# libstdc++ Frequently Asked Questions

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

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?
3. Platform-Specific Issues
   1. Can libstdc++-v3 be used with <my favorite compiler>?
   2. [removed]
   3. Building under DEC OSF kills the assembler
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 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 *Cconcept and constraints in the STL...
   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?

# 1.0 General Information

## 1.1 What is libstdc++-v3?

The GNU Standard C++ Library v3, or libstdc++-2.9x, 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 current release is the eleventh snapshot. 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 below).

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 incompatabilities.

## 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, and Ulrich Drepper 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 eleventh (and latest) snapshot of libstdc++-v3 is available via ftp. The filename is libstdc++-2.92.tar.gz.

The 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 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 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 FAQ 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.

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!

# 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 useable 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 Building DEC OSF kills the assembler

The `atomicity.h` header for the Alpha processor currently uses pseudo-operators which the DEC assembler doesn't understand (in particular, .subsection and .previous). The simple solution is to install GNU `as` and arrange for the GCC build to use it (or merge the sources and build it during the bootstrap).

Anyone who knows the DEC assembler well enough to provide the equivalent of these two pseudos would win praise and accolades from many.

# 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. Work has already gone into the source tree to make this less painful for the next release.

Please do not report this as a bug. We know about it. 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.

    New:
    ---
    - preliminary doxygen documentation has been added. Running "make
      doxygen" in the libstdc++-v3 build directory will generate HTML
      documentation that can be used to cross-reference names and files in
      the library.
    - a dejagnu based testing framework has been added
    - a new implementation of the concept checking code has been ported
      from the boost libraries.
    - support for -fno-exceptions has been added
    - stdexcept was re-written
    - using deprecated or antiquated headers now gives a warning
    - the stdio interface to iostreams has been tweaked, and now works
      with synchronized c/c++ io
    - new libsupc++ routines implementing the IA-64 C++ ABI.
    - HPUX configuration files
    - support for AIX added
    - a lot of bugs were fixed.
    - preliminary named locales implemented
    - portability improvements made to generation of <limits>
    - speedups to improve configuration time.
    - DJGPP support added.
    - support for dlopening shared libstdc++

## 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.

- As of 2.91, these bugs have all been fixed. We look forward to new ones, well, not exactly...

## 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 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. There are multiple solutions under discussion.

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 (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 sums things up here.

### 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`, 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.

Currently our header files are installed in `${prefix}/include/g++-v3` (see the 'v'?). This may change with the next release of GCC, as it may be too confusing, but the question has not yet been decided.

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.

## 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++ will eventually be 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.

The FAQ for SGI's STL (one jump off of their main page) is recommended reading.

## 5.4 Extensions and Backward Compatibility

Although you can specify `-I` options to make the preprocessor search the g++-v3/ext and /backward directories, it is better to refer to files there by their path, as in:

           #include <ext/hash_map>

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?

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. This requirement includes both read and write access to objects; do not assume that two threads may read a shared standard container 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!

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