concept_checks">
<para>
Errors about <emphasis>*Concept</emphasis> and
<emphasis>constraints</emphasis> in the STL
</para>
</question>
<answer xml: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 in the
<link linkend="std.diagnostics.concept_checking">Diagnostics</link>.
chapter of the manual.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.dlopen_crash">
<question xml:id="q-dlopen_crash">
<para>
Program crashes when using library code in a
dynamically-loaded library
</para>
</question>
<answer xml: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 class="normal">
// 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 xml:id="faq.memory_leaks">
<question xml:id="q-memory_leaks">
<para>
<quote>Memory leaks</quote> in containers
</para>
</question>
<answer xml: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
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://valgrind.org/">valgrind</link>.
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 xml:id="faq.list_size_on">
<question xml:id="q-list_size_on">
<para>
list::size() is O(n)!
</para>
</question>
<answer xml:id="a-list_size_on">
<para>
See
the <link linkend="std.containers">Containers</link>
chapter.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.easy_to_fix">
<question xml:id="q-easy_to_fix">
<para>
Aw, that's easy to fix!
</para>
</question>
<answer xml: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 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/contribute.html">submitting
patches</link> 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 testsuite -
but only if such a test exists.
</para>
</answer>
</qandaentry>
</qandadiv>
<!-- Miscellaneous -->
<qandadiv xml:id="faq.misc" xreflabel="Miscellaneous">
<qandaentry xml:id="faq.iterator_as_pod">
<question xml:id="faq.iterator_as_pod_q">
<para>
string::iterator is not char*; vector<T>::iterator is not T*
</para>
</question>
<answer xml:id="faq.iterator_as_pod_a">
<para>
If you have code that depends on container<T> 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>&*i</varname>. Future revisions
of the Standard are expected to bless this usage for
vector<> (but not for basic_string<>).
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.what_is_next">
<question xml:id="q-what_is_next">
<para>
What's next after libstdc++?
</para>
</question>
<answer xml: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
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">
The C++ Library Technical Report 1</link>.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.sgi_stl">
<question xml:id="q-sgi_stl">
<para>
What about the STL from SGI?
</para>
</question>
<answer xml:id="a-sgi_stl">
<para>
The <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.sgi.com/tech/stl/">STL from SGI</link>,
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 "rope" class (which is included as an
optional extension), nor is <classname>valarray</classname> and some others.
Classes like <classname>vector<></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 xml:id="faq.extensions_and_backwards_compat">
<question xml:id="q-extensions_and_backwards_compat">
<para>
Extensions and Backward Compatibility
</para>
</question>
<answer xml: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 xml:id="faq.tr1_support">
<question xml:id="q-tr1_support">
<para>
Does libstdc++ support TR1?
</para>
</question>
<answer xml: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
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">
Technical Report 1</link>.
</para>
<para>
The implementation status of TR1 in libstdc++ can be tracked <link linkend="status.iso.tr1">on the TR1 status
page</link>.
</para>
</answer>
</qandaentry>
<qandaentry xml:id="faq.get_iso_cxx">
<question xml:id="q-get_iso_cxx">
<para>How do I get a copy of the ISO C++ Standard?
</para>
</question>
<answer xml: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 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.ansi.org">here</link>. (And if
you've already registered with them, clicking this link will take
you to directly to the place where you can
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://webstore.ansi.org/RecordDetail.aspx?sku=ISO%2FIEC+14882:2003">buy the standard on-line</link>.
</para>
<para>
Who is your country's member body? Visit the
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.iso.ch/">ISO homepage</link> 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 xml:id="faq.what_is_abi">
<question xml:id="q-what_is_abi">
<para>
What's an ABI and why is it so messy?
</para>
</question>
<answer xml: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 xml:id="faq.size_equals_capacity">
<question xml:id="q-size_equals_capacity">
<para>
How do I make std::vector<T>::capacity() == std::vector<T>::size?
</para>
</question>
<answer xml:id="a-size_equals_capacity">
<para>
The standard idiom for deallocating a <classname>vector<T></classname>'s
unused memory is to create a temporary copy of the vector and swap their
contents, e.g. for <classname>vector<T> v</classname>
</para>
<literallayout class="normal">
std::vector<T>(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>
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