Subversion Repositories scarts

   #[1]GNU C++ Standard Library [2]Copyright

                     libstdc++ Frequently Asked Questions

   The latest version of this document is always available at

   [3]http://gcc.gnu.org/onlinedocs/libstdc++/faq/. The main

   documentation page is at

   [4]http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html.

   To the [5]libstdc++-v3 homepage.

     _________________________________________________________________

                                   Questions

    1. [6]General Information

         1. [7]What is libstdc++-v3?

         2. [8]Why should I use libstdc++?

         3. [9]Who's in charge of it?

         4. [10]How do I get libstdc++?

         5. [11]When is libstdc++ going to be finished?

         6. [12]How do I contribute to the effort?

         7. [13]What happened to libg++? I need that!

         8. [14]What if I have more questions?

         9. [15]What are the license terms for libstdc++-v3?

    2. [16]Installation

         1. [17]How do I install libstdc++-v3?

         2. [18][removed]

         3. [19]What is this CVS thing that you keep mentioning?

         4. [20]How do I know if it works?

         5. [21]This library is HUGE! And what's libsupc++?

         6. [22]Why do I get an error saying libstdc++.so.X is missing

            when I run my program?

    3. [23]Platform-Specific Issues

         1. [24]Can libstdc++-v3 be used with ?

         2. [25][removed]

         3. [26][removed]

         4. [27]I can't use 'long long' on Solaris

         5. [28]_XOPEN_SOURCE / _GNU_SOURCE / etc is always defined

         6. [29]OS X ctype.h is broken! How can I hack it?

         7. [30]Threading is broken on i386

         8. [31]Recent GNU/Linux glibc required?

         9. [32]Can't use wchar_t/wstring on FreeBSD

        10. [33]MIPS atomic operations

    4. [34]Known Bugs and Non-Bugs

         1. [35]What works already?

         2. [36]Bugs in gcc/g++ (not libstdc++-v3)

         3. [37]Bugs in the C++ language/lib specification

         4. [38]Things in libstdc++ that only look like bugs

               o [39]reopening a stream fails

               o [40]-Weffc++ complains too much

               o [41]"ambiguous overloads" after including an old-style

                 header

               o [42]The g++-3 headers are not ours

               o [43]compilation errors from streambuf.h

               o [44]errors about *Concept and constraints in the STL...

               o [45]program crashes when using library code in a

                 dynamically-loaded library

               o [46]"memory leaks" in containers

         5. [47]Aw, that's easy to fix!

    5. [48]Miscellaneous

         1. [49]string::iterator is not char*; vector::iterator is not

            T*

         2. [50]What's next after libstdc++-v3?

         3. [51]What about the STL from SGI?

         4. [52]Extensions and Backward Compatibility

         5. [53]Does libstdc++ support TR1?

         6. [54]Is libstdc++-v3 thread-safe?

         7. [55]How do I get a copy of the ISO C++ Standard?

         8. [56]What's an ABI and why is it so messy?

         9. [57]How do I make std::vector::capacity() ==

            std::vector::size?

     _________________________________________________________________

                            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. 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 [58]1.4 below).

   The older libstdc++-v2 project is no longer maintained; the code has

   been completely replaced and rewritten. [59]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

   [60]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/ compiler (gcc, g++, etc) is

   widely considered to be one of the leading compilers in the world. Its

   development is overseen by the [61]GCC team. All of the rapid

   development and near-legendary [62]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 [63]homepage.

   If you have questions, ideas, code, or are just curious, sign up!

     _________________________________________________________________

1.4 How do I get libstdc++?

   The [64]homepage has instructions for retrieving the latest CVS

   sources, and for browsing the CVS sources over the web.

   Stable versions of libstdc++-v3 are included with releases of [65]the

   GCC compilers.

   The subset commonly known as the Standard Template Library (chapters

   23 through 25, mostly) is adapted from the final release of the SGI

   STL, with extensive changes.

     _________________________________________________________________

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 [66]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 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 started

   such a project yet.

   (The [67]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 [68]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 [69]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 [70]Phil

   Edwards or [71]Gabriel Dos Reis.

     _________________________________________________________________

1.9 What are the license terms for libstdc++-v3?

   See [72]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 required for GCC 3.4 and later.

     * The GNU Autotools are needed if you are messing with the configury

       or makefiles.

   The file [73]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 [74]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 [75]CVS entry in the GNU

   software catalogue has a better description as well as a [76]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, but you do

   need DejaGNU, as described [77]here.

   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!

     _________________________________________________________________

2.5 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 [78]clause 18 of the standard,

   e.g., new and delete), then try linking against libsupc++.a (Using gcc

   instead of g++ and explicitly linking in -lsupc++ 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.

     _________________________________________________________________

2.6 Why do I get an error saying libstdc++.so.X is missing when I run my

program?

   Depending on your platform and library version, the message might be

   similar to one of the following:

    ./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

   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

   LD_LIBRARY_PATH environment variable, which is a colon-separated list

   of directories in which the linker will search for shared libraries:

    LD_LIBRARY_PATH=${prefix}/lib:$LD_LIBRARY_PATH

    export LD_LIBRARY_PATH

   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.

   See the man pages for ld(1), ldd(1) and ldconfig(8) for more

   information. The dynamic linker has different names on different

   platforms but the man page is usually called something such as ld.so /

   rtld / dld.so.

     _________________________________________________________________

                         3.0 Platform-Specific Issues

3.1 Can libstdc++-v3 be used with ?

   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 [79]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. [80]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?

   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.

   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 [81]this short thread ("_GLIBCPP_USE_WCHAR_T undefined in

   FreeBSD's c++config.h?").

     _________________________________________________________________

3.10 MIPS atomic operations

   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.

   mips*-*-linux* continues to use the MIPS II routines, and more work in

   this area is expected.

     _________________________________________________________________

                          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 [82]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 ,

   ending with a message, "bits/basic_file.h:52: parse error before `{'

   token." Please read [83]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?

   Short answer: Pretty much everything works except for some corner

   cases. Also, localization is incomplete. For whether it works well, or

   as you expect it to work, see 5.2.

   Long answer: See the docs/html/17_intro/CHECKLIST file, which is badly

   outdated... Also see the RELEASE-NOTES file, which is kept more up to

   date.

     _________________________________________________________________

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 [84]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, [85]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 [86]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 [87]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 [88]here.

   Some of these have resulted in [89]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 

    ...

    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 [90]proposed LWG resolution in 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. Update: for GCC 4.0 we implemented the resolution of

   [91]DR #409 and open() now calls clear() on success!

   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  header), then you will suddenly be faced with huge

   numbers of ambiguity errors. This was discussed on the -v3 list;

   Nathan Myers [92]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 [93]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 [94]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 [95]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 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

   "memory leaks" in containers A few people have reported that the

   standard containers appear to leak memory when tested with memory

   checkers such as [96]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 [97]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

   [98]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++ [99]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 [100]testsuite -- but only if such a test

   exists.

     _________________________________________________________________

                               5.0 Miscellaneous

5.1 string::iterator is not char*; vector::iterator is not T*

   If you have code that depends on container 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 [101]4.3. 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 [102]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.

    5. There is an effort underway to add significant extensions to the

       standard library specification. The latest version of this effort

       is described in [103]The C++ Library Technical Report 1. See

       [104]5.5.

   [105]This question about the next libstdc++ prompted some brief but

   interesting [106]speculation.

     _________________________________________________________________

5.3 What about the STL from SGI?

   The [107]STL from SGI, 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.

   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 we have made

   significant changes to them since then.

   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 

   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.,

   , .

   At this time most of the features of the SGI STL extension have been

   replaced by standardized libraries. In particular, the unordered_map

   and unordered_set containers of TR1 are suitable replacement for the

   non-standard hash_map and hash_set containers in the SGI STL. See

   [108]5.5 for more details.

   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 

        namespace Sgi { using ::hash_map; }; // inherit globals

      #else

        #include 

        #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 my_map;

   This is a bit cleaner than defining typedefs for all the

   instantiations you might need.

   Note: explicit template specializations must be declared in the same

   namespace as the original template. This means you cannot use a

   namespace alias when declaring an explicit specialization.

   Extensions to the library have [109]their own page.

     _________________________________________________________________

5.5 Does libstdc++ support TR1?

   The C++ Standard Library Technical Report adds many new features to

   the library. The latest version of this effort is described in

   [110]Technical Report 1.

   libstdc++ strives to implement all of TR1. An [111]overview of the

   implementation status is available.

   Briefly, the features of TR1 and the current status are:

   Reference_wrapper - Complete - Useful to pass references to functions

   that take their parameters by value.

   Reference-counted smart pointers - Complete - The shared_ptr and

   weak_ptr allow several object to know about a pointer and whether it

   is valid. When the last reference to the pointer is destroyed the

   pointer is freed.

   Function objects - Complete - Function return types (i.e, result_of),

   the functions template mem_fn (a generalization of mem_fun and

   mem_fun_red), function object binders (e.g, bind, a generalization of

   bind1st and bind2nd), and polymorhpic function wrappers (e.g, class

   template function).

   Type traits - Complete - The type_traits class gives templates the

   ability to probe information about the input type and enable

   type-dependent logic to be performed without the need of template

   specializations.

   Fixed-size arrays - Complete - The array class implements small

   fixed-sized arrays with container semantics.

   Unordered containers - Complete - The unordered_set, unordered_map,

   unordered_multiset, and unordered_multimap containers are hashed

   versions of the map, set, multimap, and multiset containers

   respectively. These classes are suitable replacements for the SGI STL

   hash_map and hash_set extensions.

   Tuples - Complete - The tuple class implements small heterogeneous

   arrays. This is an enhanced pair. In fact, the standard pair is

   enhanced with a tuple interface.

   C99 compatibility - Under construction - There are many features

   designed to minimize the divergence of the C and the C++ languages.

   Special functions - Under construction - Twenty-three mathematical

   functions familiar to physicists and engineers are included:

   cylindrical and spherical Bessel and Neumann functions, hypergeometric

   functions, Laguerre polynomials, Legendre functions, elliptic

   integrals, exponential integrals and the Riemann zeta function all for

   your computing pleasure.

   A regular expression engine This library provides for regular

   expression objects with traversal of text with return of

   subexpressions.

   A random number engine This library contains randow number generators

   with several different choices of distribution.

     _________________________________________________________________

5.6 Is libstdc++-v3 thread-safe?

   libstdc++-v3 strives to be thread-safe when all of the following

   conditions are met:

     * The system's libc is itself thread-safe,

     * gcc -v reports a thread model other than 'single',

     * [pre-3.3 only] a non-generic implementation of atomicity.h exists

       for the architecture in question.

   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, i.e., treat library objects

   like any other shared resource. 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 [112]17 (library introduction), [113]23 (containers), and

   [114]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 [115]here. (And if you've already registered with them, clicking

   this link will take you to directly to the place where you can

   [116]buy the standard on-line.

   Who is your country's member body? Visit the [117]ISO homepage and

   find out!

     _________________________________________________________________

5.8 What's an ABI and why is it so messy?