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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - min_element doesn't produce the first minimum element in some cases"
href="http://llvm.org/bugs/show_bug.cgi?id=19547">19547</a>
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<th>Summary</th>
<td>min_element doesn't produce the first minimum element in some cases
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>3.4
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
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<th>Reporter</th>
<td>kremenek@apple.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu, mclow.lists@gmail.com
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<th>Classification</th>
<td>Unclassified
</td>
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<p>
<div>
<pre>min_element is defined as calling it's comparator with the current min as the
first argument & the current iteration element as the second one. However, it
does the reverse which leads to an unstable result for equal elements.
$ cat test.cpp
#include <algorithm>
#include <array>
#include <iostream>
struct X {
X(int x) : _x(x) {}
bool operator<=(const X& x) const
{
return _x <= x._x;
}
int _x;
};
int main()
{
std::array<X, 5> x = {1, 1, 2, 4, 4};
auto min = std::min_element(x.begin(), x.end(), [](const X& x1, const X&
x2) {
return x1 <= x2;
});
if (min == x.begin()) {
std::cerr << "min_element OK\n";
} else {
std::cerr << "min_element isn't stable\n";
}
}
$ clang++ -stdlib=libc++ test.cpp -std=c++11
$ ./a.out
min_element isn't stable</pre>
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