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

  <p>libc++ is a new implementation of the C++ standard library, targeting
     C++0X.</p>

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

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

    <ul>
        <li>Correctness as defined by the (currently draft) C++0X 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++'0x?</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++'0x, 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++'0x, 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++'0x support.  Our experience (and the
      experience of libstdc++ developers) is that adding support for C++0x (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++-4.2 and
      clang (lack of C++0X language support disables some functionality).</p>

    <ul>
     <li>Mac OS X i386</li>
     <li>Mac OS X x86_64</li>
    </ul>

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

   <p>libc++ is still under development.  It has about 98% of
      <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3126.pdf">N3126</a>
      implemented/tested.  C++'98 support is fully featured, and most of C++'0x
      support is as well.  The only major missing piece of C++'0x support is
      <code>&lt;atomic&gt;</code>.</p>

   <p><a href="libcxx_by_chapter.pdf">Here</a> is a by-chapter breakdown of what
   is passing tests and what isn't. This chart is currently based on testing
   against g++-4.4.0 with -std=c++0x. </p>

   <p>
   A Windows port is underway.  <a href="results.Windows.html">Here</a> are
   recent test results.
   </p>

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

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

  <p>To check out the code, use:</p>

  <ul>
  <li><code>svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx</code></li>
  </ul>

  <p>
     On Mac OS 10.7 (Lion), the easiest way to get this library is to install
     Xcode 4.2 or later.  However if you want to install tip-of-trunk from here
     (getting the bleeding edge), read on.
  </p>

  <p>To build on Mac OS X 10.6 (Snow Leopard), you need a helper library and header 
    <a href="http://home.roadrunner.com/~hinnant/libcppabi.zip">found here</a>.
    cp cxxabi.h to /usr/include, and cp libc++abi.dylib to /usr/lib.  On Mac OS
    X 10.7 (Lion) and later, this helper library and header are already installed
    for you.
    <p>
    
  <p>
     Next:
  </p>
  
  <ul>
    <li><code>cd libcxx/lib</code></li>
    <li><code>export TRIPLE=-apple-</code></li>
    <li>Only on 10.6: <code>export MACOSX_DEPLOYMENT_TARGET=10.6</code></li>
    <li><code>./buildit</code></li>
  </ul>
  
  <p>
     That should result in a libc++.1.dylib.  To install it I like to use links
     instead of copying, but either should work:
  </p>

  <ul>
    <li><code>cd /usr/lib</code></li>
    <li><code>sudo ln -sf path-to-libcxx/lib/libc++.1.dylib libc++.1.dylib</code></li>
    <li><code>sudo ln -sf libc++.1.dylib libc++.dylib</code></li>
    <li><code>cd /usr/include/c++</code></li>
    <li><code>sudo ln -sf path-to-libcxx/include v1</code></li>
  </ul>

  <p>
  To use with clang you can:
  </p>

  <ul>
    <li><code>clang++ -stdlib=libc++ test.cpp</code></li>
    <li><code>clang++ -std=c++0x -stdlib=libc++ test.cpp</code></li>
  </ul>

  <p>To run the libc++ test suit (recommended):</p>

  <ul>
  <li><code>cd libcxx/test</code></li>
  <li><code>./testit</code></li>
     <ul>
       <li>On Mac OS 10.6, to work around bugs in libc headers like
           math.h and inttypes.h, add "-U__STRICT_ANSI__" and
           "-D__STDC_FORMAT_MACROS" to the command line with:
           <blockquote>
             <pre>export OPTIONS="-std=c++0x -stdlib=libc++ -U__STRICT_ANSI__ -D__STDC_FORMAT_MACROS"</pre>
           </blockquote></li>
       <li>People porting libc++ to other OSes will likely have to
           define similar macros.</li>
     </ul>
  </ul>

  <p>Send discussions to the
  (<a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev">clang mailing list</a>).</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>
</ul>

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