# libstdc++ Frequently Asked Questions

The latest version of this document is always available at http://gcc.gnu.org/onlinedocs/libstdc++/faq/. The main documentation page is at http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html.

To the libstdc++-v3 homepage.

# Questions

1. General Information
   1. What is libstdc++-v3?
   2. Why should I use libstdc++?
   3. Who's in charge of it?
   4. How do I get libstdc++?
   5. When is libstdc++ going to be finished?
   6. How do I contribute to the effort?
   7. What happened to libg++? I need that!
   8. What if I have more questions?
   9. What are the license terms for libstdc++-v3?
2. Installation
   1. How do I install libstdc++-v3?
   2. [removed]
   3. What is this CVS thing that you keep mentioning?
   4. How do I know if it works?
   5. This library is HUGE! And what's libsupc++?
   6. Why do I get an error saying `libstdc++.so.X` is missing when I run my program?
3. Platform-Specific Issues
   1. Can libstdc++-v3 be used with <my favorite compiler>?
   2. [removed]
   3. [removed]
   4. I can't use 'long long' on Solaris
   5. `_XOPEN_SOURCE` / `_GNU_SOURCE` / etc is always defined
   6. OS X ctype.h is broken! How can I hack it?
   7. Threading is broken on i386
   8. Recent GNU/Linux glibc required?
   9. Can't use wchar_t/wstring on FreeBSD
   10. MIPS atomic operations
4. Known Bugs and Non-Bugs
   1. What works already?
   2. Bugs in gcc/g++ (not libstdc++-v3)
   3. Bugs in the C++ language/lib specification
   4. Things in libstdc++ that only look like bugs
      - reopening a stream fails
      - -Weffc++ complains too much
      - "ambiguous overloads" after including an old-style header
      - The g++-3 headers are not ours
      - compilation errors from streambuf.h
      - errors about *Concept and constraints in the STL...
      - program crashes when using library code in a dynamically-loaded library
      - "memory leaks" in containers
   5. Aw, that's easy to fix!
5. Miscellaneous
   1. string::iterator is not char*; vector<T>::iterator is not T*
   2. What's next after libstdc++-v3?
   3. What about the STL from SGI?
   4. Extensions and Backward Compatibility
   5. [removed]
   6. Is libstdc++-v3 thread-safe?
   7. How do I get a copy of the ISO C++ Standard?
   8. What's an ABI and why is it so messy?
   9. How do I make std::vector<T>::capacity() == std::vector<T>::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. As the library reaches stable plateaus, it is captured in a snapshot and released. The latest release is 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 1.4 below).

The older libstdc++-v2 project is no longer maintained; the code has been completely replaced and rewritten. 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 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 GCC team. All of the rapid development and near-legendary 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 homepage. If you have questions, ideas, code, or are just curious, sign up!

## 1.4 How do I get libstdc++?

The 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 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.

## 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 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 started such a project yet.

(The 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 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 `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 Phil Edwards or Gabriel Dos Reis.

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

See 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 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 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 CVS entry in the GNU software catalogue has a better description as well as a 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 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 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 <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 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. 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 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 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 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...

What follows 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:

## 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 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, 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 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 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 here. Some of these have resulted in 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 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.

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 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 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 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 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 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 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 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++ 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 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 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.

This question about the next libstdc++ prompted some brief but interesting speculation.

## 5.3 What about the STL from SGI?

The 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 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 <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.

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 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?

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 17 (library introduction), 23 (containers), and 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 here. (And if you've already registered with them, clicking this link will take you to directly to the place where you can buy the standard on-line.

Who is your country's member body? Visit the 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.

## 5.9 How do I make std::vector<T>::capacity() == std::vector<T>::size()?

The standard idiom for deallocating a `std::vector<T>`'s unused memory is to create a temporary copy of the vector and swap their contents, e.g. for `std::vector<T> v`

         std::vector<T>(v).swap(v);

The copy will take O(n) time and the swap is constant time.

See Shrink-to-fit strings for a similar solution for strings.

See license.html for copying conditions. Comments and suggestions are welcome, and may be sent to the libstdc++ mailing list.