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   <!--*********************************************************************-->
   <h1>"libc++" C++ Standard Library</h1>
   <!--*********************************************************************-->

   <p>libc++ is an implementation of the C++ standard library, targeting
      C++11, C++14 and above.</p>

   <p>All of the code in libc++ is <a
      href="https://llvm.org/docs/DeveloperPolicy.html#license">dual licensed</a>
      under the MIT license and the UIUC License (a BSD-like license).</p>

   <!--=====================================================================-->
   <h2>New Documentation Coming Soon!</h2>
   <!--=====================================================================-->

   <p> Looking for documentation on how to use, build and test libc++? If so
       checkout the new libc++ documentation.</p>

   <p><a href="https://libcxx.llvm.org/docs/">
       Click here for the new libc++ documentation.</a></p>

   <!--=====================================================================-->
   <h2 id="goals">Features and Goals</h2>
   <!--=====================================================================-->

     <ul>
         <li>Correctness as defined by the C++11 standard.</li>
         <li>Fast execution.</li>
         <li>Minimal memory use.</li>
         <li>Fast compile times.</li>
         <li>ABI compatibility with gcc's libstdc++ for some low-level features
             such as exception objects, rtti and memory allocation.</li>
         <li>Extensive unit tests.</li>
     </ul>

   <!--=====================================================================-->
   <h2 id="why">Why a new C++ Standard Library for C++11?</h2>
   <!--=====================================================================-->

   <p>After its initial introduction, many people have asked "why start a new
      library instead of contributing to an existing library?" (like Apache's
      libstdcxx, GNU's libstdc++, STLport, etc).  There are many contributing
      reasons, but some of the major ones are:</p>

   <ul>
   <li><p>From years of experience (including having implemented the standard
       library before), we've learned many things about implementing
       the standard containers which require ABI breakage and fundamental changes
       to how they are implemented.  For example, it is generally accepted that
       building std::string using the "short string optimization" instead of
       using Copy On Write (COW) is a superior approach for multicore
       machines (particularly in C++11, which has rvalue references).  Breaking
       ABI compatibility with old versions of the library was
       determined to be critical to achieving the performance goals of
       libc++.</p></li>

   <li><p>Mainline libstdc++ has switched to GPL3, a license which the developers
       of libc++ cannot use.  libstdc++ 4.2 (the last GPL2 version) could be
       independently extended to support C++11, but this would be a fork of the
       codebase (which is often seen as worse for a project than starting a new
       independent one).  Another problem with libstdc++ is that it is tightly
        integrated with G++ development, tending to be tied fairly closely to the
        matching version of G++.</p>
     </li>

   <li><p>STLport and the Apache libstdcxx library are two other popular
       candidates, but both lack C++11 support.  Our experience (and the
       experience of libstdc++ developers) is that adding support for C++11 (in
       particular rvalue references and move-only types) requires changes to
       almost every class and function, essentially amounting to a rewrite.
       Faced with a rewrite, we decided to start from scratch and evaluate every
       design decision from first principles based on experience.</p>

       <p>Further, both projects are apparently abandoned: STLport 5.2.1 was
       released in Oct'08, and STDCXX 4.2.1 in May'08.</p>

     </ul>

   <!--=====================================================================-->
   <h2 id="requirements">Platform Support</h2>
   <!--=====================================================================-->

   <p>
     libc++ is known to work on the following platforms, using g++ and
     clang. Note that functionality provided by &lt;atomic&gt; is only functional with
     clang.
   </p>

   <ul>
     <li>Mac OS X i386</li>
     <li>Mac OS X x86_64</li>
     <li>FreeBSD 10+ i386</li>
     <li>FreeBSD 10+ x86_64</li>
     <li>FreeBSD 10+ ARM</li>
     <li>Linux i386</li>
     <li>Linux x86_64</li>
   </ul>

   <!--=====================================================================-->
   <h2 id="dir-structure">Current Status</h2>
   <!--=====================================================================-->

    <p>libc++ is a 100% complete C++11 implementation on Apple's OS X. </p>
    <p>LLVM and Clang can self host in C++ and C++11 mode with libc++ on Linux.</p>
    <p>libc++ is also a 100% complete C++14 implementation. A list of new features and
       changes for C++14 can be found <a href="cxx1y_status.html">here</a>.</p>
    <p>libc++'s C++17 implementation is not yet complete. A list of features and changes
       for C++17 can be found <a href="cxx1z_status.html">here</a>.</p>
    <p>A list of features and changes for the next C++ standard, known here as
       "C++2a" (probably to be C++20) can be found <a href="cxx2a_status.html">here</a>.</p>
    <p>Implementation of the post-C++14 Technical Specifications is in progress. A list of features
       and the current status of these features can be found <a href="ts1z_status.html">here</a>.</p>
    <p>As features get moved from the Technical Specifications into the main standard, we
       will (after a period for migration) remove them from the TS implementation. This
       process is detailed <a href="DesignDocs/ExperimentalFeatures.html">here</a>.</p>

    <!--======================================================================-->
    <h2 id="buildbots">Build Bots</h2>
    <!--======================================================================-->
    <p>The latest libc++ build results can be found at the following locations.</p>
    <ul>
       <li><a href="http://lab.llvm.org:8011/console">
         Buildbot libc++ builders
       </a></li>
       <li><a href="http://lab.llvm.org:8080/green/view/Libcxx/">
         Jenkins libc++ builders
       </a></li>
     </ul>

   <!--=====================================================================-->
   <h2>Get it and get involved!</h2>
   <!--=====================================================================-->

   <p>First please review our
      <a href="https://llvm.org/docs/DeveloperPolicy.html">Developer's Policy</a>.

     The documentation for building and using libc++ can be found below.
     <ul>
       <li><a href="https://libcxx.llvm.org/docs/UsingLibcxx.html">
         <b>Using libc++</b></a>
           Documentation on using the library in your programs</li>
       <li><a href="https://libcxx.llvm.org/docs/BuildingLibcxx.html">
         <b>Building libc++</b></a>
           Documentation on building the library using CMake</li>
       <li><a href="https://libcxx.llvm.org/docs/TestingLibcxx.html">
         <b>Testing libc++</b></a>
           Documentation for developers wishing to test the library</li>
     </ul>

   <!--=====================================================================-->
   <h3>Notes and Known Issues</h3>
   <!--=====================================================================-->

   <p>
     <ul>
       <li>
         Building libc++ with <code>-fno-rtti</code> is not supported. However
         linking against it with <code>-fno-rtti</code> is supported.
       </li>
     </ul>
   </p>

   <p>Send discussions to the
     <a href="https://lists.llvm.org/mailman/listinfo/libcxx-dev">libc++ mailing list</a>.</p>

   <!--=====================================================================-->
   <h2>Bug reports and patches</h2>
   <!--=====================================================================-->

   <p>
   If you think you've found a bug in libc++, please report it using
   the <a href="https://bugs.llvm.org/">LLVM Bugzilla</a>. If you're not sure, you
   can post a message to the <a href="https://lists.llvm.org/mailman/listinfo/libcxx-dev">libcxx-dev</a>
   mailing list or on IRC.
   </p>

   <p>
   If you want to contribute a patch to libc++, the best place for that is
   <a href="https://llvm.org/docs/Phabricator.html">Phabricator</a>. Please
   add libcxx-commits as a subscriber.
   </p>

   <!--=====================================================================-->
   <h2>Design Documents</h2>
   <!--=====================================================================-->

 <ul>
 <li><a href="atomic_design.html"><tt>&lt;atomic&gt;</tt></a></li>
 <li><a href="type_traits_design.html"><tt>&lt;type_traits&gt;</tt></a></li>
 <li><a href="https://cplusplusmusings.wordpress.com/2012/07/05/clang-and-standard-libraries-on-mac-os-x/">Excellent notes by Marshall Clow</a></li>
 </ul>

 </div>
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	<label>libc++ Info</label>
	<a href="/index.html">About</a>
	</div>

	<div class="submenu">
	<label>Quick Links</label>
	<a href="https://libcxxabi.llvm.org/">libc++abi</a>
	<a href="https://lists.llvm.org/mailman/listinfo/libcxx-dev">libcxx-dev</a>
	<a href="https://lists.llvm.org/mailman/listinfo/libcxx-commits">libcxx-commits</a>
	<a href="https://bugs.llvm.org/">Bug Reports</a>
	<a href="https://github.com/llvm/llvm-project/tree/master/libcxx/">Browse Sources</a>
	</div>
	</div>

	<div id="content">
	<!--*********************************************************************-->
	<h1>"libc++" C++ Standard Library</h1>
	<!--*********************************************************************-->

	<p>libc++ is an implementation of the C++ standard library, targeting
	C++11, C++14 and above.</p>

	<p>All of the code in libc++ is <a
	href="https://llvm.org/docs/DeveloperPolicy.html#license">dual licensed</a>
	under the MIT license and the UIUC License (a BSD-like license).</p>

	<!--=====================================================================-->
	<h2>New Documentation Coming Soon!</h2>
	<!--=====================================================================-->

	<p> Looking for documentation on how to use, build and test libc++? If so
	checkout the new libc++ documentation.</p>

	<p><a href="https://libcxx.llvm.org/docs/">
	Click here for the new libc++ documentation.</a></p>

	<!--=====================================================================-->
	<h2 id="goals">Features and Goals</h2>
	<!--=====================================================================-->

	<ul>
	<li>Correctness as defined by the C++11 standard.</li>
	<li>Fast execution.</li>
	<li>Minimal memory use.</li>
	<li>Fast compile times.</li>
	<li>ABI compatibility with gcc's libstdc++ for some low-level features
	such as exception objects, rtti and memory allocation.</li>
	<li>Extensive unit tests.</li>
	</ul>

	<!--=====================================================================-->
	<h2 id="why">Why a new C++ Standard Library for C++11?</h2>
	<!--=====================================================================-->

	<p>After its initial introduction, many people have asked "why start a new
	library instead of contributing to an existing library?" (like Apache's
	libstdcxx, GNU's libstdc++, STLport, etc). There are many contributing
	reasons, but some of the major ones are:</p>

	<ul>
	<li><p>From years of experience (including having implemented the standard
	library before), we've learned many things about implementing
	the standard containers which require ABI breakage and fundamental changes
	to how they are implemented. For example, it is generally accepted that
	building std::string using the "short string optimization" instead of
	using Copy On Write (COW) is a superior approach for multicore
	machines (particularly in C++11, which has rvalue references). Breaking
	ABI compatibility with old versions of the library was
	determined to be critical to achieving the performance goals of
	libc++.</p></li>

	<li><p>Mainline libstdc++ has switched to GPL3, a license which the developers
	of libc++ cannot use. libstdc++ 4.2 (the last GPL2 version) could be
	independently extended to support C++11, but this would be a fork of the
	codebase (which is often seen as worse for a project than starting a new
	independent one). Another problem with libstdc++ is that it is tightly
	integrated with G++ development, tending to be tied fairly closely to the
	matching version of G++.</p>
	</li>

	<li><p>STLport and the Apache libstdcxx library are two other popular
	candidates, but both lack C++11 support. Our experience (and the
	experience of libstdc++ developers) is that adding support for C++11 (in
	particular rvalue references and move-only types) requires changes to
	almost every class and function, essentially amounting to a rewrite.
	Faced with a rewrite, we decided to start from scratch and evaluate every
	design decision from first principles based on experience.</p>

	<p>Further, both projects are apparently abandoned: STLport 5.2.1 was
	released in Oct'08, and STDCXX 4.2.1 in May'08.</p>

	</ul>

	<!--=====================================================================-->
	<h2 id="requirements">Platform Support</h2>
	<!--=====================================================================-->

	<p>
	libc++ is known to work on the following platforms, using g++ and
	clang. Note that functionality provided by <atomic> is only functional with
	clang.
	</p>

	<ul>
	<li>Mac OS X i386</li>
	<li>Mac OS X x86_64</li>
	<li>FreeBSD 10+ i386</li>
	<li>FreeBSD 10+ x86_64</li>
	<li>FreeBSD 10+ ARM</li>
	<li>Linux i386</li>
	<li>Linux x86_64</li>
	</ul>

	<!--=====================================================================-->
	<h2 id="dir-structure">Current Status</h2>
	<!--=====================================================================-->

	<p>libc++ is a 100% complete C++11 implementation on Apple's OS X. </p>
	<p>LLVM and Clang can self host in C++ and C++11 mode with libc++ on Linux.</p>
	<p>libc++ is also a 100% complete C++14 implementation. A list of new features and
	changes for C++14 can be found <a href="cxx1y_status.html">here</a>.</p>
	<p>libc++'s C++17 implementation is not yet complete. A list of features and changes
	for C++17 can be found <a href="cxx1z_status.html">here</a>.</p>
	<p>A list of features and changes for the next C++ standard, known here as
	"C++2a" (probably to be C++20) can be found <a href="cxx2a_status.html">here</a>.</p>
	<p>Implementation of the post-C++14 Technical Specifications is in progress. A list of features
	and the current status of these features can be found <a href="ts1z_status.html">here</a>.</p>
	<p>As features get moved from the Technical Specifications into the main standard, we
	will (after a period for migration) remove them from the TS implementation. This
	process is detailed <a href="DesignDocs/ExperimentalFeatures.html">here</a>.</p>

	<!--======================================================================-->
	<h2 id="buildbots">Build Bots</h2>
	<!--======================================================================-->
	<p>The latest libc++ build results can be found at the following locations.</p>
	<ul>
	<li><a href="http://lab.llvm.org:8011/console">
	Buildbot libc++ builders
	</a></li>
	<li><a href="http://lab.llvm.org:8080/green/view/Libcxx/">
	Jenkins libc++ builders
	</a></li>
	</ul>

	<!--=====================================================================-->
	<h2>Get it and get involved!</h2>
	<!--=====================================================================-->

	<p>First please review our
	<a href="https://llvm.org/docs/DeveloperPolicy.html">Developer's Policy</a>.

	The documentation for building and using libc++ can be found below.
	<ul>
	<li><a href="https://libcxx.llvm.org/docs/UsingLibcxx.html">
	<b>Using libc++</b></a>
	Documentation on using the library in your programs</li>
	<li><a href="https://libcxx.llvm.org/docs/BuildingLibcxx.html">
	<b>Building libc++</b></a>
	Documentation on building the library using CMake</li>
	<li><a href="https://libcxx.llvm.org/docs/TestingLibcxx.html">
	<b>Testing libc++</b></a>
	Documentation for developers wishing to test the library</li>
	</ul>

	<!--=====================================================================-->
	<h3>Notes and Known Issues</h3>
	<!--=====================================================================-->

	<p>
	<ul>
	<li>
	Building libc++ with <code>-fno-rtti</code> is not supported. However
	linking against it with <code>-fno-rtti</code> is supported.
	</li>
	</ul>
	</p>

	<p>Send discussions to the
	<a href="https://lists.llvm.org/mailman/listinfo/libcxx-dev">libc++ mailing list</a>.</p>

	<!--=====================================================================-->
	<h2>Bug reports and patches</h2>
	<!--=====================================================================-->

	<p>
	If you think you've found a bug in libc++, please report it using
	the <a href="https://bugs.llvm.org/">LLVM Bugzilla</a>. If you're not sure, you
	can post a message to the <a href="https://lists.llvm.org/mailman/listinfo/libcxx-dev">libcxx-dev</a>
	mailing list or on IRC.
	</p>

	<p>
	If you want to contribute a patch to libc++, the best place for that is
	<a href="https://llvm.org/docs/Phabricator.html">Phabricator</a>. Please
	add libcxx-commits as a subscriber.
	</p>

	<!--=====================================================================-->
	<h2>Design Documents</h2>
	<!--=====================================================================-->

	<ul>
	<li><a href="atomic_design.html"><tt><atomic></tt></a></li>
	<li><a href="type_traits_design.html"><tt><type_traits></tt></a></li>
	<li><a href="https://cplusplusmusings.wordpress.com/2012/07/05/clang-and-standard-libraries-on-mac-os-x/">Excellent notes by Marshall Clow</a></li>
	</ul>

	</div>
	</body>
	</html>