I'am trying to use std::unordered_set in cross-platform C++ application. It compiles and works like a charm in Visual C++ under Windows, but generates a fatal compilation error on clang under Mac OS X.
I want to know why it happens and what is the right way to get this working.
Example code:
//
// Clang build cmdline:
// $ clang++ ./set.cpp -Wall -Werror -Wfatal-errors -std=c++11 -stdlib=libc++ -o set.out
//
#include <iostream>
#include <unordered_set>
struct Point {
int x, y;
Point(int x = 0, int y = 0) {
this->x = x;
this->y = y;
}
bool operator==(Point const& p) const {
return this->x == p.x && this->y == p.y;
}
operator std::size_t () const {
return std::hash<int>()(x) ^ std::hash<int>()(y);
}
};
typedef std::unordered_set<Point> points_set_t;
int main() {
Point point1(1, 5);
Point point2(1, 1);
Point point3(1, 5);
points_set_t points;
points.insert(point1);
points.insert(point2);
points.insert(point3);
for (points_set_t::const_iterator it = points.begin(); it != points.end(); it++) {
std::cout << it->x << ":" << it->y << std::endl;
}
}
Clang output:
In file included from ./set.cpp:6:
In file included from /usr/bin/../lib/c++/v1/iostream:38:
In file included from /usr/bin/../lib/c++/v1/ios:216:
In file included from /usr/bin/../lib/c++/v1/__locale:15:
In file included from /usr/bin/../lib/c++/v1/string:434:
In file included from /usr/bin/../lib/c++/v1/algorithm:591:
/usr/bin/../lib/c++/v1/type_traits:748:38: fatal error: implicit instantiation of undefined template 'std::__1::hash<Point>'
: public integral_constant<bool, __is_empty(_Tp)> {};
^
/usr/bin/../lib/c++/v1/memory:1948:40: note: in instantiation of template class 'std::__1::is_empty<std::__1::hash<Point> >'
requested here
bool = is_empty<_T2>::value
^
/usr/bin/../lib/c++/v1/memory:1970:44: note: in instantiation of default argument for '__libcpp_compressed_pair_switch<unsigned
long, std::__1::hash<Point>, false, false>' required here
template <class _T1, class _T2, unsigned = __libcpp_compressed_pair_switch<_T1, _T2>::value>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/c++/v1/memory:2354:15: note: in instantiation of default argument for '__libcpp_compressed_pair_imp<unsigned long,
std::__1::hash<Point> >' required here
: private __libcpp_compressed_pair_imp<_T1, _T2>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/c++/v1/__hash_table:527:55: note: in instantiation of template class 'std::__1::__compressed_pair<unsigned long,
std::__1::hash<Point> >' requested here
__compressed_pair<size_type, hasher> __p2_;
^
/usr/bin/../lib/c++/v1/unordered_set:330:13: note: in instantiation of template class 'std::__1::__hash_table<Point,
std::__1::hash<Point>, std::__1::equal_to<Point>, std::__1::allocator<Point> >' requested here
__table __table_;
^
./set.cpp:28:18: note: in instantiation of template class 'std::__1::unordered_set<Point, std::__1::hash<Point>,
std::__1::equal_to<Point>, std::__1::allocator<Point> >' requested here
points_set_t points;
^
/usr/bin/../lib/c++/v1/memory:3076:29: note: template is declared here
template <class _Tp> struct hash;
^
1 error generated.
UPD Working implementation using @mfontanini's suggestion: https://gist.github.com/vbo/6090142.
In order to make std::unordered_set work with your Point class, you can provide a std::hash specialization for it:
namespace std
{
template<>
struct hash<Point> {
size_t operator()(const Point &pt) const {
return std::hash<int>()(pt.x) ^ std::hash<int>()(pt.y);
}
};
}
You could also change std::unordered_set's second template parameter(it defaults to std::hash<Point>), which indicates a functor type that returns the required hash.
It seems like you tried providing this hash implementation via a user defined conversion to size_t, but that won't work. The fact that it works in VC is caused by some bug in their implementation.
来源：https://stackoverflow.com/questions/17885563/using-c11-unordered-set-in-visual-c-and-clang