The following code defines a unordered_set
.
The code compiles just fine.
But using the lambda function instead of functor throw when calling find:
libc++abi.dylib: terminate called throwing an exception
#include <unordered_set>
class pair_hash {
public:
size_t operator() (const std::pair<int, int> &x) const {
return std::hash<int>()(x.first) ^ std::hash<int>()(x.second);
}
};
int main() {
std::unordered_set<std::pair<int, int>, pair_hash> temp;
temp.find(std::make_pair(0,0));
std::function<std::size_t(std::pair<int , int>)> fpair_hash;
fpair_hash = [](const std::pair<int, int>& v) -> std::size_t
{
return std::hash<int>()(v.first) ^ std::hash<int>()(v.second);
};
std::unordered_set<std::pair<int, int>, decltype(fpair_hash)> temp2;
//why does this not work?
temp2.find(std::make_pair(0,0));
return 0;
}
clang++ -std=c++11 -stdlib=libc++ -o test test.cpp
decltype(fpair_hash)
is std::function<std::size_t(std::pair<int , int>)>
so you are just building set with empty hash function.
You need to provide your function to constructor of std::unordered_set
:
std::unordered_set<std::pair<int, int>, decltype(fpair_hash)> temp2(10, fpair_hash);
This should make it work, but using std::function
will have overhead of polymorphic call and you probably don't need it:
auto fpair_hash = [](const std::pair<int, int>& v) -> std::size_t
{
return std::hash<int>()(v.first) ^ std::hash<int>()(v.second);
};
Finally, your hash function is not really good - it maps all pairs (x, x)
to 0
. Perhaps using something like x * 17 + y * 13
instead of x ^ y
will reduce probability of collisions.