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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - bad code with hand-crafted offsetof"
href="https://bugs.llvm.org/show_bug.cgi?id=40831">40831</a>
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<th>Summary</th>
<td>bad code with hand-crafted offsetof
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>stsp2@yandex.ru
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>As offsetof doesn't work with template arguments,
I have to create the one by hands:
---
#include <cstddef>
#include <iostream>
class L {};
template <typename T, L (T::*M)[0]>
struct offset_of {
constexpr operator size_t() const {
return (std::uintptr_t)&(((T*)nullptr)->*M);
}
};
template <typename T, L (T::*M)[0]>
struct B {
char aa[10];
static const int off = offset_of<T, M>();
};
struct A {
char a;
L _mark[0];
B<A, &A::_mark> b;
};
int main()
{
A a;
std::cout << "size " << sizeof(A) << " off " << a.b.off << std::endl;
return 0;
}
---
This example works perfectly with gcc:
size 11 off 1
But behaves badly when compiled with clang:
size 1 off 1
(and sometimes even segfaults, it seems)
Besides fixing the bug, I wonder if it would be
possible to somehow fix offsetof() to allow the
use of templated args? Otherwise people have to
apply the horrible hacks like this.</pre>
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