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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - weak_ptr(weak_ptr const &) can segfault in the presence of virtual bases"
href="https://bugs.llvm.org/show_bug.cgi?id=41114">41114</a>
</td>
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<th>Summary</th>
<td>weak_ptr(weak_ptr const &) can segfault in the presence of virtual bases
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>arthur.j.odwyer@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
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<p>
<div>
<pre>cat >test.cc <<EOF
#include <memory>
#include <stdio.h>
struct A { int i = 0; virtual ~A() {} };
struct B : public virtual A { char d[100000]; };
int main() {
std::weak_ptr<B> p = std::shared_ptr<B>(new B);
puts("hello world");
std::weak_ptr<A> q = p;
}
EOF
clang++ -std=c++11 test.cc
./a.out
Segmentation fault
What's going on here is that the converting constructor weak_ptr(const
weak_ptr&) is implemented as
weak_ptr<_Tp>::weak_ptr(shared_ptr<_Yp> const& __r,
typename enable_if<is_convertible<_Yp*, _Tp*>::value,
__nat*>::type)
_NOEXCEPT
: __ptr_(__r.__ptr_), // LINE XXX
__cntrl_(__r.__cntrl_)
{
if (__cntrl_)
__cntrl_->__add_weak();
}
When `_Tp` is a virtual base of `_Yp`, the conversion on line XXX generally
needs to dereference `__r.__ptr_` to compute the appropriate offset. This is
fine if the weak_ptr stores a null pointer value, and it's fine if the weak_ptr
is not-expired; but if the weak_ptr is expired (or empty) and stores a non-null
pointer value, then we dereference into freed memory and segfault.
The solution adopted by both libstdc++ and MSVC is to implement this
constructor as essentially
weak_ptr<_Tp>::weak_ptr(shared_ptr<_Yp> const& __r,
typename enable_if<is_convertible<_Yp*, _Tp*>::value,
__nat*>::type)
_NOEXCEPT
: weak_ptr(__r.lock())
{}</pre>
</div>
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