# 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?
2. Installation
   1. How do I install libstdc++-v3?
   2. Is this a drop-in replacement for the libstdc++ that's shipped with g++?
   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. Building under Cygwin hangs/explodes!?
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
      The g++-3 headers are not ours
   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. Compiling with "-fnew-abi"
   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++-2.90.x/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 tenth 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 tenth (and latest) snapshot of libstdc++-v3 is available via ftp.

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 SGI STL, which is also an ongoing work.

## 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.

# 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 release of libstdc++.
- A recent release of GCC (version 2.95 works). Note that building GCC is much easier and more automated than building the GCC 2.[78] series was.
- If you plan on hacking around with the makefiles, you will need the tools autoconfand automake.
- GNU Make is the only make that supports these 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.

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 Is this a drop-in replacement for the libstdc++ that's shipped with g++?

Yes, as of 2.90.8, it is intended as such. And as of 2.91, libstdc++-v3 is the library that's shipped with g++, so much of this answer has become moot.

## 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 ("`gmake install`") to run the testsuite. Note that 2.91 does not use DejaGNU yet.

To run the testsuite on the library after building it, use "gmake check" while in your build directory. To run the testsuite on the library after building and installing it, use "gmake check-install" instead.

The testsuite subdirectory in your build directory will then contain three files of the form YYYYMMDD-mkcheck*.txt. One of them (-mkcheck.txt itself) contains the results of the tests; this can be mailed to the list. The other files (-mkchecklog.txt and -mkcheckfiles.txt) contain messages from the compiler while building the test programs, and a list of the tests to be run, respectively.

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 Building under Cygwin hangs/explodes!?

Sometimes, yes. You're probably in the middle of generating the `numeric_limits` specializations when it hangs, right? Thought so...

The `<limits>` header and its associated library code are platform-specific. These files get generated from scratch during installation, and it is this generator that is hanging. More specifically, the only sure way to determine what the `numeric_limits<T>::traps` boolean should be is to actually divide by zero and see if it is trapped or not.

Under NT, this will occasionally just hang. On those occasions when the test does not hang, the zero-division is in fact trapped. That doesn't prevent hanging elsewhere.

You have two options. You can get a newer cygwin1.dll (see the Cygwin paragraph in the installation instructions). Or you can get a prebuilt set of bits/std_limits.h and src/limitsMEMBERS.cc files from Mumit Khan's Cygwin-related website.

# 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.

## 4.1 What works already?

This is a verbatim clip from the "Status" section of the RELEASE-NOTES for the latest snapshot.

    New:
    - namespace std:: is now on by default.
    - choice of "C" include strategies, including the shadow header work,
      or generic global to std mapping of required "C" types.
    - cpu/atomicity.h tweaks, additions of ia64 and arm support.
    - abstraction of atomicity.h header to support notion of os/atomicity.h files.
    - addition of backward header bits
    - use of system_header pragma
    - Conditional use of -Werror
    - preliminary support for new g++ diagnostics capabilities, including
      -fdiagnostics-show-location=once
    - pedantic and shadow argument warning fixes
    - Ugly, yet correct mechanism for dealing with "C" math adopted,
      including the use of builtins.
    - updates and configure/build work to support new libtool
    - addition of strstream
    - valarray work
    - complex work
    - update to SGI STL 3.3
    - libio sync between glibc/libstdc++-v3. Some divergence since initial
      merge, but sources remain quite close.
    - IO fixes for alpha
    - wide character work for IO when using libio
    - addition of c_io_stdio and "C" IO abstraction layer.
    - auto_ptr fixes, testsuite additions
    - Attempts to use -ffunction-sections -fdata-sections and
      --gc-sections, depending on use of GNU ld and specific features. As of
      late, --gc-sections has been disabled due to problems with it throwing
      away initialization sections. This work is ongoing.
    - long double support
    - sub directory removal, coherent organization of cpu and os-specific
      files, consolidation of include directories, integration of the C++
      support bits for operator new/delete,exceptions, etc. All includes
      are now either in the include/* hierarchy or in libsupc++'s sub directory.
    - Support for more platforms, including irix and bsd variants.
    - filebuf tweaks to deal with variable-size buffers.
    - filebuf implementation for putbackc, etc. al.
    - ctype rewritten. Includes ctype, ctype, and others.
    - codecvt rewritten. Includes codecvt,
      codecvt. In addition,
      implementation-defined conversions using iconv are now supported with
      the __enc_traits partial-specialization of the State template
      parameter of the codecvt class. In this manner, conversions between
      encodings such as UCS4, USC2, UNICODE, UNICODEBIG, UNICODELITTLE, etc
      can be performed.
    - preliminary work on named locales
    - preliminary documentation for locale implementation has been established.
    - Many, many bug fixes.
    - Many, many testsuite additions and consistent VERIFY usage.
    - work on mkcheck to make it more flexible, use libtool, etc.

## 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.

## 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!

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.

### 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.

## 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 of course send the patch to our mailing list, not the GCC 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.

## 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.

The ISO Committee will meet periodically to review Defect Reports in the C++ Standard. Undoubtably 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.

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 is merged into libstdc++-v3 with changes as necessary. Currently release 3.3 is being used. Changes in the STL usually produce some weird bugs and lots of changes in the rest of the libstd++ source as we scramble to keep up. :-)

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 Compiling with "-fnew-abi"

Towards the end of July 1999, this subject was brought up again on the mailing list under a different name. The related thread (by the name HOWTO-honor-std) is very instructive. More info is at the end of RELEASE-NOTES.

This functionality is now automated and turned on by default.

## 5.6 Is libstdc++-v3 thread-safe?

Quick answer: no, as of 2.91 (tenth snapshot), the library is not appropriate for multithreaded access. The string class is MT-safe.

This is assuming that your idea of "multithreaded" is the same as ours... The general question of multithreading and libstdc++-v3 is addressed in the chapter-specific advice for Library Introduction. Threadsafe containers are covered in more detail in the Received Wisdom section on containers.

## 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!

Comments and suggestions are welcome, and may be sent to Phil Edwards or Gabriel Dos Reis.
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