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7  <title>"libc++abi" C++ Standard Library Support</title>
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15    <a href="http://llvm.org/">LLVM Home</a>
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19    <label>libc++abi Info</label>
20    <a href="/index.html">About</a>
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24    <label>Quick Links</label>
25    <a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev">cfe-dev</a>
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27    <a href="http://llvm.org/bugs/">Bug Reports</a>
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34  <!--*********************************************************************-->
35  <h1>"libc++abi" C++ Standard Library Support</h1>
36  <!--*********************************************************************-->
37
38  <p>libc++abi is a new implementation of low level support for a standard
39     C++ library.</p>
40
41  <p>All of the code in libc++abi is <a
42     href="http://llvm.org/docs/DeveloperPolicy.html#license">dual licensed</a>
43     under the MIT license and the UIUC License (a BSD-like license).</p>
44
45  <!--=====================================================================-->
46  <h2 id="goals">Features and Goals</h2>
47  <!--=====================================================================-->
48
49    <ul>
50        <li>Correctness as defined by the C++11 standard.</li>
51        <li>Provide a portable sublayer to ease the porting of <a href="http://libcxx.llvm.org/">libc++</a></li>
52        <li>On Mac OS X, be ABI compatible with the existing low-level support.</li>
53    </ul>
54
55  <!--=====================================================================-->
56  <h2 id="requirements">Platform Support</h2>
57  <!--=====================================================================-->
58
59   <p>libc++abi is known to work on the following platforms, using clang.</p>
60
61    <ul>
62    <li>Darwin</li>
63    </ul>
64
65  <!--=====================================================================-->
66  <h2 id="dir-structure">Current Status</h2>
67  <!--=====================================================================-->
68
69   <p>libc++abi is complete.  <a href="spec.html">Here</a> is a
70   list of functionality.</p>
71
72  <!--=====================================================================-->
73  <h2>Get it and get involved!</h2>
74  <!--=====================================================================-->
75
76  <p>To check out the code, use:</p>
77
78  <ul>
79  <li><code>svn co http://llvm.org/svn/llvm-project/libcxxabi/trunk libcxxabi</code></li>
80  </ul>
81
82  <p>Send discussions to the
83  (<a href="http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev">clang mailing list</a>).</p>
84
85  <!--=====================================================================-->
86  <h2>Frequently asked questions</h2>
87  <!--=====================================================================-->
88
89  <p>Q: Why are the destructors for the standard exception classes defined in libc++abi?
90     They're just empty, can't they be defined inline?</p>
91  <p>A: The destructors for them live in libc++abi because they are "key" functions.
92     The Itanium ABI describes a "key" function as the first virtual declared.
93     And wherever the key function is defined, that is where the <code>type_info</code> gets defined.
94     And in libc++ types are the same type if and only if they have the same <code>type_info</code>
95     (as in there must be only one type info per type in the entire application).
96     And on OS X, libstdc++ and libc++ share these exception types.
97     So to be able to throw in one dylib and catch in another (a <code>std::exception</code> for example), 
98     there must be only one <code>std::exception type_info</code> in the entire app.
99     That typeinfo gets laid down beside <code>~exception()</code> in libc++abi (for both libstdc++ and libc++).</p>
100     <p>--Howard Hinnant</p>
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