C++ unordered_set of vectors

Question

Can I create a unordered_set of vectors in C++? something like this

std::unordered_set<std::vector<int>> s1; 

because I know that is possible with the "set" class of the std lib but seems that it doesn't work for the unordered version thanks

Update: this is the exactly code that I'm trying to use

typedef int CustomerId; typedef std::vector<CustomerId> Route; typedef std::unordered_set<Route> Plan;  // ... in the main Route r1 = { 4, 5, 2, 10 }; Route r2 = { 1, 3, 8 , 6 }; Route r3 = { 9, 7 }; Plan p = { r1, r2 }; 

and it's all right if I use set, but I receive a compilation error when try to use the unordered version

main.cpp:46:11: error: non-aggregate type 'Route' (aka 'vector<CustomerId>') cannot be initialized with an initializer list     Route r3 = { 9, 7 }; 

Answer 1

Sure you can. You'll have to come up with a hash though, since the default one (std::hash<std::vector<int>>) will not be implemented. For example, based on this answer, we can build:

struct VectorHash {     size_t operator()(const std::vector<int>& v) const {         std::hash<int> hasher;         size_t seed = 0;         for (int i : v) {             seed ^= hasher(i) + 0x9e3779b9 + (seed<<6) + (seed>>2);         }         return seed;     } }; 

And then:

using MySet = std::unordered_set<std::vector<int>, VectorHash>; 

You could also, if you so choose, instead add a specialization to std::hash<T> for this type (note this could be undefined behavior with std::vector<int>, but is definitely okay with a user-defined type):

namespace std {     template <>     struct hash<std::vector<int>> {         size_t operator()(const vector<int>& v) const {             // same thing         }     }; }  using MySet = std::unordered_set<std::vector<int>>; 

Answer 2

As an alternative to a custom-written hasher, Boost provides a hasher for many standard library types. This should work in your case:

#include <boost/container_hash/hash.hpp>  std::unordered_set<   std::vector<int>,   boost::hash<std::vector<int>> > s1; 

Reference: https://www.boost.org/doc/libs/1_78_0/doc/html/hash/reference.html

In older Boost versions, the header file was boost/functional/hash.hpp.