# libstdc++ Frequently Asked Questions

The latest version of this document is always available at http://sourceware.cygnus.com/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
   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, 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 ninth 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 (EGCS) or Linux. Benjamin Kosnik, Gabriel Dos Reis, Nathan Myers, 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 ninth (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 in a recent Usenet article.

## 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++@sourceware.cygnus.com`.

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.

The installation instructions cover this in more detail, but replacing the older library requires rebuilding some of the code that comes with g++. You will need sources for the 2.95.2 compiler in order to build this snapshot. Building the library on its own and then using -I/-L will no longer work.

After the 2.90.8 snapshot, the library sources were integrated into the compiler sources. Future releases of the compiler will ship with libstdc++-v3.

## 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 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.

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 are using the libgcc.a-rebuilding method to enable std:: you might find that the testsuite starts dying with nasty linker errors. This is symptomatic of the rebuilt libgcc.a not being installed; the previous one is still in use.

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 wait until the signal-handling in Cygwin is reworked and released; this is planned to take "a few weeks" and will result in a fix sometime in late December or early January. 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:
    ---
    - MT safe string. Supported CPUs are alpha, powerpc, x86, sparc32 and sparc64.
    - Configure support for --enable-threads=posix, as well as initial IO
      locking implementation.
    - Support for native building on Solaris 2.5.1, Solaris 2.6, Solaris
      2.7, cygwin, [alpha, powerpc, x86]-linux, and preliminary support for
      Irix and Aix4.2, Aix 4.3 hosts.
    - --enable-namespaces is on by default.
    - Configure and Makefile support for "drop-in" replacement to
      libstdc++-v2 completed. It is now possible to bootstrap g++, and
      have g++ find libstdc++-v3 headers and libraries by default.
    - Synched with CVS egcs libio.
    - Cygwin native compiling supported.
    - Cross compiling and embedded targets (newlib) with multilibs support added.
    - SGI's strstream implementation has been added.
    - Copyright on all sources assigned to the FSF.
    - Configure, build and install documentation has been added.
    - Support to enable long long has been added.
    - More valarray improvements.
    - Extractors and inserters for std::complex have been added.
    - Extractors and inserters for void* have been fixed.
    - autoconf macros are now in _GLIBCPP_ namespace.
    - group checking for num_get implemented.
    - Many, many bug fixes.

## 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.

- When using associative containers (like `map`), and compiling with `-O3` (or `-finline-functions`) and `-Winline`, I get a zillion errors like:

         .../include/g++/stl_tree.h: In function `int __black_count(struct __rb_tree_node_base *, struct __rb_tree_node_base *)':
         .../include/g++/stl_tree.h:1045: warning: can't inline call to `int __black_count(struct __rb_tree_node_base *, struct __rb_tree_node_base *)'
         .../include/g++/stl_tree.h:1053: warning: called from here

  This has been discussed a number of times; the problem is that __black_count is marked inline but is also a recursive function. As of 12 July 1999, it has been rewritten into an optimized non-recursive form, so fresh checkouts/releases should no longer see this warning. (The compiler can usually figure out how to make that transformation on its own.)

- The reason that EGCS 1.1.2 cannot be used to build the library is that lookups do not work properly, and so the compiler will ICE when building the locale-related source files. This has been fixed in GCC 2.95.

## 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.

## 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.

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 3.2 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>

## 5.5 Compiling with "-fnew-abi"

The library mostly works if you compile it (and programs you link with it) using "-fnew-abi -fno-honor-std" on a vanilla GCC compiler. However, some features, such as RTTI and error handlers, might not link properly with a vanilla libgcc built in GCC under the old ABI. If you rebuild libgcc using the "-f" flags above, you can get both complete language support and full benefits of -fnew-abi -- short mangled symbol names, far more efficient exception handling, and empty base optimization, to name a few. (Note that the new ABI may change from one GCC snapshot to the next, so you would have to rebuild all your libraries each time you get a new compiler snapshot.)

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 2.90.8 (ninth 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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