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 };

Afflictive

Did you mean something like std::unordered_set<std::vector<int>> actually? – Ddene

I'm sorry, I have digit the incorrect class name, I mean exactly unordered_set – Afflictive

How doesn't it work? What's the problem you're having? – Clevie

What does doesn't work mean? Please post an SSCCE – Klarrisa

Yes you can have an std::unordered_set<std::vector<some_type>> – Shaduf

I've updated the post to clarify the error – Afflictive

@CattaniSimone Uh... what please? What compiler are you using that r3 fails but not the Plan constructor? – Ahl

@Ahl - happens with me too when using clang. – Tourney

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>>;

Ahl

The second half is probably UB. As far as the library is concerned, I don't think std::vector<int> counts as a user-defined type. – Corrupt

Why is the default one (std::hash<std::vector<int>>) not used? – Rog

@Rog There is no std::hash<std::vector<int>> - it's not provided by the standard library. – Ahl

How does std::set<vector<int>> work then if there is no default hash? – Camber

@Camber std::set isn't a hash table, it's a binary tree. It uses < – Ahl

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.

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