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<h1>"libc++abi" C++ Standard Library Support</h1>
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<p>libc++abi is a new implementation of low level support for a standard
C++ library.</p>
<p>All of the code in libc++abi 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>
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<h2 id="goals">Features and Goals</h2>
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<ul>
<li>Correctness as defined by the C++11 standard.</li>
<li>Provide a portable sublayer to ease the porting of <a href="http://libcxx.llvm.org/">libc++</a></li>
<li>On Mac OS X, be ABI compatible with the existing low-level support.</li>
</ul>
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<h2 id="requirements">Platform Support</h2>
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<p>libc++abi is known to work on the following platforms, using clang.</p>
<ul>
<li>Darwin</li>
</ul>
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<h2 id="dir-structure">Current Status</h2>
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<p>libc++abi is complete. <a href="spec.html">Here</a> is a
list of functionality.</p>
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<h2>Get it and get involved!</h2>
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<p>To check out the code, use:</p>
<ul>
<li><code>svn co http://llvm.org/svn/llvm-project/libcxxabi/trunk libcxxabi</code></li>
</ul>
<p>To build:</p>
<ul>
<li>Check out libcxxabi into <code>llvm/projects</code></li>
<li><code>cd llvm</code></li>
<li><code>mkdir build && cd build</code></li>
<li><code>cmake .. # on linux you may need to prefix with CC=clang CXX=clang++</code></li>
<li><code>make</code></li>
</ul>
<p>To do a standalone build:</p>
<ul>
<li>
Check out the <a href="http://libcxx.llvm.org">libcxx source</a> tree.
</li>
<li><code>cd libcxxabi</code></li>
<li><code>mkdir build && cd build</code></li>
<li><code>cmake -DLIBCXXABI_LIBCXX_PATH=path/to/libcxx .. # on
linux you may need -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++</code></li>
<li><code>make</code></li>
</ul>
<p> By default CMake uses <code>llvm-config</code> to locate the required
LLVM sources. If CMake cannot find <code>llvm-config</code> then you must
configure CMake using either of the following options.
</p>
<ul>
<li><code>-DLLVM_CONFIG_PATH=path/to/llvm-config</code></li>
<li><code>-DLLVM_PATH=path/to/llvm-source-root</code></li>
</ul>
</p>
<p>To run the tests:</p>
<ul>
<li><code>make check-cxxabi</code></li>
</ul>
<p>Note: in a standalone build, the system's libc++ will be used for tests. If
the system's libc++ was statically linked against libc++abi (or linked against
a different ABI library), this may interfere with test results.</p>
<p>Send discussions to the
(<a href="http://lists.llvm.org/mailman/listinfo/cfe-dev">clang mailing list</a>).</p>
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<h2>Frequently asked questions</h2>
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<p>Q: Why are the destructors for the standard exception classes defined in libc++abi?
They're just empty, can't they be defined inline?</p>
<p>A: The destructors for them live in libc++abi because they are "key" functions.
The Itanium ABI describes a "key" function as the first virtual declared.
And wherever the key function is defined, that is where the <code>type_info</code> gets defined.
And in libc++ types are the same type if and only if they have the same <code>type_info</code>
(as in there must be only one type info per type in the entire application).
And on OS X, libstdc++ and libc++ share these exception types.
So to be able to throw in one dylib and catch in another (a <code>std::exception</code> for example),
there must be only one <code>std::exception type_info</code> in the entire app.
That typeinfo gets laid down beside <code>~exception()</code> in libc++abi (for both libstdc++ and libc++).</p>
<p>--Howard Hinnant</p>
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