libstdc++-v3/doc/html/faq/index.html

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  19 <h1 class="centered">libstdc++ Frequently Asked Questions</h1>
  20
  21 <p class="fineprint"><em>
  22    The latest version of this document is always available at
  23    <a href="http://gcc.gnu.org/onlinedocs/libstdc++/faq/">
  24    http://gcc.gnu.org/onlinedocs/libstdc++/faq/</a>.  The main documentation
  25    page is at
  26    <a href="http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html">
  27    http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html</a>.
  28 </em></p>
  29
  30 <p><em>
  31    To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++ homepage</a>.
  32 </em></p>
  33
  34 <!-- ####################################################### -->
  35 <hr />
  36 <h1>Questions</h1>
  37 <ol>
  38    <li><a href="#1_0">General Information</a>
  39    <!-- I suspect these will mostly be links to/into existing documents. -->
  40    <ol>
  41       <li><a href="#1_1">What is libstdc++?</a> </li>
  42       <li><a href="#1_2">Why should I use libstdc++?</a> </li>
  43       <li><a href="#1_3">Who's in charge of it?</a> </li>
  44       <li><a href="#1_4">[removed]</a> </li>
  45       <li><a href="#1_5">When is libstdc++ going to be finished?</a> </li>
  46       <li><a href="#1_6">How do I contribute to the effort?</a> </li>
  47       <li><a href="#1_7">What happened to libg++?  I need that!</a> </li>
  48       <li><a href="#1_8">What if I have more questions?</a> </li>
  49       <li><a href="#1_9">What are the license terms for libstdc++?</a> </li>
  50    </ol>
  51    </li>
  52
  53    <li><a href="#2_0">Installation</a>
  54       <ol>
  55          <li><a href="#2_1">How do I install libstdc++?</a> </li>
  56          <li><a href="#2_2">[removed]</a> </li>
  57          <li><a href="#2_3">What is this SVN thing that you keep
  58                             mentioning?</a> </li>
  59          <li><a href="#2_4">How do I know if it works?</a> </li>
  60          <li><a href="#2_5">This library is HUGE!  And what's libsupc++?</a> </li>
  61          <li><a href="#2_6">Why do I get an error saying
  62                             <code>libstdc++.so.X</code> is missing when I
  63                             run my program?</a> </li>
  64       </ol>
  65    </li>
  66
  67    <li><a href="#3_0">Platform-Specific Issues</a>
  68       <ol>
  69          <li><a href="#3_1">Can libstdc++ be used with &lt;my
  70                             favorite compiler&gt;?</a> </li>
  71          <li><a href="#3_2">[removed]</a> </li>
  72          <li><a href="#3_3">[removed]</a> </li>
  73          <li><a href="#3_4">I can't use 'long long' on Solaris</a> </li>
  74          <li><a href="#3_5"><code>_XOPEN_SOURCE</code> /
  75                             <code>_GNU_SOURCE</code> / etc is always defined</a>
  76          </li>
  77          <li><a href="#3_6">OS X ctype.h is broken!  How can I hack it?</a></li>
  78          <li><a href="#3_7">Threading is broken on i386</a></li>
  79          <li><a href="#3_8">Recent GNU/Linux glibc required?</a></li>
  80          <li><a href="#3_9">Can't use wchar_t/wstring on FreeBSD</a></li>
  81          <li><a href="#3_10">MIPS atomic operations</a></li>
  82       </ol>
  83    </li>
  84
  85    <li><a href="#4_0">Known Bugs and Non-Bugs</a>
  86       <ol>
  87          <li><a href="#4_1">What works already?</a> </li>
  88          <li><a href="#4_2">Bugs in gcc/g++ (not libstdc++)</a> </li>
  89          <li><a href="#4_3">Bugs in the C++ language/lib specification</a> </li>
  90          <li><a href="#4_4">Things in libstdc++ that only look like bugs</a><ul>
  91            <li><a href="#4_4_iostreamclear">reopening a stream fails</a> </li>
  92            <li><a href="#4_4_Weff">-Weffc++ complains too much</a> </li>
  93            <li><a href="#4_4_rel_ops">&quot;ambiguous overloads&quot;
  94                                after including an old-style header</a> </li>
  95            <li><a href="#4_4_interface">The g++-3 headers are
  96                                <strong>not ours</strong></a> </li>
  97            <li><a href="#4_4_glibc">compilation errors from streambuf.h</a> </li>
  98            <li><a href="#4_4_checks">errors about <em>*Concept</em> and
  99                                <em>constraints</em> in the STL...</a> </li>
 100            <li><a href="#4_4_dlsym">program crashes when using library code
 101                                in a dynamically-loaded library</a> </li>
 102            <li><a href="#4_4_leak">"memory leaks" in containers</a> </li>
 103            <li><a href="#4_4_list_size">list::size() is O(n)!</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++?</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">Does libstdc++ support TR1?</a> </li>
 118          <li><a href="#5_6">Is libstdc++ 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++?</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.
 138          For those who want to see exactly how
 139          far the project has come, or just want the latest
 140          bleeding-edge code, the up-to-date source is available over
 141          anonymous SVN, and can even be browsed over the
 142          <a href="http://gcc.gnu.org/svn.html">web</a>.
 143       </p>
 144
 145 <hr />
 146    <h2><a name="1_2">1.2 Why should I use libstdc++?</a></h2>
 147       <p>The completion of the ISO C++ standardization gave the
 148          C++ community a powerful set of reuseable tools in the form
 149          of the C++ Standard Library.  However, all existing C++
 150          implementations are (as the Draft Standard used to say)
 151          &quot;incomplet and incorrekt,&quot; and many suffer from
 152          limitations of the compilers that use them.
 153       </p>
 154       <p>The GNU C/C++/FORTRAN/&lt;pick-a-language&gt; compiler
 155          (<code>gcc</code>, <code>g++</code>, etc) is widely considered to be
 156          one of the leading compilers in the world.  Its development
 157          is overseen by the
 158          <a href="http://gcc.gnu.org/">GCC team</a>.  All of
 159          the rapid development and near-legendary
 160          <a href="http://gcc.gnu.org/buildstat.html">portability</a>
 161          that are the hallmarks of an open-source project are being
 162          applied to libstdc++.
 163       </p>
 164       <p>That means that all of the Standard classes and functions
 165          (such as <code>string</code>, <code>vector&lt;&gt;</code>, iostreams,
 166          and algorithms) will be freely available and fully compliant.
 167          Programmers will no longer need to &quot;roll their own&quot;
 168          nor be worried about platform-specific incompatibilities.
 169       </p>
 170
 171 <hr />
 172    <h2><a name="1_3">1.3 Who's in charge of it?</a></h2>
 173       <p>The libstdc++ project is contributed to by several developers
 174          all over the world, in the same way as GCC or Linux.
 175          Benjamin Kosnik, Gabriel Dos Reis, Phil Edwards, Ulrich Drepper,
 176          Loren James Rittle, and Paolo Carlini are the lead maintainers of
 177          the SVN archive.
 178       </p>
 179       <p>Development and discussion is held on the libstdc++ mailing
 180          list.  Subscribing to the list, or searching the list
 181          archives, is open to everyone.  You can read instructions for
 182          doing so on the <a href="http://gcc.gnu.org/libstdc++/">homepage</a>.
 183          If you have questions, ideas, code, or are just curious, sign up!
 184       </p>
 185
 186 <hr />
 187    <h2><a name="1_4">1.4 How do I get libstdc++?</a></h2>
 188
 189       <p>Stable versions of libstdc++-v3 are included with releases of
 190          <a href="http://gcc.gnu.org/releases.html">the GCC compilers</a>.
 191       </p>
 192
 193 <hr />
 194    <h2><a name="1_5">1.5 When is libstdc++ going to be finished?</a></h2>
 195 <!--      <p>Nathan Myers gave the best of all possible answers in <a
 196          href="http://www.deja.com/getdoc.xp?AN=469581698&fmt=text">a
 197          Usenet article</a>.</p>
 198 which is no longer available, thanks deja...-->
 199       <p>Nathan Myers gave the best of all possible answers, responding to a
 200          Usenet article asking this question:  <em>Sooner, if you help.</em>
 201       </p>
 202
 203 <hr />
 204    <h2><a name="1_6">1.6 How do I contribute to the effort?</a></h2>
 205       <p>Here is <a href="../17_intro/contribute.html">a
 206          page devoted to this topic</a>.  Subscribing to the mailing
 207          list (see above, or the homepage) is a very good idea if you
 208          have something to contribute, or if you have spare time and
 209          want to help.  Contributions don't have to be in the form of
 210          source code; anybody who is willing to help write
 211          documentation, for example, or has found a bug in code that
 212          we all thought was working, is more than welcome!
 213       </p>
 214
 215 <hr />
 216    <h2><a name="1_7">1.7 What happened to libg++?  I need that!</a></h2>
 217       <p>The most recent libg++ README states that libg++ is no longer
 218          being actively maintained.  It should not be used for new
 219          projects, and is only being kicked along to support older code.
 220       </p>
 221       <p>The libg++ was designed and created when there was no Standard
 222          to provide guidance.  Classes like linked lists are now provided
 223          for by <code>list&lt;T&gt;</code> and do not need to be created by
 224          <code>genclass</code>.  (For that matter, templates exist now and
 225          are well-supported, whereas genclass (mostly) predates them.)
 226       </p>
 227       <p>There are other classes in libg++ that are not specified in the
 228          ISO Standard (e.g., statistical analysis).  While there are a
 229          lot of really useful things that are used by a lot of people
 230          (e.g., statistics :-), the Standards Committee couldn't include
 231          everything, and so a lot of those &quot;obvious&quot; classes
 232          didn't get included.
 233       </p>
 234       <p>Since libstdc++ is an implementation of the Standard Library, we
 235          have no plans at this time to include non-Standard utilities
 236          in the implementation, however handy they are.  (The extensions
 237          provided in the SGI STL aren't maintained by us and don't get
 238          a lot of our attention, because they don't require a lot of our
 239          time.)  It is entirely plausible that the &quot;useful stuff&quot;
 240          from libg++ might be extracted into an updated utilities library,
 241          but nobody has started such a project yet.
 242       </p>
 243       <p>(The <a href="http://www.boost.org/">Boost</a> site houses free
 244          C++ libraries that do varying things, and happened to be started
 245          by members of the Standards Committee.  Certain &quot;useful
 246          stuff&quot; classes will probably migrate there.)
 247       </p>
 248       <p>For the bold and/or desperate, the
 249          <a href="http://gcc.gnu.org/extensions.html">GCC extensions page</a>
 250          describes where to find the last libg++ source.
 251       </p>
 252
 253 <hr />
 254    <h2><a name="1_8">1.8 What if I have more questions?</a></h2>
 255       <p>If you have read the README file, and your
 256          question remains unanswered, then just ask the mailing list.
 257          At present, you do not need to be subscribed to the list to
 258          send a message to it.  More information is available on the
 259          homepage (including how to browse the list archives); to send
 260          to the list, use <a href="mailto:libstdc++@gcc.gnu.org">
 261          <code>libstdc++@gcc.gnu.org</code></a>.
 262       </p>
 263       <p>If you have a question that you think should be included here,
 264          or if you have a question <em>about</em> a question/answer here,
 265          contact <a href="mailto:pme@gcc.gnu.org">Phil Edwards</a>
 266          or <a href="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</a>.
 267       </p>
 268
 269 <hr />
 270    <h2><a name="1_9">1.9 What are the license terms for libstdc++?</a></h2>
 271       <p>See <a href="../17_intro/license.html">our license description</a>
 272          for these and related questions.
 273       </p>
 274
 275 <hr />
 276 <h1><a name="2_0">2.0 Installation</a></h1>
 277    <h2><a name="2_1">2.1 How do I install libstdc++?</a></h2>
 278       <p>Complete instructions are not given here (this is a FAQ, not
 279          an installation document), but the tools required are few:
 280       </p>
 281          <ul>
 282             <li> A 3.x or later release of GCC.  Either install a suitable
 283                  package for your system, or compile GCC from the sources.
 284                  Note that building GCC
 285                  is much easier and more automated than building the GCC
 286                  2.[78] series was.  If you are using GCC 2.95, you can
 287                  still build earlier snapshots of libstdc++ but you
 288                  should consult the documentation that comes with the
 289                  sources, the instructions are no longer included here.
 290             </li>
 291             <li> GNU Make is required to build GCC 3.4 and later.
 292             </li>
 293             <li> The GNU Autotools are needed if you are messing with
 294                  the configury or makefiles.
 295             </li>
 296          </ul>
 297       <p>The file <a href="../documentation.html#2">documentation.html</a>
 298          links to documentation of the steps necessary to build, install,
 299          and use the library.  Instructions for configuring the library
 300          with flags such as --enable-threads are there also.
 301       </p>
 302       <p>The top-level install.html file contains
 303          the exact build and installation instructions.  You may wish to
 304          browse those files over ViewVC ahead of time to get a feel for
 305          what's required.
 306       </p>
 307
 308 <hr />
 309    <h2><a name="2_2">2.2 [removed]</a></h2>
 310       <p>This question has become moot and has been removed.  The stub
 311          is here to preserve numbering (and hence links/bookmarks).
 312       </p>
 313
 314 <hr />
 315    <h2><a name="2_3">2.3 What is this SVN thing that you
 316                          keep mentioning?</a></h2>
 317       <p><em>Subversion</em> is one of several revision control packages.
 318          It was selected for GNU projects because it's free (speech), free (beer),
 319          and very high quality.  The <a href="http://subversion.tigris.org">
 320          Subversion home page</a> has a better description.
 321       </p>
 322       <p>The &quot;anonymous client checkout&quot; feature of SVN is
 323          similar to anonymous FTP in that it allows anyone to retrieve
 324          the latest libstdc++ sources.
 325       </p>
 326       <p>After the first of April, American users will have a
 327          &quot;/pharmacy&quot; command-line option...
 328          <!-- wonder how long that'll live -->
 329       </p>
 330
 331 <hr />
 332    <h2><a name="2_4">2.4 How do I know if it works?</a></h2>
 333       <p>libstdc++ comes with its own testsuite.  You do not need
 334          to actually install the library (&quot;<code>make
 335          install</code>&quot;) to run the testsuite, but you do need
 336          DejaGNU, as described
 337          <a href="http://gcc.gnu.org/install/test.html">here</a>.
 338       </p>
 339       <p>To run the testsuite on the library after building it, use
 340          &quot;make check&quot; while in your build directory.  To run
 341          the testsuite on the library after building and installing it,
 342          use &quot;make check-install&quot; instead.
 343       </p>
 344       <p>If you find bugs in the testsuite programs themselves, or if you
 345          think of a new test program that should be added to the suite,
 346          <strong>please</strong> write up your idea and send it to the list!
 347       </p>
 348
 349 <hr />
 350    <h2><a name="2_5">2.5 This library is HUGE!  And what's libsupc++?</a></h2>
 351       <p>Usually the size of libraries on disk isn't noticeable.  When a
 352          link editor (or simply &quot;linker&quot;) pulls things from a
 353          static archive library, only the necessary object files are copied
 354          into your executable, not the entire library.  Unfortunately, even
 355          if you only need a single function or variable from an object file,
 356          the entire object file is extracted.  (There's nothing unique to C++
 357          or libstdc++ about this; it's just common behavior, given here
 358          for background reasons.)
 359       </p>
 360       <p>Some of the object files which make up libstdc++.a are rather large.
 361          If you create a statically-linked executable with
 362          <code> -static</code>, those large object files are suddenly part
 363          of your executable.  Historically the best way around this was to
 364          only place a very few functions (often only a single one) in each
 365          source/object file; then extracting a single function is the same
 366          as extracting a single .o file.  For libstdc++ this is only
 367          possible to a certain extent; the object files in question contain
 368          template classes and template functions, pre-instantiated, and
 369          splitting those up causes severe maintenance headaches.
 370       </p>
 371       <p>It's not a bug, and it's not really a problem.  Nevertheless, some
 372          people don't like it, so here are two pseudo-solutions:
 373       </p>
 374       <p>If the only functions from libstdc++.a which you need are
 375          language support functions (those listed in <a
 376          href="../18_support/howto.html">clause 18</a> of the
 377          standard, e.g., <code>new</code> and <code>delete</code>),
 378          then try linking against <code>libsupc++.a</code> (Using
 379          <code>gcc</code> instead of <code>g++</code> and explicitly
 380          linking in <code>-lsupc++</code> for the final link step will
 381          do it).  This library contains only those support routines,
 382          one per object file.  But if you are using anything from the
 383          rest of the library, such as IOStreams or vectors, then
 384          you'll still need pieces from <code>libstdc++.a</code>.
 385       </p>
 386       <p>The second method is one we hope to incorporate into the library
 387          build process.  Some platforms can place each function and variable
 388          into its own section in a .o file.  The GNU linker can then perform
 389          garbage collection on unused sections; this reduces the situation
 390          to only copying needed functions into the executable, as before,
 391          but all happens automatically.
 392       </p>
 393       <p>Unfortunately the garbage collection in GNU ld is buggy; sections
 394          (corresponding to functions and variables) which <em>are</em> used
 395          are mistakenly removed, leading to horrible crashes when your
 396          executable starts up.  For the time being, this feature is not used
 397          when building the library.
 398       </p>
 399
 400 <hr />
 401    <h2><a name="2_6">2.6 Why do I get an error saying
 402                          <code>libstdc++.so.X</code> is missing when I run
 403                          my program?</a></h2>
 404       <p>Depending on your platform and library version, the message might
 405          be similar to one of the following:
 406       </p>
 407       <pre>
 408     ./a.out: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory
 409
 410     /usr/libexec/ld-elf.so.1: Shared object "libstdc++.so.6" not found </pre>
 411
 412       <p>This doesn't mean that the shared library isn't installed, only
 413          that the dynamic linker can't find it. When a dynamically-linked
 414          executable is run the linker finds and loads the required shared
 415          libraries by searching a pre-configured list of directories. If
 416          the directory where you've installed libstdc++ is not in this
 417          list then the libraries won't be found. The simplest way to fix
 418          this is to use the <code>LD_LIBRARY_PATH</code> environment
 419          variable, which is a colon-separated list of directories in which
 420          the linker will search for shared libraries:
 421       </p>
 422       <pre>
 423     LD_LIBRARY_PATH=${prefix}/lib:$LD_LIBRARY_PATH
 424     export LD_LIBRARY_PATH </pre>
 425       <p>The exact environment variable to use will depend on your platform,
 426          e.g. DYLD_LIBRARY_PATH for Darwin,
 427          LD_LIBRARY_PATH_32/LD_LIBRARY_PATH_64 for Solaris 32-/64-bit,
 428          LD_LIBRARYN32_PATH/LD_LIBRARY64_PATH for Irix N32/64-bit ABIs
 429          and SHLIB_PATH for HP-UX.
 430       </p>
 431       <p>See the man pages for <code>ld(1)</code>, <code>ldd(1)</code> and
 432          <code>ldconfig(8)</code> for more information. The dynamic linker
 433          has different names on different platforms but the man page is
 434          usually called something such as <code>ld.so / rtld / dld.so</code>.
 435       </p>
 436
 437 <hr />
 438 <h1><a name="3_0">3.0 Platform-Specific Issues</a></h1>
 439    <h2><a name="3_1">3.1 Can libstdc++ be used with &lt;my
 440                          favorite compiler&gt;?</a></h2>
 441       <p>Probably not.  Yet.</p>
 442       <p>Because GCC advances so rapidly, development and testing of
 443          libstdc++ is being done almost entirely under that compiler.
 444          If you are curious about whether other, lesser compilers
 445          (*grin*) support libstdc++, you are more than welcome to try.
 446          Configuring and building the library (see above) will still
 447          require certain tools, however.  Also keep in mind that
 448          <em>building</em> libstdc++ does not imply that your compiler
 449          will be able to <em>use</em> all of the features found in the
 450          C++ Standard Library.
 451       </p>
 452       <p>Since the goal of ISO Standardization is for all C++
 453          implementations to be able to share code, the final libstdc++
 454          should, in theory, be usable under any ISO-compliant
 455          compiler.  It will still be targeted and optimized for
 456          GCC/g++, however.
 457       </p>
 458
 459 <hr />
 460    <h2><a name="3_2">3.2 [removed]</a></h2>
 461       <p>This question has become moot and has been removed.  The stub
 462          is here to preserve numbering (and hence links/bookmarks).
 463       </p>
 464
 465 <hr />
 466    <h2><a name="3_3">3.3 [removed]</a></h2>
 467       <p>This question has become moot and has been removed.  The stub
 468          is here to preserve numbering (and hence links/bookmarks).
 469       </p>
 470
 471 <hr />
 472    <h2><a name="3_4">3.4 I can't use 'long long' on Solaris</a></h2>
 473       <p>By default we try to support the C99 <code>long long</code> type.
 474          This requires that certain functions from your C library be present.
 475       </p>
 476       <p>Up through release 3.0.2 the tests performed were too general, and
 477          this feature was disabled when it did not need to be.  The most
 478          commonly reported platform affected was Solaris.
 479       </p>
 480       <p>This has been fixed for 3.0.3 and onwards.
 481       </p>
 482
 483 <hr />
 484    <h2><a name="3_5">3.5 <code>_XOPEN_SOURCE</code> / <code>_GNU_SOURCE</code>
 485                          / etc is always defined</a></h2>
 486       <p>On Solaris, g++ (but not gcc) always defines the preprocessor
 487          macro <code>_XOPEN_SOURCE</code>.  On GNU/Linux, the same happens
 488          with <code>_GNU_SOURCE</code>.  (This is not an exhaustive list;
 489          other macros and other platforms are also affected.)
 490       </p>
 491       <p>These macros are typically used in C library headers, guarding new
 492          versions of functions from their older versions.  The C++ standard
 493          library includes the C standard library, but it requires the C90
 494          version, which for backwards-compatibility reasons is often not the
 495          default for many vendors.
 496       </p>
 497       <p>More to the point, the C++ standard requires behavior which is only
 498          available on certain platforms after certain symbols are defined.
 499          Usually the issue involves I/O-related typedefs.  In order to
 500          ensure correctness, the compiler simply predefines those symbols.
 501       </p>
 502       <p>Note that it's not enough to #define them only when the library is
 503          being built (during installation).  Since we don't have an 'export'
 504          keyword, much of the library exists as headers, which means that
 505          the symbols must also be defined as your programs are parsed and
 506          compiled.
 507       </p>
 508       <p>To see which symbols are defined, look for CPLUSPLUS_CPP_SPEC in
 509          the gcc config headers for your target (and try changing them to
 510          see what happens when building complicated code).  You can also run
 511          <code>&quot;g++ -E -dM - &lt; /dev/null&quot;</code> to display
 512          a list of predefined macros for any particular installation.
 513       </p>
 514       <p>This has been discussed on the mailing lists
 515          <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>.
 516       </p>
 517       <p>This method is something of a wart.  We'd like to find a cleaner
 518          solution, but nobody yet has contributed the time.
 519       </p>
 520
 521 <hr />
 522    <h2><a name="3_6">3.6 OS X ctype.h is broken!  How can I hack it?</a></h2>
 523       <p>This is a long-standing bug in the OS X support.  Fortunately,
 524          the patch is quite simple, and well-known.
 525          <a href="http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html"> Here's a
 526          link to the solution.</a>
 527       </p>
 528
 529 <hr />
 530    <h2><a name="3_7">3.7 Threading is broken on i386</a></h2>
 531       <p>Support for atomic integer operations is/was broken on i386
 532          platforms.  The assembly code accidentally used opcodes that are
 533          only available on the i486 and later.  So if you configured GCC
 534          to target, for example, i386-linux, but actually used the programs
 535          on an i686, then you would encounter no problems.  Only when
 536          actually running the code on a i386 will the problem appear.
 537       </p>
 538       <p>This is fixed in 3.2.2.
 539       </p>
 540
 541 <hr />
 542    <h2><a name="3_8">3.8 Recent GNU/Linux glibc required?</a></h2>
 543       <p>When running on GNU/Linux, libstdc++ 3.2.1 (shared library version
 544          5.0.1) and later uses localization and formatting code from the system
 545          C library (glibc) version 2.2.5.  That version of glibc is over a
 546          year old and contains necessary bugfixes.  Many GNU/Linux distros make
 547          glibc version 2.3.x available now.
 548       </p>
 549       <p>The guideline is simple:  the more recent the C++ library, the
 550          more recent the C library.  (This is also documented in the main
 551          GCC installation instructions.)
 552       </p>
 553
 554 <hr />
 555    <h2><a name="3_9">3.9 Can't use wchar_t/wstring on FreeBSD</a></h2>
 556       <p>At the moment there are a few problems in FreeBSD's support for
 557          wide character functions, and as a result the libstdc++ configury
 558          decides that wchar_t support should be disabled.  Once the underlying
 559          problems are fixed in FreeBSD (soon), the library support will
 560          automatically enable itself.
 561       </p>
 562       <p>You can fix the problems yourself, and learn more about the situation,
 563          by reading
 564          <a href="http://gcc.gnu.org/ml/libstdc++/2003-02/subjects.html#00286">
 565          this short thread</a> (&quot;_GLIBCPP_USE_WCHAR_T undefined in
 566          FreeBSD's c++config.h?&quot;).
 567       </p>
 568
 569 <hr />
 570    <h2><a name="3_10">3.10 MIPS atomic operations</a></h2>
 571       <p>The atomic locking routines for MIPS targets requires MIPS II
 572          and later.  A patch went in just after the 3.3 release to
 573          make mips* use the generic implementation instead.  You can also
 574          configure for mipsel-elf as a workaround.
 575       </p>
 576       <p>mips*-*-linux* continues to use the MIPS II routines, and more
 577          work in this area is expected.
 578       </p>
 579
 580 <hr />
 581 <h1><a name="4_0">4.0 Known Bugs and Non-Bugs</a></h1>
 582    <em>Note that this section can get rapidly outdated -- such is the
 583        nature of an open-source project.  For the latest information, join
 584        the mailing list or look through GCC bugzilla.</em>
 585
 586    <p>For 3.0.1, the most common &quot;bug&quot; is an apparently missing
 587       &quot;<code>../</code>&quot; in include/Makefile, resulting in files
 588       like gthr.h and gthr-single.h not being found.  Please read
 589       <a href="http://gcc.gnu.org/install/configure.html">the configuration
 590       instructions for GCC</a>,
 591       specifically the part about configuring in a separate build directory,
 592       and how strongly recommended it is.  Building in the source directory
 593       is fragile, is rarely tested, and tends to break, as in this case.
 594       This was fixed for 3.0.2.
 595    </p>
 596
 597    <p>For 3.1, the most common &quot;bug&quot; is a parse error when using
 598       <code>&lt;fstream&gt;</code>, ending with a message,
 599       &quot;<code>bits/basic_file.h:52: parse error before `{'
 600       token</code>.&quot;  Please read
 601       <a href="http://gcc.gnu.org/install/">the installation instructions for
 602       GCC</a>, specifically the part about not installing newer versions on
 603       top of older versions.  If you install 3.1 over a 3.0.x release, then
 604       the wrong basic_file.h header will be found (its location changed
 605       between releases).
 606    </p>
 607
 608    <p><strong>Please do not report these as bugs.  We know about them.</strong>
 609       Reporting this -- or any other problem that's already been fixed --
 610       hinders the development of GCC, because we have to take time to
 611       respond to your report.  Thank you.
 612    </p>
 613
 614 <hr />
 615    <h2><a name="4_1">4.1 What works already?</a></h2>
 616       <p>Short answer:  Pretty much everything <em>works</em> except for some
 617          corner cases.  Also, localization is incomplete.  For whether it works
 618          well, or as you expect it to work, see 5.2.
 619       </p>
 620       <p>Long answer: See the implementation status pages for C++98,
 621          TR1, and C++0x.
 622       </p>
 623
 624 <hr />
 625    <h2><a name="4_2">4.2 Bugs in gcc/g++ (not libstdc++)</a></h2>
 626       <p>This is by no means meant to be complete nor exhaustive, but
 627          mentions some problems that users may encounter when building
 628          or using libstdc++.  If you are experiencing one of these
 629          problems, you can find more information on the libstdc++ and
 630          the GCC mailing lists.
 631       </p>
 632       <p>Before reporting a bug, examine the
 633          <a href="http://gcc.gnu.org/bugs.html">bugs database</a> with the
 634          category set to &quot;libstdc++&quot;.
 635       </p>
 636       <ul>
 637          <li>Debugging is problematic, due to bugs in line-number generation
 638              (mostly fixed in the compiler) and gdb lagging behind the
 639              compiler (lack of personnel).  We recommend configuring the
 640              compiler using <code>--with-dwarf2</code> if the DWARF2
 641              debugging format is not already the default on your platform.
 642              Also,
 643 <a href="http://gcc.gnu.org/ml/libstdc++/2002-02/msg00034.html">changing your
 644              GDB settings</a> can have a profound effect on your C++ debugging
 645              experiences.  :-)</li>
 646       </ul>
 647
 648 <hr />
 649    <h2><a name="4_3">4.3 Bugs in the C++ language/lib specification</a></h2>
 650       <p>Yes, unfortunately, there are some.  In a
 651          <a href="http://gcc.gnu.org/ml/libstdc++/1998/msg00006.html">message
 652          to the list</a>, Nathan Myers announced that he has started a list of
 653          problems in the ISO C++ Standard itself, especially with
 654          regard to the chapters that concern the library.  The list
 655          itself is
 656          <a href="http://www.cantrip.org/draft-bugs.txt">posted on his
 657          website</a>.  Developers who are having problems interpreting
 658          the Standard may wish to consult his notes.
 659       </p>
 660       <p>For those people who are not part of the ISO Library Group
 661          (i.e., nearly all of us needing to read this page in the first
 662          place :-), a public list of the library defects is occasionally
 663          published <a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/">here</a>.
 664          Some of these have resulted in <a href="#5_2">code changes</a>.
 665       </p>
 666
 667 <hr />
 668    <h2><a name="4_4">4.4 Things in libstdc++ that only look like bugs</a></h2>
 669       <p>There are things which are not bugs in the compiler (4.2) nor
 670          the language specification (4.3), but aren't really bugs in
 671          libstdc++, either.  Really!  Please do not report these as bugs.
 672       </p>
 673       <p><a name="4_4_Weff"><strong>-Weffc++</strong></a>
 674          The biggest of these is the quadzillions of warnings about the
 675          library headers emitted when <code>-Weffc++</code> is used.  Making
 676          libstdc++ &quot;-Weffc++-clean&quot; is not a goal of the project,
 677          for a few reasons.  Mainly, that option tries to enforce
 678          object-oriented programming, while the Standard Library isn't
 679          necessarily trying to be OO.
 680       </p>
 681       <p><a name="4_4_iostreamclear"><strong>reopening a stream fails</strong>
 682          </a> Did I just say that -Weffc++ was our biggest false-bug report?
 683          I lied.  (It used to be.)  Today it seems to be reports that after
 684          executing a sequence like
 685       </p>
 686       <pre>
 687     #include &lt;fstream&gt;
 688     ...
 689     std::fstream  fs(&quot;a_file&quot;);
 690     // .
 691     // . do things with fs...
 692     // .
 693     fs.close();
 694     fs.open(&quot;a_new_file&quot;);</pre>
 695       <p>all operations on the re-opened <code>fs</code> will fail, or at
 696          least act very strangely.  Yes, they often will, especially if
 697          <code>fs</code> reached the EOF state on the previous file.  The
 698          reason is that the state flags are <strong>not</strong> cleared
 699          on a successful call to open().  The standard unfortunately did
 700          not specify behavior in this case, and to everybody's great sorrow,
 701          the <a href="../ext/howto.html#5">proposed LWG resolution in
 702          DR #22</a> is to leave the flags unchanged.  You must insert a call
 703          to <code>fs.clear()</code> between the calls to close() and open(),
 704          and then everything will work like we all expect it to work.
 705          <strong>Update:</strong> for GCC 4.0 we implemented the resolution
 706          of <a href="../ext/howto.html#5">DR #409</a> and open() now calls
 707          <code>clear()</code> on success!
 708       </p>
 709       <p><a name="4_4_rel_ops"><strong>rel_ops</strong></a>
 710          Another is the <code>rel_ops</code> namespace and the template
 711          comparison operator functions contained therein.  If they become
 712          visible in the same namespace as other comparison functions
 713          (e.g., '<code>using</code>' them and the &lt;iterator&gt; header),
 714          then you will suddenly be faced with huge numbers of ambiguity
 715          errors.  This was discussed on the -v3 list; Nathan Myers
 716          <a href="http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html">sums
 717          things up here</a>.  The collisions with vector/string iterator
 718          types have been fixed for 3.1.  <!-- more links to email here -->
 719       </p>
 720       <h3><a name="4_4_interface">The g++-3 headers are <em>not ours</em></a></h3>
 721       <p>If you have found an extremely broken header file which is
 722          causing problems for you, look carefully before submitting a
 723          &quot;high&quot; priority bug report (which you probably shouldn't
 724          do anyhow; see the last paragraph of the page describing
 725          <a href="http://gcc.gnu.org/bugs.html">the GCC bug database</a>).
 726       </p>
 727       <p>If the headers are in <code>${prefix}/include/g++-3</code>, or if
 728          the installed library's name looks like <code>libstdc++-2.10.a</code>
 729          or <code>libstdc++-libc6-2.10.so</code>, then you are using the old
 730          libstdc++-v2 library, which is nonstandard and unmaintained.  Do not
 731          report problems with -v2 to the -v3 mailing list.
 732       </p>
 733       <p>For GCC versions 3.0 and 3.1 the libstdc++ header files are
 734          installed in <code>${prefix}/include/g++-v3</code> (see the 'v'?).
 735          Starting with version 3.2 the headers are installed in
 736          <code>${prefix}/include/c++/${version}</code> as this prevents
 737          headers from previous versions being found by mistake.
 738       </p>
 739       <p><a name="4_4_glibc"><strong>glibc</strong></a>
 740          If you're on a GNU/Linux system and have just upgraded to
 741          glibc 2.2, but are still using gcc 2.95.2, then you should have
 742          read the glibc FAQ, specifically 2.34:
 743       </p>
 744       <pre>
 745 2.34.   When compiling C++ programs, I get a compilation error in streambuf.h.
 746
 747 {BH} You are using g++ 2.95.2? After upgrading to glibc 2.2, you need to
 748 apply a patch to the include files in /usr/include/g++, because the fpos_t
 749 type has changed in glibc 2.2.  The patch is at
 750 http://clisp.cons.org/~haible/gccinclude-glibc-2.2-compat.diff
 751    </pre>
 752       <p>Note that 2.95.x shipped with the
 753          <a href="#4_4_interface">old v2 library</a> which is no longer
 754          maintained.  Also note that gcc 2.95.3 fixes this problem, but
 755          requires a separate patch for libstdc++.
 756       </p>
 757       <p><a name="4_4_checks"><strong>concept checks</strong></a>
 758          If you see compilation errors containing messages about
 759          <code> <em>foo</em>Concept </code>and a<code> constraints </code>
 760          member function, then most likely you have violated one of the
 761          requirements for types used during instantiation of template
 762          containers and functions.  For example, EqualityComparableConcept
 763          appears if your types must be comparable with == and you have not
 764          provided this capability (a typo, or wrong visibility, or you
 765          just plain forgot, etc).
 766       </p>
 767       <p>More information, including how to optionally enable/disable the
 768          checks, is available
 769          <a href="../19_diagnostics/howto.html#3">here</a>.
 770       </p>
 771       <p><a name="4_4_dlsym"><strong>dlopen/dlsym</strong></a>
 772          If you are using the C++ library across dynamically-loaded
 773          objects, make certain that you are passing the correct options
 774          when compiling and linking:
 775       </p>
 776       <pre>
 777     // compile your library components
 778     g++ -fPIC -c a.cc
 779     g++ -fPIC -c b.cc
 780     ...
 781     g++ -fPIC -c z.cc
 782
 783     // create your library
 784     g++ -fPIC -shared -rdynamic -o libfoo.so a.o b.o ... z.o
 785
 786     // link the executable
 787     g++ -fPIC -rdynamic -o foo ... -L. -lfoo -ldl</pre>
 788       <p><a name="4_4_leak"><strong>"memory leaks" in containers</strong></a>
 789          A few people have reported that the standard containers appear
 790          to leak memory when tested with memory checkers such as
 791          <a href="http://developer.kde.org/~sewardj/">valgrind</a>.
 792          The library's default allocators keep free memory in a pool
 793          for later reuse, rather than returning it to the OS.  Although
 794          this memory is always reachable by the library and is never
 795          lost, memory debugging tools can report it as a leak.  If you
 796          want to test the library for memory leaks please read
 797          <a href="../debug.html#mem">Tips for memory leak hunting</a>
 798          first.
 799       </p>
 800
 801       <p><a name="4_4_list_size"><strong>list::size() is O(n)!</strong></a>
 802          See the <a href='../23_containers/howto.html#6'>Containers</a>
 803          chapter.
 804       </p>
 805 <hr />
 806    <h2><a name="4_5">4.5 Aw, that's easy to fix!</a></h2>
 807       <p>If you have found a bug in the library and you think you have
 808          a working fix, then send it in!  The main GCC site has a page
 809          on <a href="http://gcc.gnu.org/contribute.html">submitting
 810          patches</a> that covers the procedure, but for libstdc++ you
 811          should also send the patch to our mailing list in addition to
 812          the GCC patches mailing list.  The libstdc++
 813          <a href="../17_intro/contribute.html">contributors' page</a>
 814          also talks about how to submit patches.
 815       </p>
 816       <p>In addition to the description, the patch, and the ChangeLog
 817          entry, it is a Good Thing if you can additionally create a small
 818          test program to test for the presence of the bug that your
 819          patch fixes.  Bugs have a way of being reintroduced; if an old
 820          bug creeps back in, it will be caught immediately by the
 821          <a href="#2_4">testsuite</a> -- but only if such a test exists.
 822       </p>
 823
 824 <hr />
 825 <h1><a name="5_0">5.0 Miscellaneous</a></h1>
 826    <h2><a name="5_1">5.1 string::iterator is not char*;
 827                      vector&lt;T&gt;::iterator is not T*</a></h2>
 828       <p>If you have code that depends on container&lt;T&gt; iterators
 829          being implemented as pointer-to-T, your code is broken.
 830       </p>
 831       <p>While there are arguments for iterators to be implemented in
 832          that manner, A) they aren't very good ones in the long term,
 833          and B) they were never guaranteed by the Standard anyway.  The
 834          type-safety achieved by making iterators a real class rather
 835          than a typedef for <code>T*</code> outweighs nearly all opposing
 836          arguments.
 837       </p>
 838       <p>Code which does assume that a vector iterator <code> i </code>
 839          is a pointer can often be fixed by changing <code> i </code> in
 840          certain expressions to <code> &amp;*i </code>.  Future revisions
 841          of the Standard are expected to bless this usage for
 842          vector&lt;&gt; (but not for basic_string&lt;&gt;).
 843       </p>
 844
 845 <hr />
 846    <h2><a name="5_2">5.2 What's next after libstdc++?</a></h2>
 847       <p>Hopefully, not much.  The goal of libstdc++ is to produce
 848          a fully-compliant, fully-portable Standard Library.  After that,
 849          we're mostly done:  there won't <em>be</em> any more compliance
 850          work to do.  However:
 851       </p>
 852       <ol>
 853       <li><p>The ISO Committee will meet periodically to review Defect Reports
 854          in the C++ Standard.  Undoubtedly some of these will result in
 855          changes to the Standard, which will be reflected in patches to
 856          libstdc++.  Some of that is already happening, see <a href="#4_3">4.3</a>.  Some of
 857          those changes are being predicted by the library maintainers, and
 858          we add code to the library based on what the current proposed
 859          resolution specifies.  Those additions are listed in
 860          <a href="../ext/howto.html#5">the extensions page</a>.
 861       </p></li>
 862       <li><p>Performance tuning.  Lots of performance tuning was done for the
 863          3.x releases, including memory expansion in container classes and
 864          buffer usage in synchronized stream objects.
 865          Later performance-related work includes "move semantics"
 866          for containers and (optional) non-reference-counted strings (which
 867          can give performance benefits for multithreaded programs.)
 868       </p></li>
 869       <li><p>An ABI for libstdc++ is being developed, so that
 870          multiple binary-incompatible copies of the library can be replaced
 871          with a single backwards-compatible library, like libgcc_s.so is.
 872       </p></li>
 873       <li><p>The current libstdc++ contains extensions to the Library which
 874          must be explicitly requested by client code (for example, the
 875          hash tables from SGI).  Other extensions may be added to
 876          libstdc++ if they seem to be &quot;standard&quot; enough.
 877          (For example, the &quot;long long&quot; type from C99.)
 878          Bugfixes and rewrites (to improve or fix thread safety, for
 879          instance) will of course be a continuing task.
 880       </p></li>
 881       <li><p>There is an effort underway to add significant extensions to
 882          the standard library specification.  The latest version of this effort is
 883          described in
 884          <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">
 885          The C++ Library Technical Report 1</a>.
 886          See <a href="#5_5">5.5</a>.
 887       </p></li>
 888       </ol>
 889       <p><a href="http://gcc.gnu.org/ml/libstdc++/1999/msg00080.html">This
 890          question</a> about the next libstdc++ prompted some brief but
 891          interesting
 892          <a href="http://gcc.gnu.org/ml/libstdc++/1999/msg00084.html">speculation</a>.
 893       </p>
 894
 895 <hr />
 896    <h2><a name="5_3">5.3 What about the STL from SGI?</a></h2>
 897       <p>The <a href="http://www.sgi.com/tech/stl/">STL from SGI</a>,
 898          version 3.3, was the final merge of the STL codebase.  The
 899          code in libstdc++ contains many fixes and changes, and
 900          the SGI code is no longer under active
 901          development.  We expect that no future merges will take place.
 902       </p>
 903       <p>In particular, <code>string</code> is not from SGI and makes no
 904          use of their &quot;rope&quot; class (which is included as an
 905          optional extension), nor is <code>valarray</code> and some others.
 906          Classes like <code>vector&lt;&gt;</code> are, however we have
 907          made significant changes to them since then.
 908       </p>
 909       <p>The FAQ for SGI's STL (one jump off of their main page) is
 910          recommended reading.
 911       </p>
 912
 913 <hr />
 914    <h2><a name="5_4">5.4 Extensions and Backward Compatibility</a></h2>
 915       <p>Headers in the <code>ext</code> and <code>backward</code>
 916          subdirectories should be referred to by their relative paths:
 917          <!-- Careful, the leading spaces in PRE show up directly. -->
 918       </p>
 919       <pre>
 920       #include &lt;backward/hash_map&gt; </pre>
 921       <p>rather than using <code>-I</code> or other options.  This is more
 922          portable and forward-compatible.  (The situation is the same as
 923          that of other headers whose directories are not searched directly,
 924          e.g., <code>&lt;sys/stat.h&gt;</code>, <code>&lt;X11/Xlib.h&gt;</code>.
 925       </p>
 926
 927       <p>At this time most of the features of the SGI STL extension have been
 928          replaced by standardized libraries.
 929          In particular, the unordered_map and unordered_set containers of TR1
 930          are suitable replacement for the non-standard hash_map and hash_set
 931          containers in the SGI STL.  See <a href="#5_5">5.5</a> for more details.
 932       </p>
 933
 934       <p>The extensions are no longer in the global or <code>std</code>
 935          namespaces, instead they are declared in the <code>__gnu_cxx</code>
 936          namespace. For maximum portability, consider defining a namespace
 937          alias to use to talk about extensions, e.g.:
 938       </p>
 939       <pre>
 940       #ifdef __GNUC__
 941       #if __GNUC__ &lt; 3
 942         #include &lt;hash_map.h&gt;
 943         namespace extension { using ::hash_map; }; // inherit globals
 944       #else
 945         #include &lt;backward/hash_map&gt;
 946         #if __GNUC__ == 3 &amp;&amp; __GNUC_MINOR__ == 0
 947           namespace extension = std;               // GCC 3.0
 948         #else
 949           namespace extension = ::__gnu_cxx;       // GCC 3.1 and later
 950         #endif
 951       #endif
 952       #else      // ...  there are other compilers, right?
 953         namespace extension = std;
 954       #endif
 955
 956       extension::hash_map&lt;int,int&gt; my_map; </pre>
 957       <p>This is a bit cleaner than defining typedefs for all the
 958          instantiations you might need.
 959       </p>
 960       <p><strong>Note:</strong> explicit template specializations must
 961          be declared in the same namespace as the original template.
 962          This means you cannot use a namespace alias when declaring
 963          an explicit specialization.
 964       </p>
 965       <p>Extensions to the library have
 966          <a href="../ext/howto.html">their own page</a>.
 967       </p>
 968
 969 <hr />
 970    <h2><a name="5_5">5.5 Does libstdc++ support TR1?</a></h2>
 971
 972       <p>The C++ Standard Library Technical Report adds many new features to
 973          the library.  The latest version of this effort is described in
 974          <a href=
 975          "http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">
 976          Technical Report 1</a>.
 977       </p>
 978
 979       <p>libstdc++ strives to implement all of TR1.
 980          An <a href="../ext/tr1.html">overview</a> of the implementation status
 981          is available.
 982       </p>
 983
 984       <p>Briefly, the features of TR1 and the current status are:
 985       </p>
 986
 987       <p><strong>Reference_wrapper - Complete -</strong>
 988          Useful to pass references to functions that take their parameters
 989          by value.
 990       </p>
 991
 992       <p><strong>Reference-counted smart pointers - Complete -</strong>
 993          The shared_ptr and weak_ptr allow several object to know about a
 994          pointer and whether it is valid.  When the last reference to the
 995          pointer is destroyed the pointer is freed.
 996       </p>
 997
 998       <p><strong>Function objects - Complete -</strong>
 999          Function return types (i.e., result_of), the functions template
1000          mem_fn (a generalization of mem_fun and mem_fun_red), function
1001          object binders (e.g., bind, a generalization of bind1st and bind2nd),
1002          and polymorphic function wrappers (e.g, class template function).
1003       </p>
1004
1005       <p><strong>Type traits - Complete -</strong>
1006          The type_traits class gives templates the ability to probe
1007          information about the input type and enable type-dependent logic
1008          to be performed without the need of template specializations.
1009       </p>
1010
1011       <p><strong>A random number engine - Complete -</strong>
1012          This library contains random number generators with several different
1013          choices of distribution.
1014       </p>
1015
1016       <p><strong>Tuples - Complete -</strong>
1017          The tuple class implements small heterogeneous arrays.  This is an
1018          enhanced pair.  In fact, the standard pair is enhanced with a tuple
1019          interface.
1020       </p>
1021
1022       <p><strong>Fixed-size arrays - Complete -</strong>
1023          The array class implements small fixed-sized arrays with container
1024          semantics.
1025       </p>
1026
1027       <p><strong>Unordered containers - Complete -</strong>
1028          The unordered_set, unordered_map, unordered_multiset, and
1029          unordered_multimap containers are hashed versions of the map, set,
1030          multimap, and multiset containers respectively.  These classes are
1031          suitable replacements for the SGI STL hash_map and hash_set
1032          extensions.
1033       </p>
1034
1035       <p><strong>C99 compatibility - Under construction - </strong>
1036          There are many features designed to minimize the divergence of the C
1037          and the C++ languages.
1038       </p>
1039
1040       <p><strong>Special functions - Complete - </strong>
1041          Twenty-three mathematical functions familiar to physicists and
1042          engineers are included:  cylindrical and spherical Bessel and Neumann
1043          functions, hypergeometric functions, Laguerre polynomials, Legendre
1044          functions, elliptic integrals, exponential integrals and the Riemann
1045          zeta function all for your computing pleasure.
1046       </p>
1047
1048       <p><strong>A regular expression engine</strong>
1049          This library provides for regular expression objects with traversal
1050          of text with return of subexpressions.
1051       </p>
1052
1053 <hr />
1054    <h2><a name="5_6">5.6 Is libstdc++ thread-safe?</a></h2>
1055       <p>The library strives to be thread-safe when all of the following
1056          conditions are met:
1057       </p>
1058       <ul>
1059        <li>The system's libc is itself thread-safe,</li>
1060        <li>The compiler in use reports a thread model other than 'single'. This can be tested via output from <code>gcc -v</code>. Multi-thread capable versions of gcc output something like this:
1061 <pre>
1062 %gcc -v
1063 Using built-in specs.
1064 ...
1065 Thread model: posix
1066 gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)
1067 </pre>
1068
1069 <p>Look for "Thread model" lines that aren't equal to "single."</p>
1070  </li>
1071        <li>Requisite command-line flags are used for atomic operations and threading. Examples of this include <code>-pthread</code> and <code>-march=native</code>, although specifics vary depending on the host environment. See <a href="http://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html">Machine Dependent Options</a>.</li>
1072        <li>An implementation of atomicity.h functions
1073            exists for the architecture in question. See the internals documentation for more <a href="../ext/concurrence.html">details</a>.</li>
1074
1075       </ul>
1076       <p>The user-code must guard against concurrent method calls which may
1077          access any particular library object's state.  Typically, the
1078          application programmer may infer what object locks must be held
1079          based on the objects referenced in a method call.  Without getting
1080          into great detail, here is an example which requires user-level
1081          locks:
1082       </p>
1083       <pre>
1084      library_class_a shared_object_a;
1085
1086      thread_main () {
1087        library_class_b *object_b = new library_class_b;
1088        shared_object_a.add_b (object_b);   // must hold lock for shared_object_a
1089        shared_object_a.mutate ();          // must hold lock for shared_object_a
1090      }
1091
1092      // Multiple copies of thread_main() are started in independent threads.</pre>
1093       <p>Under the assumption that object_a and object_b are never exposed to
1094          another thread, here is an example that should not require any
1095          user-level locks:
1096       </p>
1097       <pre>
1098      thread_main () {
1099        library_class_a object_a;
1100        library_class_b *object_b = new library_class_b;
1101        object_a.add_b (object_b);
1102        object_a.mutate ();
1103      } </pre>
1104       <p>All library objects are safe to use in a multithreaded program as
1105          long as each thread carefully locks out access by any other
1106          thread while it uses any object visible to another thread, i.e.,
1107          treat library objects like any other shared resource.  In general,
1108          this requirement includes both read and write access to objects;
1109          unless otherwise documented as safe, do not assume that two threads
1110          may access a shared standard library object at the same time.
1111       </p>
1112       <p>See chapters <a href="../17_intro/howto.html#3">17</a> (library
1113          introduction), <a href="../23_containers/howto.html#3">23</a>
1114          (containers), and <a href="../27_io/howto.html#9">27</a> (I/O) for
1115          more information.
1116       </p>
1117
1118 <hr />
1119    <h2><a name="5_7">5.7 How do I get a copy of the ISO C++ Standard?</a></h2>
1120       <p>Copies of the full ISO 14882 standard are available on line via the
1121          ISO mirror site for committee members.  Non-members, or those who
1122          have not paid for the privilege of sitting on the committee and
1123          sustained their two-meeting commitment for voting rights, may get a
1124          copy of the standard from their respective national standards
1125          organization.  In the USA, this national standards organization is
1126          ANSI and their website is right <a href="http://www.ansi.org">here</a>.
1127          (And if you've already registered with them, clicking this link will
1128          take you to directly to the place where you can
1129 <a href="http://webstore.ansi.org/ansidocstore/product.asp?sku=ISO%2FIEC+14882%3A2003">buy
1130          the standard on-line</a>.
1131       </p>
1132       <p>Who is your country's member body?  Visit the
1133          <a href="http://www.iso.ch/">ISO homepage</a> and find out!
1134       </p>
1135       <p>The 2003 version of the standard (the 1998 version plus TC1) is
1136          available in print, ISBN 0-470-84674-7.
1137       </p>
1138
1139 <hr />
1140    <h2><a name="5_8">5.8 What's an ABI and why is it so messy?</a></h2>
1141       <p>&quot;ABI&quot; stands for &quot;Application Binary Interface.&quot;
1142          Conventionally, it refers to a great mass of details about how
1143          arguments are arranged on the call stack and/or in registers, and
1144          how various types are arranged and padded in structs.  A single CPU
1145          design may suffer multiple ABIs designed by different development
1146          tool vendors who made different choices, or even by the same vendor
1147          for different target applications or compiler versions.  In ideal
1148          circumstances the CPU designer presents one ABI and all the OSes and
1149          compilers use it.  In practice every ABI omits details that compiler
1150          implementers (consciously or accidentally) must choose for themselves.
1151       </p>
1152       <p>That ABI definition suffices for compilers to generate code so a
1153          program can interact safely with an OS and its lowest-level libraries.
1154          Users usually want an ABI to encompass more detail, allowing libraries
1155          built with different compilers (or different releases of the same
1156          compiler!) to be linked together.  For C++, this includes many more
1157          details than for C, and CPU designers (for good reasons elaborated
1158          below) have not stepped up to publish C++ ABIs.  The details include
1159          virtual function implementation, struct inheritance layout, name
1160          mangling, and exception handling.  Such an ABI has been defined for
1161          GNU C++, and is immediately useful for embedded work relying only on
1162          a &quot;free-standing implementation&quot; that doesn't include (much
1163          of) the standard library.  It is a good basis for the work to come.
1164       </p>
1165       <p>A useful C++ ABI must also incorporate many details of the standard
1166          library implementation.  For a C ABI, the layouts of a few structs
1167          (such as FILE, stat, jmpbuf, and the like) and a few macros suffice.
1168          For C++, the details include the complete set of names of functions
1169          and types used, the offsets of class members and virtual functions,
1170          and the actual definitions of all inlines.  C++ exposes many more
1171          library details to the caller than C does.  It makes defining
1172          a complete ABI a much bigger undertaking, and requires not just
1173          documenting library implementation details, but carefully designing
1174          those details so that future bug fixes and optimizations don't
1175          force breaking the ABI.
1176       </p>
1177       <p>There are ways to help isolate library implementation details from the
1178          ABI, but they trade off against speed.  Library details used in
1179          inner loops (e.g., getchar) must be exposed and frozen for all
1180          time, but many others may reasonably be kept hidden from user code,
1181          so they may later be changed.  Deciding which, and implementing
1182          the decisions, must happen before you can reasonably document a
1183          candidate C++ ABI that encompasses the standard library.
1184       </p>
1185
1186 <hr />
1187    <h2><a name="5_9">5.9 How do I make std::vector&lt;T&gt;::capacity()
1188                      == std::vector&lt;T&gt;::size()?</a> </h2>
1189    <!-- referenced by 21_strings/howto.html#6 -->
1190    <p>The standard idiom for deallocating a <code>std::vector&lt;T&gt;</code>'s
1191       unused memory is to create a temporary copy of the vector and swap their
1192       contents, e.g. for <code>std::vector&lt;T&gt; v</code>
1193    </p>
1194    <pre>
1195      std::vector&lt;T&gt;(v).swap(v);
1196    </pre>
1197    <p>The copy will take O(n) time and the swap is constant time.
1198    </p>
1199    <p>See <a href='../21_strings/howto.html#6'>Shrink-to-fit strings</a> for
1200       a similar solution for strings.
1201    </p>
1202
1203 <!-- ####################################################### -->
1204
1205 <hr />
1206 <p class="fineprint"><em>
1207 See <a href="../17_intro/license.html">license.html</a> for copying conditions.
1208 Comments and suggestions are welcome, and may be sent to
1209 <a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
1210 </em></p>
1211
1212
1213 </body>
1214 </html>
1215