Frequently Asked Questions
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# Frequently Asked Questions

Copyright © 2008-2014 FSF

  1.1. What is libstdc++?
  1.2. Why should I use libstdc++?
  1.3. Who's in charge of it?
  1.4. When is libstdc++ going to be finished?
  1.5. How do I contribute to the effort?
  1.6. What happened to the older libg++? I need that!
  1.7. What if I have more questions?
  2.1. What are the license terms for libstdc++?
  2.2. So any program which uses libstdc++ falls under the GPL?
  2.3. How is that different from the GNU {Lesser,Library} GPL?
  2.4. I see. So, what restrictions are there on programs that use the library?
  3.1. How do I install libstdc++?
  3.2. How does one get current libstdc++ sources?
  3.3. How do I know if it works?
  3.4. How do I insure that the dynamically linked library will be found?
  3.5. What's libsupc++?
  3.6. This library is HUGE!
  4.1. Can libstdc++ be used with non-GNU compilers?
  4.2. No 'long long' type on Solaris?
  4.3. _XOPEN_SOURCE and _GNU_SOURCE are always defined?
  4.4. Mac OS X ctype.h is broken! How can I fix it?
  4.5. Threading is broken on i386?
  4.6. MIPS atomic operations
  4.7. Recent GNU/Linux glibc required?
  4.8. Can't use wchar_t/wstring on FreeBSD
  5.1. What works already?
  5.2. Bugs in the ISO C++ language or library specification
  5.3. Bugs in the compiler (gcc/g++) and not libstdc++
  6.1. Reopening a stream fails
  6.2. -Weffc++ complains too much
  6.3. Ambiguous overloads after including an old-style header
  6.4. The g++-3 headers are not ours
  6.5. Errors about *Concept and constraints in the STL
  6.6. Program crashes when using library code in a dynamically-loaded library
  6.7. “Memory leaks” in containers
  6.8. list::size() is O(n)!
  6.9. Aw, that's easy to fix!
  7.1. string::iterator is not char*; vector<T>::iterator is not T*
  7.2. What's next after libstdc++?
  7.3. What about the STL from SGI?
  7.4. Extensions and Backward Compatibility
  7.5. Does libstdc++ support TR1?
  7.6. How do I get a copy of the ISO C++ Standard?
  7.7. What's an ABI and why is it so messy?
  7.8. How do I make std::vector<T>::capacity() == std::vector<T>::size?

1.1. What is libstdc++?
1.2. Why should I use libstdc++?
1.3. Who's in charge of it?
1.4. When is libstdc++ going to be finished?
1.5. How do I contribute to the effort?
1.6. What happened to the older libg++? I need that!
1.7. What if I have more questions?

1.1. | What is libstdc++?
| The GNU Standard C++ Library v3 is an ongoing project to implement the ISO 14882 Standard C++ library as described in clauses 17 through 30 and annex D. For those who want to see exactly how far the project has come, or just want the latest bleeding-edge code, the up-to-date source is available over anonymous SVN, and can be browsed over the web.
1.2. | Why should I use libstdc++?

The completion of the initial ISO C++ standardization effort gave the C++ community a powerful set of reuseable tools in the form of the C++ Standard Library. However, for several years C++ implementations were (as the Draft Standard used to say) “incomplet and incorrekt”, and many suffered from limitations of the compilers that used them.

The GNU compiler collection (gcc, g++, etc) is widely considered to be one of the leading compilers in the world. Its development is overseen by the GCC team. All of the rapid development and near-legendary portability that are the hallmarks of an open-source project are applied to libstdc++.

All of the standard classes and functions from C++98/C++03 (such as `string`, `vector<>`, iostreams, algorithms etc.) are freely available and atempt to be fully compliant. Work is ongoing to complete support for the current revision of the ISO C++ Standard.

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 the Linux kernel. The current maintainers are listed in the `MAINTAINERS` file (look for "c++ runtime libs").

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 GCC mailing lists page. If you have questions, ideas, code, or are just curious, sign up!

1.4. | 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.
1.5. | How do I contribute to the effort?

See the Contributing section in the manual. 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 and is willing to provide details, is more than welcome!

1.6. | What happened to the older libg++? I need that!
| The last libg++ README states “This package is considered obsolete and is no longer being developed.” It should not be used for new projects, and won't even compile with recent releases of GCC (or most other C++ compilers). More information can be found in the Backwards Compatibility section of the libstdc++ manual.
1.7. | What if I have more questions?

If you have read the documentation, 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 a message 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, please send email to the libstdc++ mailing list, as above.

2.1. What are the license terms for libstdc++?
2.2. So any program which uses libstdc++ falls under the GPL?
2.3. How is that different from the GNU {Lesser,Library} GPL?
2.4. I see. So, what restrictions are there on programs that use the library?

2.1. | What are the license terms for libstdc++?
| See our license description for these and related questions.
2.2. | So any program which uses libstdc++ falls under the GPL?
| No. The special exception permits use of the library in proprietary applications.
2.3. | How is that different from the GNU {Lesser,Library} GPL?

The LGPL requires that users be able to replace the LGPL code with a modified version; this is trivial if the library in question is a C shared library. But there's no way to make that work with C++, where much of the library consists of inline functions and templates, which are expanded inside the code that uses the library. So to allow people to replace the library code, someone using the library would have to distribute their own source, rendering the LGPL equivalent to the GPL.

2.4. | I see. So, what restrictions are there on programs that use the library?
| None. We encourage such programs to be released as free software, but we won't punish you or sue you if you choose otherwise.

3.1. How do I install libstdc++?
3.2. How does one get current libstdc++ sources?
3.3. How do I know if it works?
3.4. How do I insure that the dynamically linked library will be found?
3.5. What's libsupc++?
3.6. This library is HUGE!

3.1. | How do I install libstdc++?

Often libstdc++ comes pre-installed as an integral part of many existing GNU/Linux and Unix systems, as well as many embedded development tools. It may be necessary to install extra development packages to get the headers, or the documentation, or the source: please consult your vendor for details.

To build and install from the GNU GCC sources, please consult the setup documentation for detailed instructions. You may wish to browse those files ahead of time to get a feel for what's required.

3.2. | How does one get current libstdc++ sources?

Libstdc++ sources for all official releases can be obtained as part of the GCC sources, available from various sites and mirrors. A full list of download sites is provided on the main GCC site.

Current libstdc++ sources can always be checked out of the main GCC source repository using the appropriate version control tool. At this time, that tool is Subversion.

Subversion, or SVN, 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 Subversion home page has a better description.

The “anonymous client checkout” feature of SVN is similar to anonymous FTP in that it allows anyone to retrieve the latest libstdc++ sources.

For more information see SVN details.

3.3. | How do I know if it works?

Libstdc++ comes with its own validation testsuite, which includes conformance testing, regression testing, ABI testing, and performance testing. Please consult the testing documentation for GCC and Testing in the libstdc++ manual for more details.

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!

3.4. | How do I insure that the dynamically linked library will be found?

Depending on your platform and library version, the error 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.

If you already have an older version of libstdc++ installed then the error might look like one of the following instead:

        ./a.out: /usr/lib/libstdc++.so.6: version `GLIBCXX_3.4.20' not found
        ./a.out: /usr/lib/libstdc++.so.6: version `CXXABI_1.3.8' not found

This means the linker found `/usr/lib/libstdc++.so.6` but that library belongs to an older version of GCC than was used to compile and link the program `a.out` (or some part of it). The program depends on code defined in the newer libstdc++ that belongs to the newer version of GCC, so the linker must be told how to find the newer libstdc++ shared library.

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:

        export LD_LIBRARY_PATH=${prefix}/lib:$LD_LIBRARY_PATH

Here the shell variable `${prefix}` is assumed to contain the directory prefix where GCC was installed to. The directory containing the library might depend on whether you want the 32-bit or 64-bit copy of the library, so for example would be `${prefix}/lib64` on some systems. 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, and `SHLIB_PATH` for HP-UX.

See the man pages for ld, ldd and ldconfig 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` or `dld.so`.

Using `LD_LIBRARY_PATH` is not always the best solution, Finding Dynamic or Shared Libraries in the manual gives some alternatives.

3.5. | What's libsupc++?

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`, which is a subset of `libstdc++.a`. (Using gcc instead of g++ and explicitly linking in `libsupc++.a` via `-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`.

3.6. | This library is HUGE!

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++ 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++ 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.

On supported platforms, libstdc++ takes advantage of garbage collection in the GNU linker to get a result similar to separating each symbol into a separate source and object files. On these platforms, GNU ld 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.

4.1. Can libstdc++ be used with non-GNU compilers?
4.2. No 'long long' type on Solaris?
4.3. _XOPEN_SOURCE and _GNU_SOURCE are always defined?
4.4. Mac OS X ctype.h is broken! How can I fix it?
4.5. Threading is broken on i386?
4.6. MIPS atomic operations
4.7. Recent GNU/Linux glibc required?
4.8. Can't use wchar_t/wstring on FreeBSD

4.1. | Can libstdc++ be used with non-GNU compilers?

Perhaps.

Since the goal of ISO Standardization is for all C++ implementations to be able to share code, libstdc++ should be usable under any ISO-compliant compiler, at least in theory.

However, the reality is that libstdc++ is targeted and optimized for GCC/G++. This means that often libstdc++ uses specific, non-standard features of G++ that are not present in older versions of proprietary compilers. It may take as much as a year or two after an official release of GCC that contains these features for proprietary tools to support these constructs.

Recent versions of libstdc++ are known to work with the Clang compiler. In the near past, specific released versions of libstdc++ have been known to work with versions of the EDG C++ compiler, and vendor-specific proprietary C++ compilers such as the Intel ICC C++ compiler.

4.2. | No 'long long' type 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 platform-specific tests performed by libstdc++ were too general, resulting in a conservative approach to enabling the long long code paths. The most commonly reported platform affected was Solaris.

This has been fixed for libstdc++ releases greater than 3.0.3.

4.3. | `_XOPEN_SOURCE` and `_GNU_SOURCE` are 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++98 standard library includes the C standard library, but it requires the C90 version, which for backwards-compatibility 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.

4.4. | Mac OS X `ctype.h` is broken! How can I fix it?

### Note

This answer is old and probably no longer be relevant.

This was a long-standing bug in the OS X support. Fortunately, the patch was quite simple, and well-known.

4.5. | Threading is broken on i386?

### Note

This answer is old and probably no longer be relevant.

Support for atomic integer operations 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.

4.6. | MIPS atomic operations

### Note

This answer is old and probably no longer be relevant.

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.

The mips*-*-linux* port continues to use the MIPS II routines, and more work in this area is expected.

4.7. | 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 which contains necessary bugfixes. All GNU/Linux distros make more recent versions available now. libstdc++ 4.6.0 and later require glibc 2.3 or later for this localization and formatting code.

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.)

4.8. | Can't use wchar_t/wstring on FreeBSD

### Note

This answer is old and probably no longer be relevant.

Older versions of FreeBSD's C library do not have sufficient support for wide character functions, and as a result the libstdc++ configury decides that wchar_t support should be disabled. In addition, the libstdc++ platform checks that enabled wchar_t were quite strict, and not granular enough to detect when the minimal support to enable wchar_t and C++ library structures like `wstring` were present. This impacted Solaris, Darwin, and BSD variants, and is fixed in libstdc++ versions post 4.1.0.

5.1. What works already?
5.2. Bugs in the ISO C++ language or library specification
5.3. Bugs in the compiler (gcc/g++) and not libstdc++

5.1. | What works already?
| Short answer: Pretty much everything works except for some corner cases. Support for localization in `locale` may be incomplete on some non-GNU platforms. Also dependent on the underlying platform is support for wchar_t and long long specializations, and details of thread support. Long answer: See the implementation status pages for C++98, TR1, C++11, C++14, and C++17.
5.2. | Bugs in the ISO C++ language or library specification

Unfortunately, there are some.

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 on the WG21 website. Many of these issues have resulted in code changes in libstdc++.

If you think you've discovered a new bug that is not listed, please post a message describing your problem to the author of the library issues list.

5.3. | Bugs in the compiler (gcc/g++) and not libstdc++

On occasion, the compiler is wrong. Please be advised that this happens much less often than one would think, and avoid jumping to conclusions.

First, examine the ISO C++ standard. Second, try another compiler or an older version of the GNU compilers. Third, you can find more information on the libstdc++ and the GCC mailing lists: search these lists with terms describing your issue.

Before reporting a bug, please examine the bugs database with the category set to “g++”.

6.1. Reopening a stream fails
6.2. -Weffc++ complains too much
6.3. Ambiguous overloads after including an old-style header
6.4. The g++-3 headers are not ours
6.5. Errors about *Concept and constraints in the STL
6.6. Program crashes when using library code in a dynamically-loaded library
6.7. “Memory leaks” in containers
6.8. list::size() is O(n)!
6.9. Aw, that's easy to fix!

6.1. | Reopening a stream fails

One of the most-reported non-bug reports. 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. Update: for GCC 4.0 we implemented the resolution of DR #409 and open() now calls `clear()` on success!

6.2. | -Weffc++ complains too much

Many warnings are 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.

We do, however, try to have libstdc++ sources as clean as possible. If you see some simple changes that pacify `-Weffc++` without other drawbacks, send us a patch.

6.3. | Ambiguous overloads after including an old-style header

### Note

This answer is old and probably no longer be relevant.

Another problem 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.

6.4. | The g++-3 headers are not ours

If you are using headers 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 non-standard and unmaintained. Do not report problems with -v2 to the -v3 mailing list.

For GCC versions 3.0 and 3.1 the libstdc++ 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.

6.5. | Errors about *Concept and constraints in the STL

If you see compilation errors containing messages about foo Concept and something to do with 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 in the Diagnostics. chapter of the manual.

6.6. | Program crashes when using library code in a dynamically-loaded library
| 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
6.7. | “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. Under some configurations the library's 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.

6.8. | list::size() is O(n)!
| See the Containers chapter.
6.9. | 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.

7.1. string::iterator is not char*; vector<T>::iterator is not T*
7.2. What's next after libstdc++?
7.3. What about the STL from SGI?
7.4. Extensions and Backward Compatibility
7.5. Does libstdc++ support TR1?
7.6. How do I get a copy of the ISO C++ Standard?
7.7. What's an ABI and why is it so messy?
7.8. How do I make std::vector<T>::capacity() == std::vector<T>::size?

7.1. | 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. It's considered a feature, not a bug, that libstdc++ points this out.

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<>).

7.2. | What's next after libstdc++?
| Hopefully, not much. The goal of libstdc++ 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. There is an effort underway to add significant extensions to the standard library specification. The latest version of this effort is described in The C++ Library Technical Report 1.
7.3. | What about the STL from SGI?

The STL from SGI, version 3.3, was the final merge of the STL codebase. The code in libstdc++ contains many fixes and changes, and the SGI code is no longer under active development. We expect that no future merges will take place.

In particular, `string` is not from SGI and makes no use of their "rope" class (which is included as an optional extension), nor is `valarray` and some others. Classes like `vector<>` are, but have been extensively modified.

More information on the evolution of libstdc++ can be found at the API evolution and backwards compatibility documentation.

The FAQ for SGI's STL is still recommended reading.

7.4. | Extensions and Backward Compatibility
| See the link on backwards compatibility and link on evolution.
7.5. | Does libstdc++ support TR1?
| Yes. The C++ Standard Library Technical Report adds many new features to the library. The latest version of this effort is described in Technical Report 1. The implementation status of TR1 in libstdc++ can be tracked on the TR1 status page.
7.6. | How do I get a copy of the ISO C++ Standard?
| Please refer to the Contributing section in our manual.
7.7. | 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 most CPU designers (for good reasons elaborated below) have not stepped up to publish C++ ABIs. Such an ABI has been defined for the Itanium architecture (see C++ ABI for Itanium) and that is used by G++ and other compilers as the de facto standard ABI on many common architectures (including x86). G++ can also use the ARM architecture's EABI, for embedded systems relying only on a “free-standing implementation” that doesn't include (much of) the standard library, and the GNU EABI for hosted implementations on ARM. Those ABIs cover low-level details such as virtual function implementation, struct inheritance layout, name mangling, and exception handling.

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.

7.8. | How do I make std::vector<T>::capacity() == std::vector<T>::size?
| The standard idiom for deallocating a `vector<T>`'s unused memory is to create a temporary copy of the vector and swap their contents, e.g. for `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.

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