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  19     <label>libc++abi Info</label>
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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="https://llvm.org/docs/DeveloperPolicy.html#copyright-license-and-patents">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="https://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>For building libc++abi, please see the libc++ documentation on
  77      <a href="https://libcxx.llvm.org/BuildingLibcxx.html">building the runtimes</a>.
  78   </p>
  79
  80   <p>For getting involved with libc++abi, please see the libc++ documentation on
  81       <a href="https://libcxx.llvm.org/Contributing.html">getting involved</a>.
  82   </p>
  83
  84   <!--=====================================================================-->
  85   <h2>Frequently asked questions</h2>
  86   <!--=====================================================================-->
  87
  88   <p>Q: Why are the destructors for the standard exception classes defined in libc++abi?
  89      They're just empty, can't they be defined inline?</p>
  90   <p>A: The destructors for them live in libc++abi because they are "key" functions.
  91      The Itanium ABI describes a "key" function as the first virtual declared.
  92      And wherever the key function is defined, that is where the <code>type_info</code> gets defined.
  93      And in libc++ types are the same type if and only if they have the same <code>type_info</code>
  94      (as in there must be only one type info per type in the entire application).
  95      And on OS X, libstdc++ and libc++ share these exception types.
  96      So to be able to throw in one dylib and catch in another (a <code>std::exception</code> for example),
  97      there must be only one <code>std::exception type_info</code> in the entire app.
  98      That typeinfo gets laid down beside <code>~exception()</code> in libc++abi (for both libstdc++ and libc++).</p>
  99      <p>--Howard Hinnant</p>
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