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c++boostunordered-set
user3689963
user3689963

Reputation: 115

How to use boost hash_value in std unordered_set?

How can I use boost::hash_value<std::pair<>> in std::unordered_set<std::pair<>> without defining a functor?

The following doesn't work:

        typedef std::pair<unsigned, unsigned> S;

        typedef size_t (hash_value_sample)(const S&);
        std::function<hash_value_sample> hasher = 
                    static_cast<hash_value_sample&>(boost::hash_value<unsigned, unsigned>);

        std::unordered_set<S, decltype(hasher)> set;
        set.insert(S(10, 10));

The execution aborts.

terminate called after throwing an instance of 'std::bad_function_call'
  what():  bad_function_call
Abort

I think the above doesn't work because hasher() doesn't exist. I am using g++ -std=c++14

Upvotes: 0

Views: 772

Answers (2)

ichramm
ichramm

Reputation: 6632

This seems to work, according to my tests:

#include <iostream>
#include <unordered_set>
#include <boost/functional/hash.hpp>

typedef std::pair<unsigned, unsigned> S;
typedef boost::hash<S> hash_t;

int main() {
  std::unordered_set<S, hash_t> set;
  set.insert(S(10, 10));
  std::cout << set.count(S(10, 10)) << std::endl;
  std::cout << set.count(S(10, 9)) << std::endl;
  return 0;
}

Output:

❯ clang++ test.cpp -o test && ./test
1
0

Upvotes: 3

catnip
catnip

Reputation: 25388

You can do it with a lambda:

auto hasher = [] (S s) { return boost::hash_value (s); };
std::unordered_set<S, decltype(hasher)> set;

Live demo

Upvotes: 0

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