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Insert works with a set, but not with an unordered_set

In the following piece of code:

#include<unordered_set>
#include<iostream>
#include<utility>
#include<string>
#include<set>

using namespace std;

int main()
{
    set<pair<string, string> > g;
    pair<string, string> tmp;
    tmp.first="hello";
    tmp.second="world";
    g.insert(tmp);
}

if i change set<pair<string, string> > g;

to unordered_set<pair<string, string> > g;

i get an error when inserting a pair like:

test1.cpp:15:14: note:   candidate expects 2 arguments, 1 provided
  g.insert(tmp);
              ^

is it something on the lines "hash function cannot be defined for a pair, but only for basic datatypes"? If I am wrong, please correct me, otherwise please clarify. Thank!

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1 answer


There is no standard way to compute the hash on a pair. You must provide a hash function for your pair. For example for example: -

struct hash_pair {
    inline std::size_t operator()(const std::pair<std::string,std::string> & p) const {
        return // howsoever you want to implement.
    }
};


And then declare your std :: unordered_set as: -

std::unordered_set< std::pair<std::string, std::string>,  hash_pair> mySet;
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