CODEWITHSUNDEEP
c++c++11hashmap
Henrique Andrade
Henrique Andrade

Reputation: 991

error: hash function must be invocable with an argument of key type

I'm using a custom hash function with unordered_map, but I get this error:

error: static assertion failed: hash function must be invocable with an argument of key type

It works fine when I use the default hash function of unordered_multimap.

My code:

#include<vector>
#include<string>
#include<unordered_map>

using namespace std;

class Hasher {
public:
    size_t operator() (string const& key) const {
        size_t hash = 0;
        for(size_t i = 0; i < key.size(); i++) {
            hash += key[i] % 7;
        }
        return hash;
    }
};

int main(int argc, char const *argv[]) {

    unordered_multimap<int, int, Hasher, equal_to<int>> hashmap;

    hashmap.insert(make_pair(1, 11));
    hashmap.insert(make_pair(1, 21));
    hashmap.insert(make_pair(2, 12));
    hashmap.insert(make_pair(3, 13));

    auto range = hashmap.equal_range(1); 

    return 0;
}

Upvotes: 1

Views: 17249

Answers (1)

zhm
zhm

Reputation: 3651

You are using mismatched types in the key of unordered_map and operator() in Hasher.

You code should be (please note the comments inline):

#include<vector>
#include<string>
#include<unordered_map>

using namespace std;

class Hasher {
public:
    size_t operator() (string const& key) const {     // the parameter type should be the same as the type of key of unordered_map
        size_t hash = 0;
        for(size_t i = 0; i < key.size(); i++) {
            hash += key[i] % 7;
        }
        return hash;
    }
};

int main(int argc, char const *argv[]) {

    unordered_multimap<std::string, int, Hasher, equal_to<std::string>> hashmap;  // key should be string type

    hashmap.insert(make_pair("1", 11));   // key should be string type
    hashmap.insert(make_pair("1", 21));
    hashmap.insert(make_pair("2", 12));
    hashmap.insert(make_pair("3", 13));

    auto range = hashmap.equal_range("1");    // equal_range's parameter should be the same type as key

    return 0;
}

Upvotes: 8

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