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<!--*********************************************************************-->
<h1>"libc++" C++ Standard Library</h1>
<!--*********************************************************************-->
<p>libc++ is a new implementation of the C++ standard library, targeting
C++11.</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 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++'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++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++-4.2 and
clang (lack of C++11 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><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 clang++ with -std=c++11 on Apple's OS X. </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. However, be warned that Mac OS
10.7 will not boot without a valid copy of <code>libc++.1.dylib</code> in
<code>/usr/lib</code>.
</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++11 -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++11 -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>
<!--=====================================================================-->
<h3>Notes</h3>
<!--=====================================================================-->
<p>
Building libc++ with <code>-fno-rtti</code> is not supported. However linking
against it with <code>-fno-rtti</code> is supported.
</p>
<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><atomic></tt></a></li>
<li><a href="type_traits_design.html"><tt><type_traits></tt></a></li>
<li><a href="http://marshall.calepin.co/llvmclang-and-standard-libraries-on-mac-os-x.html">Excellent notes by Marshall Clow</a></li>
</ul>
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