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  23 <h1 class="centered">libstdc++ Frequently Asked Questions</h1>
  24
  25 <p class="fineprint"><em>
  26    The latest version of this document is always available at
  27    <a href="http://gcc.gnu.org/onlinedocs/libstdc++/faq/">
  28    http://gcc.gnu.org/onlinedocs/libstdc++/faq/</a>.  The main documentation
  29    page is at
  30    <a href="http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html">
  31    http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html</a>.
  32 </em></p>
  33
  34 <p><em>
  35    To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
  36 </em></p>
  37
  38 <!-- ####################################################### -->
  39 <hr />
  40 <h1>Questions</h1>
  41 <ol>
  42    <li><a href="#1_0">General Information</a>
  43    <!-- I suspect these will mostly be links to/into existing documents. -->
  44    <ol>
  45       <li><a href="#1_1">What is libstdc++-v3?</a> </li>
  46       <li><a href="#1_2">Why should I use libstdc++?</a> </li>
  47       <li><a href="#1_3">Who's in charge of it?</a> </li>
  48       <li><a href="#1_4">How do I get libstdc++?</a> </li>
  49       <li><a href="#1_5">When is libstdc++ going to be finished?</a> </li>
  50       <li><a href="#1_6">How do I contribute to the effort?</a> </li>
  51       <li><a href="#1_7">What happened to libg++?  I need that!</a> </li>
  52       <li><a href="#1_8">What if I have more questions?</a> </li>
  53       <li><a href="#1_9">What are the license terms for libstdc++-v3?</a> </li>
  54    </ol>
  55    </li>
  56
  57    <li><a href="#2_0">Installation</a>
  58       <ol>
  59          <li><a href="#2_1">How do I install libstdc++-v3?</a> </li>
  60          <li><a href="#2_2">[removed]</a> </li>
  61          <li><a href="#2_3">What is this CVS thing that you keep
  62                             mentioning?</a> </li>
  63          <li><a href="#2_4">How do I know if it works?</a> </li>
  64          <li><a href="#2_5">This library is HUGE!  And what's libsupc++?</a> </li>
  65       </ol>
  66    </li>
  67
  68    <li><a href="#3_0">Platform-Specific Issues</a>
  69       <ol>
  70          <li><a href="#3_1">Can libstdc++-v3 be used with &lt;my
  71                             favorite compiler&gt;?</a> </li>
  72          <li><a href="#3_2">[removed]</a> </li>
  73          <li><a href="#3_3">[removed]</a> </li>
  74          <li><a href="#3_4">I can't use 'long long' on Solaris</a> </li>
  75          <li><a href="#3_5"><code>_XOPEN_SOURCE</code> /
  76                             <code>_GNU_SOURCE</code> / etc is always defined</a>
  77          </li>
  78          <li><a href="#3_6">OS X ctype.h is broken!  How can I hack it?</a></li>
  79          <li><a href="#3_7">Threading is broken on i386</a></li>
  80          <li><a href="#3_8">Recent GNU/Linux glibc required?</a></li>
  81          <li><a href="#3_9">Can't use wchar_t/wstring on FreeBSD</a></li>
  82          <li><a href="#3_10">MIPS atomic operations</a></li>
  83       </ol>
  84    </li>
  85
  86    <li><a href="#4_0">Known Bugs and Non-Bugs</a>
  87       <ol>
  88          <li><a href="#4_1">What works already?</a> </li>
  89          <li><a href="#4_2">Bugs in gcc/g++ (not libstdc++-v3)</a> </li>
  90          <li><a href="#4_3">Bugs in the C++ language/lib specification</a> </li>
  91          <li><a href="#4_4">Things in libstdc++ that only look like bugs</a><ul>
  92            <li><a href="#4_4_iostreamclear">reopening a stream fails</a> </li>
  93            <li><a href="#4_4_Weff">-Weffc++ complains too much</a> </li>
  94            <li><a href="#4_4_rel_ops">&quot;ambiguous overloads&quot;
  95                                after including an old-style header</a> </li>
  96            <li><a href="#4_4_interface">The g++-3 headers are
  97                                <strong>not ours</strong></a> </li>
  98            <li><a href="#4_4_glibc">compilation errors from streambuf.h</a> </li>
  99            <li><a href="#4_4_checks">errors about <em>*Concept</em> and
 100                                <em>constraints</em> in the STL...</a> </li>
 101            <li><a href="#4_4_dlsym">program crashes when using library code
 102                                in a dynamically-loaded library</a> </li>
 103            <li><a href="#4_4_leak">"memory leaks" in containers</a> </li>
 104          </ul>
 105          </li>
 106          <li><a href="#4_5">Aw, that's easy to fix!</a> </li>
 107       </ol>
 108    </li>
 109
 110    <li><a href="#5_0">Miscellaneous</a>
 111       <ol>
 112          <li><a href="#5_1">string::iterator is not char*;
 113                             vector&lt;T&gt;::iterator is not T*</a> </li>
 114          <li><a href="#5_2">What's next after libstdc++-v3?</a> </li>
 115          <li><a href="#5_3">What about the STL from SGI?</a> </li>
 116          <li><a href="#5_4">Extensions and Backward Compatibility</a> </li>
 117          <li><a href="#5_5">[removed]</a> </li>
 118          <li><a href="#5_6">Is libstdc++-v3 thread-safe?</a> </li>
 119          <li><a href="#5_7">How do I get a copy of the ISO C++ Standard?</a> </li>
 120          <li><a href="#5_8">What's an ABI and why is it so messy?</a> </li>
 121          <li><a href="#5_9">How do I make std::vector&lt;T&gt;::capacity()
 122                             == std::vector&lt;T&gt;::size?</a> </li>
 123       </ol>
 124    </li>
 125
 126 </ol>
 127
 128 <hr />
 129
 130 <!-- ####################################################### -->
 131
 132 <h1><a name="1_0">1.0 General Information</a></h1>
 133 <!-- I suspect these will mostly be links to/into existing documents. -->
 134    <h2><a name="1_1">1.1 What is libstdc++-v3?</a></h2>
 135       <p>The GNU Standard C++ Library v3 is an
 136          ongoing project to implement the ISO 14882 Standard C++ library
 137          as described in chapters 17 through 27 and annex D.  As the
 138          library reaches stable plateaus, it is captured in a snapshot
 139          and released.  The latest release is
 140          <a href="http://gcc.gnu.org/libstdc++/index.html#download">the
 141          fourteenth snapshot</a> but newer versions have been included
 142          in recent GCC releases.  For those who want to see exactly how
 143          far the project has come, or just want the latest
 144          bleeding-edge code, the up-to-date source is available over
 145          anonymous CVS, and can even be browsed over the Web (see
 146          <a href="#1_4">1.4</a> below).
 147       </p>
 148       <p>The older libstdc++-v2 project is no longer maintained; the code
 149          has been completely replaced and rewritten.
 150          <a href="#4_4_interface">If you are using V2</a>, then you need to
 151          report bugs to your system vendor, not to the V3 list.
 152       </p>
 153       <p>A more formal description of the V3 goals can be found in the
 154          official <a href="../17_intro/DESIGN">design document</a>.
 155       </p>
 156
 157 <hr />
 158    <h2><a name="1_2">1.2 Why should I use libstdc++?</a></h2>
 159       <p>The completion of the ISO C++ standardization gave the
 160          C++ community a powerful set of reuseable tools in the form
 161          of the C++ Standard Library.  However, all existing C++
 162          implementations are (as the Draft Standard used to say)
 163          &quot;incomplet and incorrekt,&quot; and many suffer from
 164          limitations of the compilers that use them.
 165       </p>
 166       <p>The GNU C/C++/FORTRAN/&lt;pick-a-language&gt; compiler
 167          (<code>gcc</code>, <code>g++</code>, etc) is widely considered to be
 168          one of the leading compilers in the world.  Its development
 169          has recently been taken over by the
 170          <a href="http://gcc.gnu.org/">GCC team</a>.  All of
 171          the rapid development and near-legendary
 172          <a href="http://gcc.gnu.org/gcc-3.0/buildstat.html">portability</a>
 173          that are the hallmarks of an open-source project are being
 174          applied to libstdc++.
 175       </p>
 176       <p>That means that all of the Standard classes and functions
 177          (such as <code>string</code>, <code>vector&lt;&gt;</code>, iostreams,
 178          and algorithms) will be freely available and fully compliant.
 179          Programmers will no longer need to &quot;roll their own&quot;
 180          nor be worried about platform-specific incompatibilities.
 181       </p>
 182
 183 <hr />
 184    <h2><a name="1_3">1.3 Who's in charge of it?</a></h2>
 185       <p>The libstdc++ project is contributed to by several developers
 186          all over the world, in the same way as GCC or Linux.
 187          Benjamin Kosnik, Gabriel Dos Reis, Phil Edwards, Ulrich Drepper,
 188          Loren James Rittle, and Paolo Carlini are the lead maintainers of
 189          the CVS archive.
 190       </p>
 191       <p>Development and discussion is held on the libstdc++ mailing
 192          list.  Subscribing to the list, or searching the list
 193          archives, is open to everyone.  You can read instructions for
 194          doing so on the <a href="http://gcc.gnu.org/libstdc++/">homepage</a>.
 195          If you have questions, ideas, code, or are just curious, sign up!
 196       </p>
 197
 198 <hr />
 199    <h2><a name="1_4">1.4 How do I get libstdc++?</a></h2>
 200       <p>The <a href="http://gcc.gnu.org/libstdc++/">homepage</a>
 201          has instructions for retrieving the latest CVS sources, and for
 202          browsing the CVS sources over the web.
 203       </p>
 204       <p>Stable versions of libstdc++-v3 are included with releases of
 205          <a href="http://gcc.gnu.org/releases.html">the GCC compilers</a>.
 206       </p>
 207       <p>The subset commonly known as the Standard Template Library
 208          (chapters 23 through 25, mostly) is adapted from the final release
 209          of the SGI STL.
 210       </p>
 211
 212 <hr />
 213    <h2><a name="1_5">1.5 When is libstdc++ going to be finished?</a></h2>
 214 <!--      <p>Nathan Myers gave the best of all possible answers in <a
 215          href="http://www.deja.com/getdoc.xp?AN=469581698&fmt=text">a
 216          Usenet article</a>.</p>
 217 which is no longer available, thanks deja...-->
 218       <p>Nathan Myers gave the best of all possible answers, responding to a
 219          Usenet article asking this question:  <em>Sooner, if you help.</em>
 220       </p>
 221
 222 <hr />
 223    <h2><a name="1_6">1.6 How do I contribute to the effort?</a></h2>
 224       <p>Here is <a href="../17_intro/contribute.html">a
 225          page devoted to this topic</a>.  Subscribing to the mailing
 226          list (see above, or the homepage) is a very good idea if you
 227          have something to contribute, or if you have spare time and
 228          want to help.  Contributions don't have to be in the form of
 229          source code; anybody who is willing to help write
 230          documentation, for example, or has found a bug in code that
 231          we all thought was working, is more than welcome!
 232       </p>
 233
 234 <hr />
 235    <h2><a name="1_7">1.7 What happened to libg++?  I need that!</a></h2>
 236       <p>The most recent libg++ README states that libg++ is no longer
 237          being actively maintained.  It should not be used for new
 238          projects, and is only being kicked along to support older code.
 239       </p>
 240       <p>The libg++ was designed and created when there was no Standard
 241          to provide guidance.  Classes like linked lists are now provided
 242          for by <code>list&lt;T&gt;</code> and do not need to be created by
 243          <code>genclass</code>.  (For that matter, templates exist now and
 244          are well-supported, whereas genclass (mostly) predates them.)
 245       </p>
 246       <p>There are other classes in libg++ that are not specified in the
 247          ISO Standard (e.g., statistical analysis).  While there are a
 248          lot of really useful things that are used by a lot of people
 249          (e.g., statistics :-), the Standards Committee couldn't include
 250          everything, and so a lot of those &quot;obvious&quot; classes
 251          didn't get included.
 252       </p>
 253       <p>Since libstdc++ is an implementation of the Standard Library, we
 254          have no plans at this time to include non-Standard utilities
 255          in the implementation, however handy they are.  (The extensions
 256          provided in the SGI STL aren't maintained by us and don't get
 257          a lot of our attention, because they don't require a lot of our
 258          time.)  It is entirely plausable that the &quot;useful stuff&quot;
 259          from libg++ might be extracted into an updated utilities library,
 260          but nobody has started such a project yet.
 261       </p>
 262       <p>(The <a href="http://www.boost.org/">Boost</a> site houses free
 263          C++ libraries that do varying things, and happened to be started
 264          by members of the Standards Committee.  Certain &quot;useful
 265          stuff&quot; classes will probably migrate there.)
 266       </p>
 267       <p>For the bold and/or desperate, the
 268          <a href="http://gcc.gnu.org/extensions.html">GCC extensions page</a>
 269          describes where to find the last libg++ source.
 270       </p>
 271
 272 <hr />
 273    <h2><a name="1_8">1.8 What if I have more questions?</a></h2>
 274       <p>If you have read the README and RELEASE-NOTES files, and your
 275          question remains unanswered, then just ask the mailing list.
 276          At present, you do not need to be subscribed to the list to
 277          send a message to it.  More information is available on the
 278          homepage (including how to browse the list archives); to send
 279          to the list, use <a href="mailto:libstdc++@gcc.gnu.org">
 280          <code>libstdc++@gcc.gnu.org</code></a>.
 281       </p>
 282       <p>If you have a question that you think should be included here,
 283          or if you have a question <em>about</em> a question/answer here,
 284          contact <a href="mailto:pme@gcc.gnu.org">Phil Edwards</a>
 285          or <a href="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</a>.
 286       </p>
 287
 288 <hr />
 289    <h2><a name="1_9">1.9 What are the license terms for libstdc++-v3?</a></h2>
 290       <p>See <a href="../17_intro/license.html">our license description</a>
 291          for these and related questions.
 292       </p>
 293
 294 <hr />
 295 <h1><a name="2_0">2.0 Installation</a></h1>
 296    <h2><a name="2_1">2.1 How do I install libstdc++-v3?</a></h2>
 297       <p>Complete instructions are not given here (this is a FAQ, not
 298          an installation document), but the tools required are few:
 299       </p>
 300          <ul>
 301             <li> A 3.x release of GCC.  Note that building GCC is much
 302                  easier and more automated than building the GCC 2.[78]
 303                  series was.  If you are using GCC 2.95, you can still
 304                  build earlier snapshots of libstdc++.
 305             </li>
 306             <li> GNU Make is recommended, but should not be required.
 307             </li>
 308             <li> The GNU Autotools are needed if you are messing with
 309                  the configury or makefiles.
 310             </li>
 311          </ul>
 312       <p>The file <a href="../documentation.html">documentation.html</a>
 313          provides a good overview of the steps necessary to build, install,
 314          and use the library.  Instructions for configuring the library
 315          with new flags such as --enable-threads are there also, as well as
 316          patches and instructions for working with GCC 2.95.
 317       </p>
 318       <p>The top-level install.html and
 319          <a href="../17_intro/RELEASE-NOTES">RELEASE-NOTES</a> files contain
 320          the exact build and installation instructions.  You may wish to
 321          browse those files over CVSweb ahead of time to get a feel for
 322          what's required.  RELEASE-NOTES is located in the
 323          &quot;.../docs/17_intro/&quot; directory of the distribution.
 324       </p>
 325
 326 <hr />
 327    <h2><a name="2_2">2.2 [removed]</a></h2>
 328       <p>This question has become moot and has been removed.  The stub
 329          is here to preserve numbering (and hence links/bookmarks).
 330       </p>
 331
 332 <hr />
 333    <h2><a name="2_3">2.3 What is this CVS thing that you
 334                          keep mentioning?</a></h2>
 335       <p>The <em>Concurrent Versions System</em> is one of several revision
 336          control packages.  It was selected for GNU projects because it's
 337          free (speech), free (beer), and very high quality.  The <a
 338          href="http://www.gnu.org/software/cvs/cvs.html">CVS entry in
 339          the GNU software catalogue</a> has a better description as
 340          well as a
 341          <a href="http://www.cvshome.org/">link to the makers of CVS</a>.
 342       </p>
 343       <p>The &quot;anonymous client checkout&quot; feature of CVS is
 344          similar to anonymous FTP in that it allows anyone to retrieve
 345          the latest libstdc++ sources.
 346       </p>
 347       <p>After the first of April, American users will have a
 348          &quot;/pharmacy&quot; command-line option...
 349          <!-- wonder how long that'll live -->
 350       </p>
 351
 352 <hr />
 353    <h2><a name="2_4">2.4 How do I know if it works?</a></h2>
 354       <p>libstdc++-v3 comes with its own testsuite.  You do not need
 355          to actually install the library (&quot;<code>make
 356          install</code>&quot;) to run the testsuite, but you do need
 357          DejaGNU, as described
 358          <a href="http://gcc.gnu.org/install/test.html">here</a>.
 359       </p>
 360       <p>To run the testsuite on the library after building it, use
 361          &quot;make check&quot; while in your build directory.  To run
 362          the testsuite on the library after building and installing it,
 363          use &quot;make check-install&quot; instead.
 364       </p>
 365       <p>If you find bugs in the testsuite programs themselves, or if you
 366          think of a new test program that should be added to the suite,
 367          <strong>please</strong> write up your idea and send it to the list!
 368       </p>
 369
 370 <hr />
 371    <h2><a name="2_5">2.5 This library is HUGE!  And what's libsupc++?</a></h2>
 372       <p>Usually the size of libraries on disk isn't noticeable.  When a
 373          link editor (or simply &quot;linker&quot;) pulls things from a
 374          static archive library, only the necessary object files are copied
 375          into your executable, not the entire library.  Unfortunately, even
 376          if you only need a single function or variable from an object file,
 377          the entire object file is extracted.  (There's nothing unique to C++
 378          or libstdc++-v3 about this; it's just common behavior, given here
 379          for background reasons.)
 380       </p>
 381       <p>Some of the object files which make up libstdc++.a are rather large.
 382          If you create a statically-linked executable with
 383          <code> -static</code>, those large object files are suddenly part
 384          of your executable.  Historically the best way around this was to
 385          only place a very few functions (often only a single one) in each
 386          source/object file; then extracting a single function is the same
 387          as extracting a single .o file.  For libstdc++-v3 this is only
 388          possible to a certain extent; the object files in question contain
 389          template classes and template functions, pre-instantiated, and
 390          splitting those up causes severe maintenance headaches.
 391       </p>
 392       <p>It's not a bug, and it's not really a problem.  Nevertheless, some
 393          people don't like it, so here are two pseudo-solutions:
 394       </p>
 395       <p>If the only functions from libstdc++.a which you need are language
 396          support functions (those listed in
 397          <a href="../18_support/howto.html">clause 18</a> of the standard,
 398          e.g., <code>new</code> and <code>delete</code>), then try linking
 399          against <code>libsupc++.a</code> (usually specifying
 400          <code>-lsupc++</code> when calling g++ for the final link step will
 401          do it).  This library contains only those support routines, one per
 402          object file.  But if you are using anything from the rest of the
 403          library, such as IOStreams or vectors, then you'll still need
 404          pieces from <code>libstdc++.a</code>.
 405       </p>
 406       <p>The second method is one we hope to incorporate into the library
 407          build process.  Some platforms can place each function and variable
 408          into its own section in a .o file.  The GNU linker can then perform
 409          garbage collection on unused sections; this reduces the situation
 410          to only copying needed functions into the executable, as before,
 411          but all happens automatically.
 412       </p>
 413       <p>Unfortunately the garbage collection in GNU ld is buggy; sections
 414          (corresponding to functions and variables) which <em>are</em> used
 415          are mistakenly removed, leading to horrible crashes when your
 416          executable starts up.  For the time being, this feature is not used
 417          when building the library.
 418       </p>
 419
 420 <hr />
 421 <h1><a name="3_0">3.0 Platform-Specific Issues</a></h1>
 422    <h2><a name="3_1">3.1 Can libstdc++-v3 be used with &lt;my
 423                          favorite compiler&gt;?</a></h2>
 424       <p>Probably not.  Yet.</p>
 425       <p>Because GCC advances so rapidly, development and testing of
 426          libstdc++ is being done almost entirely under that compiler.
 427          If you are curious about whether other, lesser compilers
 428          (*grin*) support libstdc++, you are more than welcome to try.
 429          Configuring and building the library (see above) will still
 430          require certain tools, however.  Also keep in mind that
 431          <em>building</em> libstdc++ does not imply that your compiler
 432          will be able to <em>use</em> all of the features found in the
 433          C++ Standard Library.
 434       </p>
 435       <p>Since the goal of ISO Standardization is for all C++
 436          implementations to be able to share code, the final libstdc++
 437          should, in theory, be usable under any ISO-compliant
 438          compiler.  It will still be targeted and optimized for
 439          GCC/g++, however.
 440       </p>
 441
 442 <hr />
 443    <h2><a name="3_2">3.2 [removed]</a></h2>
 444       <p>This question has become moot and has been removed.  The stub
 445          is here to preserve numbering (and hence links/bookmarks).
 446       </p>
 447
 448 <hr />
 449    <h2><a name="3_3">3.3 [removed]</a></h2>
 450       <p>This question has become moot and has been removed.  The stub
 451          is here to preserve numbering (and hence links/bookmarks).
 452       </p>
 453
 454 <hr />
 455    <h2><a name="3_4">3.4 I can't use 'long long' on Solaris</a></h2>
 456       <p>By default we try to support the C99 <code>long long</code> type.
 457          This requires that certain functions from your C library be present.
 458       </p>
 459       <p>Up through release 3.0.2 the tests performed were too general, and
 460          this feature was disabled when it did not need to be.  The most
 461          commonly reported platform affected was Solaris.
 462       </p>
 463       <p>This has been fixed for 3.0.3 and onwards.
 464       </p>
 465
 466 <hr />
 467    <h2><a name="3_5">3.5 <code>_XOPEN_SOURCE</code> / <code>_GNU_SOURCE</code>
 468                          / etc is always defined</a></h2>
 469       <p>On Solaris, g++ (but not gcc) always defines the preprocessor
 470          macro <code>_XOPEN_SOURCE</code>.  On GNU/Linux, the same happens
 471          with <code>_GNU_SOURCE</code>.  (This is not an exhaustive list;
 472          other macros and other platforms are also affected.)
 473       </p>
 474       <p>These macros are typically used in C library headers, guarding new
 475          versions of functions from their older versions.  The C++ standard
 476          library includes the C standard library, but it requires the C90
 477          version, which for backwards-compatability reasons is often not the
 478          default for many vendors.
 479       </p>
 480       <p>More to the point, the C++ standard requires behavior which is only
 481          available on certain platforms after certain symbols are defined.
 482          Usually the issue involves I/O-related typedefs.  In order to
 483          ensure correctness, the compiler simply predefines those symbols.
 484       </p>
 485       <p>Note that it's not enough to #define them only when the library is
 486          being built (during installation).  Since we don't have an 'export'
 487          keyword, much of the library exists as headers, which means that
 488          the symbols must also be defined as your programs are parsed and
 489          compiled.
 490       </p>
 491       <p>To see which symbols are defined, look for CPLUSPLUS_CPP_SPEC in
 492          the gcc config headers for your target (and try changing them to
 493          see what happens when building complicated code).  You can also run
 494          <code>&quot;g++ -E -dM - &lt; /dev/null&quot;</code> to display
 495          a list of predefined macros for any particular installation.
 496       </p>
 497       <p>This has been discussed on the mailing lists
 498          <a href="http://gcc.gnu.org/cgi-bin/htsearch?method=and&amp;format=builtin-long&amp;sort=score&amp;words=_XOPEN_SOURCE+Solaris">quite a bit</a>.
 499       </p>
 500       <p>This method is something of a wart.  We'd like to find a cleaner
 501          solution, but nobody yet has contributed the time.
 502       </p>
 503
 504 <hr />
 505    <h2><a name="3_6">3.6 OS X ctype.h is broken!  How can I hack it?</a></h2>
 506       <p>This is a long-standing bug in the OS X support.  Fortunately,
 507          the patch is quite simple, and well-known.
 508          <a href="http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html"> Here's a
 509          link to the solution.</a>
 510       </p>
 511
 512 <hr />
 513    <h2><a name="3_7">3.7 Threading is broken on i386</a></h2>
 514       <p>Support for atomic integer operations is/was broken on i386
 515          platforms.  The assembly code accidentally used opcodes that are
 516          only available on the i486 and later.  So if you configured GCC
 517          to target, for example, i386-linux, but actually used the programs
 518          on an i686, then you would encounter no problems.  Only when
 519          actually running the code on a i386 will the problem appear.
 520       </p>
 521       <p>This is fixed in 3.2.2.
 522       </p>
 523
 524 <hr />
 525    <h2><a name="3_8">3.8 Recent GNU/Linux glibc required?</a></h2>
 526       <p>When running on GNU/Linux, libstdc++ 3.2.1 (shared library version
 527          5.0.1) and later uses localization and formatting code from the system
 528          C library (glibc) version 2.2.5.  That version of glibc is over a
 529          year old and contains necessary bugfixes.  Many GNU/Linux distros make
 530          glibc version 2.3.x available now.
 531       </p>
 532       <p>The guideline is simple:  the more recent the C++ library, the
 533          more recent the C library.  (This is also documented in the main
 534          GCC installation instructions.)
 535       </p>
 536
 537 <hr />
 538    <h2><a name="3_9">3.9 Can't use wchar_t/wstring on FreeBSD</a></h2>
 539       <p>At the moment there are a few problems in FreeBSD's support for
 540          wide character functions, and as a result the libstdc++ configury
 541          decides that wchar_t support should be disabled.  Once the underlying
 542          problems are fixed in FreeBSD (soon), the library support will
 543          automatically enable itself.
 544       </p>
 545       <p>You can fix the problems yourself, and learn more about the situation,
 546          by reading
 547          <a href="http://gcc.gnu.org/ml/libstdc++/2003-02/subjects.html#00286">
 548          this short thread</a> (&quot;_GLIBCPP_USE_WCHAR_T undefined in
 549          FreeBSD's c++config.h?&quot;).
 550       </p>
 551
 552 <hr />
 553    <h2><a name="3_10">3.10 MIPS atomic operations</a></h2>
 554       <p>The atomic locking routines for MIPS targets requires MIPS II
 555          and later.  A patch went in just after the 3.3 release to
 556          make mips* use the generic implementation instead.  You can also
 557          configure for mipsel-elf as a workaround.
 558       </p>
 559       <p>mips*-*-linux* continues to use the MIPS II routines, and more
 560          work in this area is expected.
 561       </p>
 562
 563 <hr />
 564 <h1><a name="4_0">4.0 Known Bugs and Non-Bugs</a></h1>
 565    <em>Note that this section can get rapdily outdated -- such is the
 566        nature of an open-source project.  For the latest information, join
 567        the mailing list or look through recent archives.   The RELEASE-
 568        NOTES and BUGS files are generally kept up-to-date.</em>
 569
 570    <p>For 3.0.1, the most common &quot;bug&quot; is an apparently missing
 571       &quot;<code>../</code>&quot; in include/Makefile, resulting in files
 572       like gthr.h and gthr-single.h not being found.  Please read
 573       <a href="http://gcc.gnu.org/install/configure.html">the configuration
 574       instructions for GCC</a>,
 575       specifically the part about configuring in a separate build directory,
 576       and how strongly recommended it is.  Building in the source directory
 577       is fragile, is rarely tested, and tends to break, as in this case.
 578       This was fixed for 3.0.2.
 579    </p>
 580
 581    <p>For 3.1, the most common &quot;bug&quot; is a parse error when using
 582       <code>&lt;fstream&gt;</code>, ending with a message,
 583       &quot;<code>bits/basic_file.h:52: parse error before `{'
 584       token</code>.&quot;  Please read
 585       <a href="http://gcc.gnu.org/install/">the installation instructions for
 586       GCC</a>, specifically the part about not installing newer versions on
 587       top of older versions.  If you install 3.1 over a 3.0.x release, then
 588       the wrong basic_file.h header will be found (its location changed
 589       between releases).
 590    </p>
 591
 592    <p><strong>Please do not report these as bugs.  We know about them.</strong>
 593       Reporting this -- or any other problem that's already been fixed --
 594       hinders the development of GCC, because we have to take time to
 595       respond to your report.  Thank you.
 596    </p>
 597
 598    <h2><a name="4_1">4.1 What works already?</a></h2>
 599       <p>Short answer:  Pretty much everything <em>works</em> except for some
 600          corner cases.  Also, localization is incomplete.  For whether it works
 601          well, or as you expect it to work, see 5.2.
 602       </p>
 603       <p>Long answer:  See the docs/html/17_intro/CHECKLIST file, which is
 604          badly outdated...
 605       </p>
 606       <p>What follows is a verbatim clip from the &quot;Status&quot; section
 607          of the RELEASE-NOTES for the latest snapshot.  For a list of
 608          fixed bugs, see that file.
 609       </p>
 610
 611 <!-- Yeah, I meant that "verbatim clip" thing literally... :-)  -->
 612
 613 <pre>
 614 New:
 615 </pre>
 616
 617
 618 <hr />
 619    <h2><a name="4_2">4.2 Bugs in gcc/g++ (not libstdc++-v3)</a></h2>
 620       <p>This is by no means meant to be complete nor exhaustive, but
 621          mentions some problems that users may encounter when building
 622          or using libstdc++.  If you are experiencing one of these
 623          problems, you can find more information on the libstdc++ and
 624          the GCC mailing lists.
 625       </p>
 626       <p>Before reporting a bug, examine the
 627          <a href="http://gcc.gnu.org/bugs.html">bugs database</a> with the
 628          category set to &quot;libstdc++&quot;.  The BUGS file in the source
 629          tree also tracks known serious problems.
 630       </p>
 631       <ul>
 632          <li>Debugging is problematic, due to bugs in line-number generation
 633              (mostly fixed in the compiler) and gdb lagging behind the
 634              compiler (lack of personnel).  We recommend configuring the
 635              compiler using <code>--with-dwarf2</code> if the DWARF2
 636              debugging format is not already the default on your platform.
 637              Also,
 638 <a href="http://gcc.gnu.org/ml/libstdc++/2002-02/msg00034.html">changing your
 639              GDB settings</a> can have a profound effect on your C++ debugging
 640              experiences.  :-)</li>
 641       </ul>
 642
 643 <hr />
 644    <h2><a name="4_3">4.3 Bugs in the C++ language/lib specification</a></h2>
 645       <p>Yes, unfortunately, there are some.  In a
 646          <a href="http://gcc.gnu.org/ml/libstdc++/1998/msg00006.html">message
 647          to the list</a>, Nathan Myers announced that he has started a list of
 648          problems in the ISO C++ Standard itself, especially with
 649          regard to the chapters that concern the library.  The list
 650          itself is
 651          <a href="http://www.cantrip.org/draft-bugs.txt">posted on his
 652          website</a>.  Developers who are having problems interpreting
 653          the Standard may wish to consult his notes.
 654       </p>
 655       <p>For those people who are not part of the ISO Library Group
 656          (i.e., nearly all of us needing to read this page in the first
 657          place :-), a public list of the library defects is occasionally
 658          published <a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/">here</a>.
 659          Some of these have resulted in <a href="#5_2">code changes</a>.
 660       </p>
 661
 662 <hr />
 663    <h2><a name="4_4">4.4 Things in libstdc++ that only look like bugs</a></h2>
 664       <p>There are things which are not bugs in the compiler (4.2) nor
 665          the language specification (4.3), but aren't really bugs in
 666          libstdc++, either.  Really!  Please do not report these as bugs.
 667       </p>
 668       <p><a name="4_4_Weff"><strong>-Weffc++</strong></a>
 669          The biggest of these is the quadzillions of warnings about the
 670          library headers emitted when <code>-Weffc++</code> is used.  Making
 671          libstdc++ &quot;-Weffc++-clean&quot; is not a goal of the project,
 672          for a few reasons.  Mainly, that option tries to enforce
 673          object-oriented programming, while the Standard Library isn't
 674          necessarily trying to be OO.
 675       </p>
 676       <p><a name="4_4_iostreamclear"><strong>reopening a stream fails</strong>
 677          </a> Did I just say that -Weffc++ was our biggest false-bug report?
 678          I lied.  (It used to be.)  Today it seems to be reports that after
 679          executing a sequence like
 680       </p>
 681       <pre>
 682     #include &lt;fstream&gt;
 683     ...
 684     std::fstream  fs(&quot;a_file&quot;);
 685     // .
 686     // . do things with fs...
 687     // .
 688     fs.close();
 689     fs.open(&quot;a_new_file&quot;);</pre>
 690       <p>all operations on the re-opened <code>fs</code> will fail, or at
 691          least act very strangely.  Yes, they often will, especially if
 692          <code>fs</code> reached the EOF state on the previous file.  The
 693          reason is that the state flags are <strong>not</strong> cleared
 694          on a successful call to open().  The standard unfortunately did
 695          not specify behavior in this case, and to everybody's great sorrow,
 696          the <a href="../ext/howto.html#5">proposed LWG resolution in
 697          DR #22</a> is to leave the flags unchanged.  You must insert a call
 698          to <code>fs.clear()</code> between the calls to close() and open(),
 699          and then everything will work like we all expect it to work.
 700       </p>
 701       <p><a name="4_4_rel_ops"><strong>rel_ops</strong></a>
 702          Another is the <code>rel_ops</code> namespace and the template
 703          comparison operator functions contained therein.  If they become
 704          visible in the same namespace as other comparison functions
 705          (e.g., '<code>using</code>' them and the &lt;iterator&gt; header),
 706          then you will suddenly be faced with huge numbers of ambiguity
 707          errors.  This was discussed on the -v3 list; Nathan Myers
 708          <a href="http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html">sums
 709          things up here</a>.  The collisions with vector/string iterator
 710          types have been fixed for 3.1.  <!-- more links to email here -->
 711       </p>
 712       <h3><a name="4_4_interface">The g++-3 headers are <em>not ours</em></a></h3>
 713       <p>If you have found an extremely broken header file which is
 714          causing problems for you, look carefully before submitting a
 715          &quot;high&quot; priority bug report (which you probably shouldn't
 716          do anyhow; see the last paragraph of the page describing
 717          <a href="http://gcc.gnu.org/gnatswrite.html">the GCC bug database</a>).
 718       </p>
 719       <p>If the headers are in <code>${prefix}/include/g++-3</code>, or if
 720          the installed library's name looks like <code>libstdc++-2.10.a</code>
 721          or <code>libstdc++-libc6-2.10.so</code>, then you are using the old
 722          libstdc++-v2 library, which is nonstandard and unmaintained.  Do not
 723          report problems with -v2 to the -v3 mailing list.
 724       </p>
 725       <p>For GCC versions 3.0 and 3.1 the libstdc++-v3 header files are
 726          installed in <code>${prefix}/include/g++-v3</code> (see the 'v'?).
 727          Starting with version 3.2 the headers are installed in
 728          <code>${prefix}/include/c++/${version}</code> as this prevents
 729          headers from previous versions being found by mistake.
 730       </p>
 731       <p><a name="4_4_glibc"><strong>glibc</strong></a>
 732          If you're on a GNU/Linux system and have just upgraded to
 733          glibc 2.2, but are still using gcc 2.95.2, then you should have
 734          read the glibc FAQ, specifically 2.34:
 735       </p>
 736       <pre>
 737 2.34.   When compiling C++ programs, I get a compilation error in streambuf.h.
 738
 739 {BH} You are using g++ 2.95.2? After upgrading to glibc 2.2, you need to
 740 apply a patch to the include files in /usr/include/g++, because the fpos_t
 741 type has changed in glibc 2.2.  The patch is at
 742 http://clisp.cons.org/~haible/gccinclude-glibc-2.2-compat.diff
 743    </pre>
 744       <p>Note that 2.95.x shipped with the
 745          <a href="#4_4_interface">old v2 library</a> which is no longer
 746          maintained.  Also note that gcc 2.95.3 fixes this problem, but
 747          requires a separate patch for libstdc++-v3.
 748       </p>
 749       <p><a name="4_4_checks"><strong>concept checks</strong></a>
 750          If you see compilation errors containing messages about
 751          <code> <em>foo</em>Concept </code>and a<code> constraints </code>
 752          member function, then most likely you have violated one of the
 753          requirements for types used during instantiation of template
 754          containers and functions.  For example, EqualityComparableConcept
 755          appears if your types must be comparable with == and you have not
 756          provided this capability (a typo, or wrong visibility, or you
 757          just plain forgot, etc).
 758       </p>
 759       <p>More information, including how to optionally enable/disable the
 760          checks, is available
 761          <a href="../19_diagnostics/howto.html#3">here</a>.
 762       </p>
 763       <p><a name="4_4_dlsym"><strong>dlopen/dlsym</strong></a>
 764          If you are using the C++ library across dynamically-loaded
 765          objects, make certain that you are passing the correct options
 766          when compiling and linking:
 767       </p>
 768       <pre>
 769     // compile your library components
 770     g++ -fPIC -c a.cc
 771     g++ -fPIC -c b.cc
 772     ...
 773     g++ -fPIC -c z.cc
 774
 775     // create your library
 776     g++ -fPIC -shared -rdynamic -o libfoo.so a.o b.o ... z.o
 777
 778     // link the executable
 779     g++ -fPIC -rdynamic -o foo ... -L. -lfoo -ldl</pre>
 780       <p><a name="4_4_leak"><strong>"memory leaks" in containers</strong></a>
 781          A few people have reported that the standard containers appear
 782          to leak memory when tested with memory checkers such as
 783          <a href="http://developer.kde.org/~sewardj/">valgrind</a>.
 784          The library's default allocators keep free memory in a pool
 785          for later reuse, rather than returning it to the OS.  Although
 786          this memory is always reachable by the library and is never
 787          lost, memory debugging tools can report it as a leak.  If you
 788          want to test the library for memory leaks please read
 789          <a href="../debug.html#mem">Tips for memory leak hunting</a>
 790          first.
 791       </p>
 792
 793 <hr />
 794    <h2><a name="4_5">4.5 Aw, that's easy to fix!</a></h2>
 795       <p>If you have found a bug in the library and you think you have
 796          a working fix, then send it in!  The main GCC site has a page
 797          on <a href="http://gcc.gnu.org/contribute.html">submitting
 798          patches</a> that covers the procedure, but for libstdc++ you
 799          should also send the patch to our mailing list in addition to
 800          the GCC patches mailing list.  The libstdc++
 801          <a href="../17_intro/contribute.html">contributors' page</a>
 802          also talks about how to submit patches.
 803       </p>
 804       <p>In addition to the description, the patch, and the ChangeLog
 805          entry, it is a Good Thing if you can additionally create a small
 806          test program to test for the presence of the bug that your
 807          patch fixes.  Bugs have a way of being reintroduced; if an old
 808          bug creeps back in, it will be caught immediately by the
 809          <a href="#2_4">testsuite</a> -- but only if such a test exists.
 810       </p>
 811
 812 <hr />
 813 <h1><a name="5_0">5.0 Miscellaneous</a></h1>
 814    <h2><a name="5_1">5.1 string::iterator is not char*;
 815                      vector&lt;T&gt;::iterator is not T*</a></h2>
 816       <p>If you have code that depends on container&lt;T&gt; iterators
 817          being implemented as pointer-to-T, your code is broken.
 818       </p>
 819       <p>While there are arguments for iterators to be implemented in
 820          that manner, A) they aren't very good ones in the long term,
 821          and B) they were never guaranteed by the Standard anyway.  The
 822          type-safety achieved by making iterators a real class rather
 823          than a typedef for <code>T*</code> outweighs nearly all opposing
 824          arguments.
 825       </p>
 826       <p>Code which does assume that a vector iterator <code> i </code>
 827          is a pointer can often be fixed by changing <code> i </code> in
 828          certain expressions to <code> &amp;*i </code>.  Future revisions
 829          of the Standard are expected to bless this usage for
 830          vector&lt;&gt; (but not for basic_string&lt;&gt;).
 831       </p>
 832
 833 <hr />
 834    <h2><a name="5_2">5.2 What's next after libstdc++-v3?</a></h2>
 835       <p>Hopefully, not much.  The goal of libstdc++-v3 is to produce
 836          a fully-compliant, fully-portable Standard Library.  After that,
 837          we're mostly done:  there won't <em>be</em> any more compliance
 838          work to do.  However:
 839       </p>
 840       <ol>
 841       <li><p>The ISO Committee will meet periodically to review Defect Reports
 842          in the C++ Standard.  Undoubtedly some of these will result in
 843          changes to the Standard, which will be reflected in patches to
 844          libstdc++.  Some of that is already happening, see 4.2.  Some of
 845          those changes are being predicted by the library maintainers, and
 846          we add code to the library based on what the current proposed
 847          resolution specifies.  Those additions are listed in
 848          <a href="../ext/howto.html#5">the extensions page</a>.
 849       </p></li>
 850       <li><p>Performance tuning.  Lots of performance tuning.  This too is
 851          already underway for post-3.0 releases, starting with memory
 852          expansion in container classes and buffer usage in synchronized
 853          stream objects.
 854       </p></li>
 855       <li><p>An ABI for libstdc++ is being developed, so that
 856          multiple binary-incompatible copies of the library can be replaced
 857          with a single backwards-compatible library, like libgcc_s.so is.
 858       </p></li>
 859       <li><p>The current libstdc++ contains extensions to the Library which
 860          must be explicitly requested by client code (for example, the
 861          hash tables from SGI).  Other extensions may be added to
 862          libstdc++-v3 if they seem to be &quot;standard&quot; enough.
 863          (For example, the &quot;long long&quot; type from C99.)
 864          Bugfixes and rewrites (to improve or fix thread safety, for
 865          instance) will of course be a continuing task.
 866       </p></li>
 867       </ol>
 868       <p><a href="http://gcc.gnu.org/ml/libstdc++/1999/msg00080.html">This
 869          question</a> about the next libstdc++ prompted some brief but
 870          interesting
 871          <a href="http://gcc.gnu.org/ml/libstdc++/1999/msg00084.html">speculation</a>.
 872       </p>
 873
 874 <hr />
 875    <h2><a name="5_3">5.3 What about the STL from SGI?</a></h2>
 876       <p>The <a href="http://www.sgi.com/tech/stl/">STL from SGI</a>,
 877          version 3.3, was the most recent merge of the STL codebase.  The
 878          code in libstdc++ contains many fixes and changes, and it is
 879          very likely that the SGI code is no longer under active
 880          development.  We expect that no future merges will take place.
 881       </p>
 882       <p>In particular, <code>string</code> is not from SGI and makes no
 883          use of their &quot;rope&quot; class (which is included as an
 884          optional extension), nor is <code>valarray</code> and some others.
 885          Classes like <code>vector&lt;&gt;</code> are, however we have
 886          made significant changes to them since then.
 887       </p>
 888       <p>The FAQ for SGI's STL (one jump off of their main page) is
 889          recommended reading.
 890       </p>
 891
 892 <hr />
 893    <h2><a name="5_4">5.4 Extensions and Backward Compatibility</a></h2>
 894       <p>Headers in the <code>ext</code> and <code>backward</code>
 895          subdirectories should be referred to by their relative paths:
 896          <!-- Careful, the leading spaces in PRE show up directly. -->
 897       </p>
 898       <pre>
 899       #include &lt;ext/hash_map&gt; </pre>
 900       <p>rather than using <code>-I</code> or other options.  This is more
 901          portable and forward-compatible.  (The situation is the same as
 902          that of other headers whose directories are not searched directly,
 903          e.g., <code>&lt;sys/stat.h&gt;</code>, <code>&lt;X11/Xlib.h&gt;</code>.
 904       </p>
 905
 906       <p>The extensions are no longer in the global or <code>std</code>
 907          namespaces, instead they are declared in the <code>__gnu_cxx</code>
 908          namespace. For maximum portability, consider defining a namespace
 909          alias to use to talk about extensions, e.g.:
 910       </p>
 911       <pre>
 912       #ifdef __GNUC__
 913       #if __GNUC__ &lt; 3
 914         #include &lt;hash_map.h&gt;
 915         namespace Sgi { using ::hash_map; }; // inherit globals
 916       #else
 917         #include &lt;ext/hash_map&gt;
 918         #if __GNUC_MINOR__ == 0
 919           namespace Sgi = std;               // GCC 3.0
 920         #else
 921           namespace Sgi = ::__gnu_cxx;       // GCC 3.1 and later
 922         #endif
 923       #endif
 924       #else      // ...  there are other compilers, right?
 925         namespace Sgi = std;
 926       #endif
 927
 928       Sgi::hash_map&lt;int,int&gt; my_map; </pre>
 929       <p>This is a bit cleaner than defining typedefs for all the
 930          instantiations you might need.
 931       </p>
 932       <p><strong>Note:</strong> explicit template specializations must
 933          be declared in the same namespace as the original template.
 934          This means you cannot use a namespace alias when declaring
 935          an explicit specialization.
 936       </p>
 937       <p>Extensions to the library have
 938          <a href="../ext/howto.html">their own page</a>.
 939       </p>
 940
 941 <hr />
 942    <h2><a name="5_5">5.5 [removed]</a></h2>
 943       <p>This question has become moot and has been removed.  The stub
 944          is here to preserve numbering (and hence links/bookmarks).
 945       </p>
 946
 947 <hr />
 948    <h2><a name="5_6">5.6 Is libstdc++-v3 thread-safe?</a></h2>
 949       <p>libstdc++-v3 strives to be thread-safe when all of the following
 950          conditions are met:
 951       </p>
 952       <ul>
 953        <li>The system's libc is itself thread-safe,</li>
 954        <li><code>gcc -v</code> reports a thread model other than 'single',</li>
 955        <li>[pre-3.3 only] a non-generic implementation of atomicity.h
 956            exists for the architecture in question.</li>
 957       </ul>
 958       <p>The user-code must guard against concurrent method calls which may
 959          access any particular library object's state.  Typically, the
 960          application programmer may infer what object locks must be held
 961          based on the objects referenced in a method call.  Without getting
 962          into great detail, here is an example which requires user-level
 963          locks:
 964       </p>
 965       <pre>
 966      library_class_a shared_object_a;
 967
 968      thread_main () {
 969        library_class_b *object_b = new library_class_b;
 970        shared_object_a.add_b (object_b);   // must hold lock for shared_object_a
 971        shared_object_a.mutate ();          // must hold lock for shared_object_a
 972      }
 973
 974      // Multiple copies of thread_main() are started in independent threads.</pre>
 975       <p>Under the assumption that object_a and object_b are never exposed to
 976          another thread, here is an example that should not require any
 977          user-level locks:
 978       </p>
 979       <pre>
 980      thread_main () {
 981        library_class_a object_a;
 982        library_class_b *object_b = new library_class_b;
 983        object_a.add_b (object_b);
 984        object_a.mutate ();
 985      } </pre>
 986       <p>All library objects are safe to use in a multithreaded program as
 987          long as each thread carefully locks out access by any other
 988          thread while it uses any object visible to another thread, i.e.,
 989          treat library objects like any other shared resource.  In general,
 990          this requirement includes both read and write access to objects;
 991          unless otherwise documented as safe, do not assume that two threads
 992          may access a shared standard library object at the same time.
 993       </p>
 994       <p>See chapters <a href="../17_intro/howto.html#3">17</a> (library
 995          introduction), <a href="../23_containers/howto.html#3">23</a>
 996          (containers), and <a href="../27_io/howto.html#9">27</a> (I/O) for
 997          more information.
 998       </p>
 999
1000 <hr />
1001    <h2><a name="5_7">5.7 How do I get a copy of the ISO C++ Standard?</a></h2>
1002       <p>Copies of the full ISO 14882 standard are available on line via the
1003          ISO mirror site for committee members.  Non-members, or those who
1004          have not paid for the privilege of sitting on the committee and
1005          sustained their two-meeting commitment for voting rights, may get a
1006          copy of the standard from their respective national standards
1007          organization.  In the USA, this national standards organization is
1008          ANSI and their website is right <a href="http://www.ansi.org">here</a>.
1009          (And if you've already registered with them, clicking this link will
1010          take you to directly to the place where you can
1011 <a href="http://webstore.ansi.org/ansidocstore/product.asp?sku=ISO%2FIEC+14882%2D1998">buy
1012          the standard on-line</a>.
1013       </p>
1014       <p>Who is your country's member body?  Visit the
1015          <a href="http://www.iso.ch/">ISO homepage</a> and find out!
1016       </p>
1017
1018 <hr />
1019    <h2><a name="5_8">5.8 What's an ABI and why is it so messy?</a></h2>
1020       <p>&quot;ABI&quot; stands for &quot;Application Binary Interface.&quot;
1021          Conventionally, it refers to a great mass of details about how
1022          arguments are arranged on the call stack and/or in registers, and
1023          how various types are arranged and padded in structs.  A single CPU
1024          design may suffer multiple ABIs designed by different development
1025          tool vendors who made different choices, or even by the same vendor
1026          for different target applications or compiler versions.  In ideal
1027          circumstances the CPU designer presents one ABI and all the OSes and
1028          compilers use it.  In practice every ABI omits details that compiler
1029          implementers (consciously or accidentally) must choose for themselves.
1030       </p>
1031       <p>That ABI definition suffices for compilers to generate code so a
1032          program can interact safely with an OS and its lowest-level libraries.
1033          Users usually want an ABI to encompass more detail, allowing libraries
1034          built with different compilers (or different releases of the same
1035          compiler!) to be linked together.  For C++, this includes many more
1036          details than for C, and CPU designers (for good reasons elaborated
1037          below) have not stepped up to publish C++ ABIs.  The details include
1038          virtual function implementation, struct inheritance layout, name
1039          mangling, and exception handling.  Such an ABI has been defined for
1040          GNU C++, and is immediately useful for embedded work relying only on
1041          a &quot;free-standing implementation&quot; that doesn't include (much
1042          of) the standard library.  It is a good basis for the work to come.
1043       </p>
1044       <p>A useful C++ ABI must also incorporate many details of the standard
1045          library implementation.  For a C ABI, the layouts of a few structs
1046          (such as FILE, stat, jmpbuf, and the like) and a few macros suffice.
1047          For C++, the details include the complete set of names of functions
1048          and types used, the offsets of class members and virtual functions,
1049          and the actual definitions of all inlines.  C++ exposes many more
1050          library details to the caller than C does.  It makes defining
1051          a complete ABI a much bigger undertaking, and requires not just
1052          documenting library implementation details, but carefully designing
1053          those details so that future bug fixes and optimizations don't
1054          force breaking the ABI.
1055       </p>
1056       <p>There are ways to help isolate library implementation details from the
1057          ABI, but they trade off against speed.  Library details used in
1058          inner loops (e.g., getchar) must be exposed and frozen for all
1059          time, but many others may reasonably be kept hidden from user code,
1060          so they may later be changed.  Deciding which, and implementing
1061          the decisions, must happen before you can reasonably document a
1062          candidate C++ ABI that encompasses the standard library.
1063       </p>
1064
1065 <hr />
1066    <h2><a name="5_9">5.9 How do I make std::vector&lt;T&gt;::capacity()
1067                      == std::vector&lt;T&gt;::size()?</a> </h2>
1068    <!-- referenced by 21_strings/howto.html#6 -->
1069    <p>The standard idiom for deallocating a <code>std::vector&lt;T&gt;</code>'s
1070       unused memory is to create a temporary copy of the vector and swap their
1071       contents, e.g. for <code>std::vector&lt;T&gt; v</code>
1072    </p>
1073    <pre>
1074      std::vector&lt;T&gt;(v).swap(v);
1075    </pre>
1076    <p>The copy will take O(n) time and the swap is constant time.
1077    </p>
1078    <p>See <a href='../21_strings/howto.html#6'>Shrink-to-fit strings</a> for
1079       a similar solution for strings.
1080    </p>
1081
1082 <!-- ####################################################### -->
1083
1084 <hr />
1085 <p class="fineprint"><em>
1086 See <a href="../17_intro/license.html">license.html</a> for copying conditions.
1087 Comments and suggestions are welcome, and may be sent to
1088 <a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
1089 </em></p>
1090
1091
1092 </body>
1093 </html>
1094