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<p class="MsoNormal">+1. I’m configured in this mode exclusively, and also prefer abort().<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Olivier<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">libcxx-dev <libcxx-dev-bounces@lists.llvm.org> on behalf of Duncan Exon Smith via libcxx-dev <libcxx-dev@lists.llvm.org><br>
<b>Reply-To: </b>Duncan Exon Smith <dexonsmith@apple.com><br>
<b>Date: </b>Monday, January 28, 2019 at 11:01 AM<br>
<b>To: </b>Louis Dionne <ldionne@apple.com><br>
<b>Cc: </b>Libc++ Dev <libcxx-dev@lists.llvm.org><br>
<b>Subject: </b>Re: [libcxx-dev] Question about the no-exceptions mode of libc++<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">FWIW, I agree ::abort is more appropriate.<o:p></o:p></p>
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<p class="MsoNormal">On 2019 Jan 28, at 10:44, Louis Dionne via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hi,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">My understanding is that libc++ has a no-exceptions mode under which we never throw and never use try-catch blocks. This mode is enabled by defining
<span style="font-family:Menlo">_LIBCPP_NO_EXCEPTIONS</span>, or by passing <span style="font-family:Menlo">
LIBCXX_ENABLE_EXCEPTIONS=OFF</span> when configuring CMake.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">However, I'd like to understand the design of this mode a bit better. It seems like we sometimes call
<span style="font-family:Menlo">abort()</span> instead of throwing, but sometimes we just swallow the would-be exception and carry on. For example, instead of throwing
<span style="font-family:Menlo">std::bad_optional_access</span>, we call <span style="font-family:Menlo">
std::abort()</span>:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> void __throw_bad_optional_access() {</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> #ifndef _LIBCPP_NO_EXCEPTIONS</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> throw bad_optional_access();</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> #else</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> _VSTD::abort();</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> #endif</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> }</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">This makes sense to me. However, in other cases, we just swallow the exception. For example, in
<span style="font-family:Menlo">std::map::at</span> when we can't find the key, we just don't throw an exception, and we dereference a null pointer unless I'm mistaken:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> template <class _Key, class _Tp, class _Compare, class _Allocator></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> const _Tp& map<_Key, _Tp, _Compare, _Allocator>::at(const key_type& __k) const {</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> __parent_pointer __parent;</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> __node_base_pointer __child = __tree_.__find_equal(__parent, __k);</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> #ifndef _LIBCPP_NO_EXCEPTIONS</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> if (__child == nullptr)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> throw out_of_range("map::at: key not found");</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> #endif // _LIBCPP_NO_EXCEPTIONS</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> return static_cast<__node_pointer>(__child)->__value_.__get_value().second;</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:Menlo"> }</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Here, when <span style="font-family:Menlo">__child == nullptr</span>, we do NOT throw an exception and we end up dereferencing it, which is UB. I haven't made a full survey of libc++, but I'd like to know whether this is by design, and
if so, what is the rationale for it. Otherwise, I'd like for us to agree on what the behaviour should be (I suggest
<span style="font-family:Menlo">std::abort()</span>) so that we can fix places that don't behave correctly.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Cheers,<o:p></o:p></p>
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<p class="MsoNormal">Louis<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
libcxx-dev mailing list<br>
<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a><br>
https://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev<o:p></o:p></p>
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