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Jonathan Wakely 408e31a2dd index.html: Explain memory "leaks" due to allocators. 
2003-03-12  Jonathan Wakely  <redi@gcc.gnu.org>

	* docs/html/faq/index.html: Explain memory "leaks" due to allocators.
	* docs/html/faq/index.txt: Regenerate.
	* docs/html/debug.html: Add a bit to allocator text and fix XHTML.

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libstdc++ Frequently Asked Questions
The latest version of this document is always available at
[1]http://gcc.gnu.org/onlinedocs/libstdc++/faq/. The main
documentation page is at
[2]http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html.
To the [3]libstdc++-v3 homepage.
_________________________________________________________________
Questions
1. [4]General Information
1. [5]What is libstdc++-v3?
2. [6]Why should I use libstdc++?
3. [7]Who's in charge of it?
4. [8]How do I get libstdc++?
5. [9]When is libstdc++ going to be finished?
6. [10]How do I contribute to the effort?
7. [11]What happened to libg++? I need that!
8. [12]What if I have more questions?
9. [13]What are the license terms for libstdc++-v3?
2. [14]Installation
1. [15]How do I install libstdc++-v3?
2. [16][removed]
3. [17]What is this CVS thing that you keep mentioning?
4. [18]How do I know if it works?
5. [19]This library is HUGE! And what's libsupc++?
3. [20]Platform-Specific Issues
1. [21]Can libstdc++-v3 be used with <my favorite compiler>?
2. [22][removed]
3. [23][removed]
4. [24]I can't use 'long long' on Solaris
5. [25]_XOPEN_SOURCE / _GNU_SOURCE / etc is always defined
6. [26]OS X ctype.h is broken! How can I hack it?
7. [27]Threading is broken on i386
8. [28]Recent GNU/Linux glibc required?
9. [29]Can't use wchar_t/wstring on FreeBSD
4. [30]Known Bugs and Non-Bugs
1. [31]What works already?
2. [32]Bugs in gcc/g++ (not libstdc++-v3)
3. [33]Bugs in the C++ language/lib specification
4. [34]Things in libstdc++ that only look like bugs
o [35]reopening a stream fails
o [36]-Weffc++ complains too much
o [37]"ambiguous overloads" after including an old-style
header
o [38]The g++-3 headers are not ours
o [39]compilation errors from streambuf.h
o [40]errors about *Concept and constraints in the STL...
o [41]program crashes when using library code in a
dynamically-loaded library
o [42]"memory leaks" in containers
5. [43]Aw, that's easy to fix!
5. [44]Miscellaneous
1. [45]string::iterator is not char*; vector<T>::iterator is not
T*
2. [46]What's next after libstdc++-v3?
3. [47]What about the STL from SGI?
4. [48]Extensions and Backward Compatibility
5. [49][removed]
6. [50]Is libstdc++-v3 thread-safe?
7. [51]How do I get a copy of the ISO C++ Standard?
8. [52]What's an ABI and why is it so messy?
_________________________________________________________________
1.0 General Information
1.1 What is libstdc++-v3?
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. As the library reaches stable plateaus, it is captured in
a snapshot and released. The latest release is [53]the fourteenth
snapshot but newer versions have been included in recent GCC releases.
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 CVS, and can even be browsed over the Web
(see [54]1.4 below).
The older libstdc++-v2 project is no longer maintained; the code has
been completely replaced and rewritten. [55]If you are using V2, then
you need to report bugs to your system vendor, not to the V3 list.
A more formal description of the V3 goals can be found in the official
[56]design document.
_________________________________________________________________
1.2 Why should I use libstdc++?
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) "incomplet and incorrekt," and many suffer from
limitations of the compilers that use them.
The GNU C/C++/FORTRAN/<pick-a-language> compiler (gcc, g++, etc) is
widely considered to be one of the leading compilers in the world. Its
development has recently been taken over by the [57]GCC team. All of
the rapid development and near-legendary [58]portability that are the
hallmarks of an open-source project are being applied to libstdc++.
That means that all of the Standard classes and functions (such as
string, vector<>, iostreams, and algorithms) will be freely available
and fully compliant. Programmers will no longer need to "roll their
own" nor be worried about platform-specific incompatibilities.
_________________________________________________________________
1.3 Who's in charge of it?
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 CVS archive.
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 [59]homepage.
If you have questions, ideas, code, or are just curious, sign up!
_________________________________________________________________
1.4 How do I get libstdc++?
The fourteenth (and latest) snapshot of libstdc++-v3 is [60]available
via ftp.
The [61]homepage has instructions for retrieving the latest CVS
sources, and for browsing the CVS sources over the web.
The subset commonly known as the Standard Template Library (chapters
23 through 25, mostly) is adapted from the final release of the SGI
STL.
_________________________________________________________________
1.5 When is libstdc++ going to be finished?
Nathan Myers gave the best of all possible answers, responding to a
Usenet article asking this question: Sooner, if you help.
_________________________________________________________________
1.6 How do I contribute to the effort?
Here is [62]a page devoted to this topic. 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, is more than welcome!
_________________________________________________________________
1.7 What happened to libg++? I need that!
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.
The libg++ was designed and created when there was no Standard to
provide guidance. Classes like linked lists are now provided for by
list<T> and do not need to be created by genclass. (For that matter,
templates exist now and are well-supported, whereas genclass (mostly)
predates them.)
There are other classes in libg++ that are not specified in the ISO
Standard (e.g., statistical analysis). While there are a lot of really
useful things that are used by a lot of people (e.g., statistics :-),
the Standards Committee couldn't include everything, and so a lot of
those "obvious" classes didn't get included.
Since libstdc++ is an implementation of the Standard Library, we have
no plans at this time to include non-Standard utilities in the
implementation, however handy they are. (The extensions provided in
the SGI STL aren't maintained by us and don't get a lot of our
attention, because they don't require a lot of our time.) It is
entirely plausable that the "useful stuff" from libg++ might be
extracted into an updated utilities library, but nobody has stated
such a project yet.
(The [63]Boost site houses free C++ libraries that do varying things,
and happened to be started by members of the Standards Committee.
Certain "useful stuff" classes will probably migrate there.)
For the bold and/or desperate, the [64]GCC extensions page describes
where to find the last libg++ source.
_________________________________________________________________
1.8 What if I have more questions?
If you have read the README and RELEASE-NOTES files, 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 to the list, use [65]libstdc++@gcc.gnu.org.
If you have a question that you think should be included here, or if
you have a question about a question/answer here, contact [66]Phil
Edwards or [67]Gabriel Dos Reis.
_________________________________________________________________
1.9 What are the license terms for libstdc++-v3?
See [68]our license description for these and related questions.
_________________________________________________________________
2.0 Installation
2.1 How do I install libstdc++-v3?
Complete instructions are not given here (this is a FAQ, not an
installation document), but the tools required are few:
* A 3.x release of GCC. Note that building GCC is much easier and
more automated than building the GCC 2.[78] series was. If you are
using GCC 2.95, you can still build earlier snapshots of
libstdc++.
* GNU Make is recommended, but should not be required.
* The GNU Autotools are needed if you are messing with the configury
or makefiles.
The file [69]documentation.html provides a good overview of the steps
necessary to build, install, and use the library. Instructions for
configuring the library with new flags such as --enable-threads are
there also, as well as patches and instructions for working with GCC
2.95.
The top-level install.html and [70]RELEASE-NOTES files contain the
exact build and installation instructions. You may wish to browse
those files over CVSweb ahead of time to get a feel for what's
required. RELEASE-NOTES is located in the ".../docs/17_intro/"
directory of the distribution.
_________________________________________________________________
2.2 [removed]
This question has become moot and has been removed. The stub is here
to preserve numbering (and hence links/bookmarks).
_________________________________________________________________
2.3 What is this CVS thing that you keep mentioning?
The Concurrent Versions System 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 [71]CVS entry in the GNU
software catalogue has a better description as well as a [72]link to
the makers of CVS.
The "anonymous client checkout" feature of CVS is similar to anonymous
FTP in that it allows anyone to retrieve the latest libstdc++ sources.
After the first of April, American users will have a "/pharmacy"
command-line option...
_________________________________________________________________
2.4 How do I know if it works?
libstdc++-v3 comes with its own testsuite. You do not need to actually
install the library ("make install") to run the testsuite.
To run the testsuite on the library after building it, use "make
check" while in your build directory. To run the testsuite on the
library after building and installing it, use "make check-install"
instead.
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, please write
up your idea and send it to the list!
_________________________________________________________________
2.4 This library is HUGE! And what's libsupc++?
Usually the size of libraries on disk isn't noticeable. When a link
editor (or simply "linker") 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++-v3 about this; it's just common behavior, given here for
background reasons.)
Some of the object files which make up libstdc++.a are rather large.
If you create a statically-linked executable with -static, 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++-v3 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.
It's not a bug, and it's not really a problem. Nevertheless, some
people don't like it, so here are two pseudo-solutions:
If the only functions from libstdc++.a which you need are language
support functions (those listed in [73]clause 18 of the standard,
e.g., new and delete), then try linking against libsupc++.a (usually
specifying -lsupc++ when calling g++ 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
libstdc++.a.
The second method is one we hope to incorporate into the library build
process. Some platforms 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.
Unfortunately the garbage collection in GNU ld is buggy; sections
(corresponding to functions and variables) which are used are
mistakenly removed, leading to horrible crashes when your executable
starts up. For the time being, this feature is not used when building
the library.
_________________________________________________________________
3.0 Platform-Specific Issues
3.1 Can libstdc++-v3 be used with <my favorite compiler>?
Probably not. Yet.
Because GCC advances so rapidly, development and testing of libstdc++
is being done almost entirely under that compiler. If you are curious
about whether other, lesser compilers (*grin*) support libstdc++, you
are more than welcome to try. Configuring and building the library
(see above) will still require certain tools, however. Also keep in
mind that building libstdc++ does not imply that your compiler will be
able to use all of the features found in the C++ Standard Library.
Since the goal of ISO Standardization is for all C++ implementations
to be able to share code, the final libstdc++ should, in theory, be
usable under any ISO-compliant compiler. It will still be targeted and
optimized for GCC/g++, however.
_________________________________________________________________
3.2 [removed]
This question has become moot and has been removed. The stub is here
to preserve numbering (and hence links/bookmarks).
_________________________________________________________________
3.3 [removed]
This question has become moot and has been removed. The stub is here
to preserve numbering (and hence links/bookmarks).
_________________________________________________________________
3.4 I can't use 'long long' on Solaris
By default we try to support the C99 long long type. This requires
that certain functions from your C library be present.
Up through release 3.0.2 the tests performed were too general, and
this feature was disabled when it did not need to be. The most
commonly reported platform affected was Solaris.
This has been fixed for 3.0.3 and onwards.
_________________________________________________________________
3.5 _XOPEN_SOURCE / _GNU_SOURCE / etc is always defined
On Solaris, g++ (but not gcc) always defines the preprocessor macro
_XOPEN_SOURCE. On GNU/Linux, the same happens with _GNU_SOURCE. (This
is not an exhaustive list; other macros and other platforms are also
affected.)
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-compatability reasons is often not the
default for many vendors.
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.
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.
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 "g++ -E -dM
- < /dev/null" to display a list of predefined macros for any
particular installation.
This has been discussed on the mailing lists [74]quite a bit.
This method is something of a wart. We'd like to find a cleaner
solution, but nobody yet has contributed the time.
_________________________________________________________________
3.6 OS X ctype.h is broken! How can I hack it?
This is a long-standing bug in the OS X support. Fortunately, the
patch is quite simple, and well-known. [75]Here's a link to the
solution.
_________________________________________________________________
3.7 Threading is broken on i386
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.
This is fixed in 3.2.2.
_________________________________________________________________
3.8 Recent GNU/Linux glibc required?
For 3.2.1 (shared library version 5.0.1) and later, the library 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.
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.)
_________________________________________________________________
3.9 Can't use wchar_t/wstring on FreeBSD
At the moment there are a few problems in FreeBSD's support for wide
character functions, and as a result the libstdc++ configury decides
that wchar_t support should be disabled. Once the underlying problems
are fixed in FreeBSD (soon), the library support will automatically
enable itself.
You can fix the problems yourself, and learn more about the situation,
by reading [76]this short thread ("_GLIBCPP_USE_WCHAR_T undefined in
FreeBSD's c++config.h?").
_________________________________________________________________
4.0 Known Bugs and Non-Bugs
Note that this section can get rapdily outdated -- such is the nature
of an open-source project. For the latest information, join the
mailing list or look through recent archives. The RELEASE- NOTES and
BUGS files are generally kept up-to-date.
For 3.0.1, the most common "bug" is an apparently missing "../" in
include/Makefile, resulting in files like gthr.h and gthr-single.h not
being found. Please read [77]the configuration instructions for GCC,
specifically the part about configuring in a separate build directory,
and how strongly recommended it is. Building in the source directory
is fragile, is rarely tested, and tends to break, as in this case.
This was fixed for 3.0.2.
For 3.1, the most common "bug" is a parse error when using <fstream>,
ending with a message, "bits/basic_file.h:52: parse error before `{'
token." Please read [78]the installation instructions for GCC,
specifically the part about not installing newer versions on top of
older versions. If you install 3.1 over a 3.0.x release, then the
wrong basic_file.h header will be found (its location changed between
releases).
Please do not report these as bugs. We know about them. Reporting this
-- or any other problem that's already been fixed -- hinders the
development of GCC, because we have to take time to respond to your
report. Thank you.
4.1 What works already?
This is a verbatim clip from the "Status" section of the RELEASE-NOTES
for the latest snapshot. For a list of fixed bugs, see that file.
New:
---
(post 3.0.97)
- more doxygen documentation
- more named locale fixups
- stdio_filebuf that takes fd, FILE
- io performance tuning
- allocation tuning, valgrind fixups
- __cxa_demangle now supported
(3.0.97)
- more doxygen documentation.
- more named locale bug fixes
- support for symbol versioning when using GNU ld >= 2.12
- wide-io
- tuning for executable size
(3.0.96)
- more doxygen documentation.
- extensions moved out of namespace std
- HPUX long long support
- more string optimizations
- support for NetBSD cross compiles
- concept_check merge from boost
- header simplification
- named locale bug shakeout
- thread testsuite
(3.0.95)
- add S390, m68k, x86-64 support.
- doxygen documentation has been extended, including man pages.
- verbose terminate handling has been added.
- some libsupc++ tweaks
- warnings for deprecated headers now active.
- dejagnu testsuite preliminary documentation.
- dejagnu testsuite default.
- dejagnu testsuite cross compiler, multilib safe.
- long long iostreams on by default, rework of ISO C99 support.
- iterator re-write and testsuites.
- container testsuites.
- allocator revamp and testsuites.
- more concept-checking work.
- basic_string optimization and MT fixes.
- new limits implementation.
- update -fno-exceptions code, verify it works.
- full named locale support fpr all facets, choice of gnu,
ieee_1003.1-200x (POSIX 2), or generic models. Full support depends
on target OS and underlying "C" library support.
_________________________________________________________________
4.2 Bugs in gcc/g++ (not libstdc++-v3)
This is by no means meant to be complete nor exhaustive, but mentions
some problems that users may encounter when building or using
libstdc++. If you are experiencing one of these problems, you can find
more information on the libstdc++ and the GCC mailing lists.
Before reporting a bug, examine the [79]bugs database with the
category set to "libstdc++". The BUGS file in the source tree also
tracks known serious problems.
* Debugging is problematic, due to bugs in line-number generation
(mostly fixed in the compiler) and gdb lagging behind the compiler
(lack of personnel). We recommend configuring the compiler using
--with-dwarf2 if the DWARF2 debugging format is not already the
default on your platform. Also, [80]changing your GDB settings can
have a profound effect on your C++ debugging experiences. :-)
_________________________________________________________________
4.3 Bugs in the C++ language/lib specification
Yes, unfortunately, there are some. In a [81]message to the list,
Nathan Myers announced that he has started a list of problems in the
ISO C++ Standard itself, especially with regard to the chapters that
concern the library. The list itself is [82]posted on his website.
Developers who are having problems interpreting the Standard may wish
to consult his notes.
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 [83]here.
Some of these have resulted in [84]code changes.
_________________________________________________________________
4.4 Things in libstdc++ that only look like bugs
There are things which are not bugs in the compiler (4.2) nor the
language specification (4.3), but aren't really bugs in libstdc++,
either. Really! Please do not report these as bugs.
-Weffc++ The biggest of these is the quadzillions of warnings about
the library headers emitted when -Weffc++ is used. Making libstdc++
"-Weffc++-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.
reopening a stream fails Did I just say that -Weffc++ was our biggest
false-bug report? I lied. (It used to be.) Today it seems to be
reports that after executing a sequence like
#include <fstream>
...
std::fstream fs("a_file");
// .
// . do things with fs...
// .
fs.close();
fs.open("a_new_file");
all operations on the re-opened fs will fail, or at least act very
strangely. Yes, they often will, especially if fs reached the EOF
state on the previous file. The reason is that the state flags are not
cleared on a successful call to open(). The standard unfortunately did
not specify behavior in this case, and to everybody's great sorrow,
the [85]proposed LWG resolution (see DR #22) is to leave the flags
unchanged. You must insert a call to fs.clear() between the calls to
close() and open(), and then everything will work like we all expect
it to work.
rel_ops Another is the rel_ops namespace and the template comparison
operator functions contained therein. If they become visible in the
same namespace as other comparison functions (e.g., 'using' 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 [86]sums things up here. The collisions with
vector/string iterator types have been fixed for 3.1.
The g++-3 headers are not ours
If you have found an extremely broken header file which is causing
problems for you, look carefully before submitting a "high" priority
bug report (which you probably shouldn't do anyhow; see the last
paragraph of the page describing [87]the GCC bug database).
If the headers are in ${prefix}/include/g++-3, or if the installed
library's name looks like libstdc++-2.10.a or libstdc++-libc6-2.10.so,
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.
For GCC versions 3.0 and 3.1 the libstdc++-v3 header files are
installed in ${prefix}/include/g++-v3 (see the 'v'?). Starting with
version 3.2 the headers are installed in
${prefix}/include/c++/${version} as this prevents headers from
previous versions being found by mistake.
glibc If you're on a GNU/Linux system and have just upgraded to glibc
2.2, but are still using gcc 2.95.2, then you should have read the
glibc FAQ, specifically 2.34:
2.34. When compiling C++ programs, I get a compilation error in streambuf.h.
{BH} You are using g++ 2.95.2? After upgrading to glibc 2.2, you need to
apply a patch to the include files in /usr/include/g++, because the fpos_t
type has changed in glibc 2.2. The patch is at
http://clisp.cons.org/~haible/gccinclude-glibc-2.2-compat.diff
Note that 2.95.x shipped with the [88]old v2 library which is no
longer maintained. Also note that gcc 2.95.3 fixes this problem, but
requires a separate patch for libstdc++-v3.
concept checks If you see compilation errors containing messages about
fooConcept and a constraints 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).
More information, including how to optionally enable/disable the
checks, is available [89]here.
dlopen/dlsym If you are using the C++ library across
dynamically-loaded objects, make certain that you are passing the
correct options when compiling and linking:
// compile the library components
g++ -fPIC -c a.cc
g++ -fPIC -c b.cc
...
g++ -fPIC -c z.cc
// create the 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
"memory leaks" in containers A few people have reported that the
standard containers appear to leak memory when tested with memory
checkers such as [90]valgrind. 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 [91]Tips for
memory leak hunting first.
_________________________________________________________________
4.5 Aw, that's easy to fix!
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
[92]submitting patches 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++ [93]contributors' page also
talks about how to submit patches.
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 [94]testsuite -- but only if such a test
exists.
_________________________________________________________________
5.0 Miscellaneous
5.1 string::iterator is not char*; vector<T>::iterator is not T*
If you have code that depends on container<T> iterators being
implemented as pointer-to-T, your code is broken.
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 T*
outweighs nearly all opposing arguments.
Code which does assume that a vector iterator i is a pointer can often
be fixed by changing i in certain expressions to &*i . Future
revisions of the Standard are expected to bless this usage for
vector<> (but not for basic_string<>).
_________________________________________________________________
5.2 What's next after libstdc++-v3?
Hopefully, not much. The goal of libstdc++-v3 is to produce a
fully-compliant, fully-portable Standard Library. After that, we're
mostly done: there won't be any more compliance work to do. However:
1. The ISO Committee will meet periodically to review Defect Reports
in the C++ Standard. Undoubtedly some of these will result in
changes to the Standard, which will be reflected in patches to
libstdc++. Some of that is already happening, see 4.2. Some of
those changes are being predicted by the library maintainers, and
we add code to the library based on what the current proposed
resolution specifies. Those additions are listed in [95]the
extensions page.
2. Performance tuning. Lots of performance tuning. This too is
already underway for post-3.0 releases, starting with memory
expansion in container classes and buffer usage in synchronized
stream objects.
3. An ABI for libstdc++ is being developed, so that multiple
binary-incompatible copies of the library can be replaced with a
single backwards-compatible library, like libgcc_s.so is.
4. The current libstdc++ contains extensions to the Library which
must be explicitly requested by client code (for example, the hash
tables from SGI). Other extensions may be added to libstdc++-v3 if
they seem to be "standard" enough. (For example, the "long long"
type from C99.) Bugfixes and rewrites (to improve or fix thread
safety, for instance) will of course be a continuing task.
[96]This question about the next libstdc++ prompted some brief but
interesting [97]speculation.
_________________________________________________________________
5.3 What about the STL from SGI?
The [98]STL from SGI, version 3.3, was the most recent merge of the
STL codebase. The code in libstdc++ contains many fixes and changes,
and it is very likely that the SGI code is no longer under active
development. We expect that no future merges will take place.
In particular, string is not from SGI and makes no use of their "rope"
class (which is included as an optional extension), nor is valarray
and some others. Classes like vector<> are, however.
The FAQ for SGI's STL (one jump off of their main page) is recommended
reading.
_________________________________________________________________
5.4 Extensions and Backward Compatibility
Headers in the ext and backward subdirectories should be referred to
by their relative paths:
#include <ext/hash_map>
rather than using -I or other options. This is more portable and
forward-compatible. (The situation is the same as that of other
headers whose directories are not searched directly, e.g.,
<sys/stat.h>, <X11/Xlib.h>.
The extensions are no longer in the global or std namespaces, instead
they are declared in the __gnu_cxx namespace. For maximum portability,
consider defining a namespace alias to use to talk about extensions,
e.g.:
#ifdef __GNUC__
#if __GNUC__ < 3
#include <hash_map.h>
namespace Sgi { using ::hash_map; }; // inherit globals
#else
#include <ext/hash_map>
#if __GNUC_MINOR__ == 0
namespace Sgi = std; // GCC 3.0
#else
namespace Sgi = ::__gnu_cxx; // GCC 3.1 and later
#endif
#endif
#else // ... there are other compilers, right?
namespace Sgi = std;
#endif
Sgi::hash_map<int,int> my_map;
This is a bit cleaner than defining typedefs for all the
instantiations you might need.
Extensions to the library have [99]their own page.
_________________________________________________________________
5.5 [removed]
This question has become moot and has been removed. The stub is here
to preserve numbering (and hence links/bookmarks).
_________________________________________________________________
5.6 Is libstdc++-v3 thread-safe?
When the system's libc is itself thread-safe, a non-generic
implementation of atomicity.h exists for the architecture, and gcc
itself reports a thread model other than single; libstdc++-v3 strives
to be thread-safe. The user-code must guard against concurrent method
calls which may access any particular library object's state.
Typically, the application programmer may infer what object locks must
be held based on the objects referenced in a method call. Without
getting into great detail, here is an example which requires
user-level locks:
library_class_a shared_object_a;
thread_main () {
library_class_b *object_b = new library_class_b;
shared_object_a.add_b (object_b); // must hold lock for shared_object_
a
shared_object_a.mutate (); // must hold lock for shared_object_
a
}
// Multiple copies of thread_main() are started in independent threads.
Under the assumption that object_a and object_b are never exposed to
another thread, here is an example that should not require any
user-level locks:
thread_main () {
library_class_a object_a;
library_class_b *object_b = new library_class_b;
object_a.add_b (object_b);
object_a.mutate ();
}
All library objects are safe to use in a multithreaded program as long
as each thread carefully locks out access by any other thread while it
uses any object visible to another thread. In general, this
requirement includes both read and write access to objects; unless
otherwise documented as safe, do not assume that two threads may
access a shared standard library object at the same time.
See chapters [100]17 (library introduction), [101]23 (containers), and
[102]27 (I/O) for more information.
_________________________________________________________________
5.7 How do I get a copy of the ISO C++ Standard?
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 [103]here. (And if you've already registered with them, clicking
this link will take you to directly to the place where you can
[104]buy the standard on-line.
Who is your country's member body? Visit the [105]ISO homepage and
find out!
_________________________________________________________________
5.8 What's an ABI and why is it so messy?
"ABI" stands for "Application Binary Interface." 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.
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
"free-standing implementation" that doesn't include (much of) the
standard library. It is a good basis for the work to come.
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.
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.
_________________________________________________________________
See [106]license.html for copying conditions. Comments and suggestions
are welcome, and may be sent to [107]the libstdc++ mailing list.
References
1. http://gcc.gnu.org/onlinedocs/libstdc++/faq/
2. http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html
3. http://gcc.gnu.org/libstdc++/
4. ../faq/index.html#1_0
5. ../faq/index.html#1_1
6. ../faq/index.html#1_2
7. ../faq/index.html#1_3
8. ../faq/index.html#1_4
9. ../faq/index.html#1_5
10. ../faq/index.html#1_6
11. ../faq/index.html#1_7
12. ../faq/index.html#1_8
13. ../faq/index.html#1_9
14. ../faq/index.html#2_0
15. ../faq/index.html#2_1
16. ../faq/index.html#2_2
17. ../faq/index.html#2_3
18. ../faq/index.html#2_4
19. ../faq/index.html#2_5
20. ../faq/index.html#3_0
21. ../faq/index.html#3_1
22. ../faq/index.html#3_2
23. ../faq/index.html#3_3
24. ../faq/index.html#3_4
25. ../faq/index.html#3_5
26. ../faq/index.html#3_6
27. ../faq/index.html#3_7
28. ../faq/index.html#3_8
29. ../faq/index.html#3_9
30. ../faq/index.html#4_0
31. ../faq/index.html#4_1
32. ../faq/index.html#4_2
33. ../faq/index.html#4_3
34. ../faq/index.html#4_4
35. ../faq/index.html#4_4_iostreamclear
36. ../faq/index.html#4_4_Weff
37. ../faq/index.html#4_4_rel_ops
38. ../faq/index.html#4_4_interface
39. ../faq/index.html#4_4_glibc
40. ../faq/index.html#4_4_checks
41. ../faq/index.html#4_4_dlsym
42. ../faq/index.html#4_4_leak
43. ../faq/index.html#4_5
44. ../faq/index.html#5_0
45. ../faq/index.html#5_1
46. ../faq/index.html#5_2
47. ../faq/index.html#5_3
48. ../faq/index.html#5_4
49. ../faq/index.html#5_5
50. ../faq/index.html#5_6
51. ../faq/index.html#5_7
52. ../faq/index.html#5_8
53. http://gcc.gnu.org/libstdc++/index.html#download
54. ../faq/index.html#1_4
55. ../faq/index.html#4_4_interface
56. ../17_intro/DESIGN
57. http://gcc.gnu.org/
58. http://gcc.gnu.org/gcc-3.0/buildstat.html
59. http://gcc.gnu.org/libstdc++/
60. http://gcc.gnu.org/libstdc++/index.html#download
61. http://gcc.gnu.org/libstdc++/
62. ../17_intro/contribute.html
63. http://www.boost.org/
64. http://gcc.gnu.org/extensions.html
65. mailto:libstdc++@gcc.gnu.org
66. mailto:pme@gcc.gnu.org
67. mailto:gdr@gcc.gnu.org
68. ../17_intro/license.html
69. ../documentation.html
70. ../17_intro/RELEASE-NOTES
71. http://www.gnu.org/software/cvs/cvs.html
72. http://www.cvshome.org/
73. ../18_support/howto.html
74. http://gcc.gnu.org/cgi-bin/htsearch?method=and&format=builtin-long&sort=score&words=_XOPEN_SOURCE+Solaris
75. http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html
76. http://gcc.gnu.org/ml/libstdc++/2003-02/subjects.html#00286
77. http://gcc.gnu.org/install/configure.html
78. http://gcc.gnu.org/install/
79. http://gcc.gnu.org/bugs.html
80. http://gcc.gnu.org/ml/libstdc++/2002-02/msg00034.html
81. http://gcc.gnu.org/ml/libstdc++/1998/msg00006.html
82. http://www.cantrip.org/draft-bugs.txt
83. http://anubis.dkuug.dk/jtc1/sc22/wg21/
84. ../faq/index.html#5_2
85. ../ext/howto.html#5
86. http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html
87. http://gcc.gnu.org/gnatswrite.html
88. ../faq/index.html#4_4_interface
89. ../19_diagnostics/howto.html#3
90. http://developer.kde.org/~sewardj/
91. ../debug.html#mem
92. http://gcc.gnu.org/contribute.html
93. ../17_intro/contribute.html
94. ../faq/index.html#2_4
95. ../ext/howto.html#5
96. http://gcc.gnu.org/ml/libstdc++/1999/msg00080.html
97. http://gcc.gnu.org/ml/libstdc++/1999/msg00084.html
98. http://www.sgi.com/Technology/STL/
99. ../ext/howto.html
100. ../17_intro/howto.html#3
101. ../23_containers/howto.html#3
102. ../27_io/howto.html#9
103. http://www.ansi.org/
104. http://webstore.ansi.org/ansidocstore/product.asp?sku=ISO%2FIEC+14882%2D1998
105. http://www.iso.ch/
106. ../17_intro/license.html
107. mailto:libstdc++@gcc.gnu.org
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