http://llvm.org/bugs/show_bug.cgi?id=19547
Bug ID: 19547
Summary: min_element doesn't produce the first minimum element
in some cases
Product: libc++
Version: 3.4
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: All Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
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
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