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

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