How do I reuse my hasher in another hasher? - c++

Question

I have been given a task to write a C++ program that stores an unordered_set of objects street. At the same time, object street is to contain some general info and an unordered_set of objects house. I have written a structure-hasher for house
struct house_hasher {
std::hash<std::string> number_hash;
std::hash<size_t> storeys_hash;
std::hash<size_t> aparts_hash;
std::hash<size_t> residents_hash;
std::hash<std::string> street_name_hash;
std::hash<double> pfsm_hash;
std::hash<double*> pea_hash;
std::hash<double*> sea_hash;
std::hash<bool*> ps_hash;
size_t operator()(const house& h) const {
const size_t coef = 2'946'901;
size_t hash_value = 0;
hash_value = (pow(coef, 8) * number_hash(h.getter_number()) +
pow(coef, 7) * storeys_hash(h.getter_storeys_n()) +
pow(coef, 6) * aparts_hash(h.getter_aparts_n()) +
pow(coef, 5) * residents_hash(h.getter_residents_n()) +
pow(coef, 4) * street_name_hash(h.getter_street_name()) +
pow(coef, 3) * pfsm_hash(h.getter_price_for_square_meter()) +
pow(coef, 2) * pea_hash(h.getter_payments_each_apartments()) +
coef * sea_hash(h.getter_square_each_apartments()) +
ps_hash(h.getter_payments_statuses()));
return hash_value;
}
};
to enable adding house to the container and it works correctly. But the thing is that I have to write a structure-hasher for object street since it must be addable as well. I assume I could easily add another structure and copy-paste the code from the initial structure to the one mentioned, so that the part, that hashes houses, repeats twice in the project but still it seems to be stupid.
What I want to do is to create one more structure-hasher for streets but to use the object of house_hasher in it and be able to hash houses like that
struct street_hasher {
std::hash<std::string> name_hash;
std::hash<size_t> number_hash;
std::hash<size_t> houses_hash;
std::hash<std::unordered_set<house, house_hasher>> uset_houses_hash;
size_t operator()(const street& s) const {
const size_t coef = 2'946'901;
size_t hash_value = 0;
size_t add_hash = 0;
house_hasher hasher_for_house;
std::unordered_set<house, house_hasher>::iterator uset_it = s.getter_street_uset_houses().begin();
for (uset_it; uset_it != s.getter_street_uset_houses().end(); ++uset_it) {
add_hash += hasher_for_house(*uset_it);
}
hash_value = (pow(coef, 3) * name_hash(s.getter_street_name()) +
pow(coef, 2) * number_hash(s.getter_street_number()) +
coef * houses_hash(s.getter_street_houses_n()) +
add_hash);
return hash_value;
}
};
However, this does not seem to be correct. VS throws an error C2280 "std::_Uhash_compare<_Kty,_Hasher,_Keyeq>::_Uhash_compare(const std::_Uhash_compare<_Kty,_Hasher,_Keyeq> &)": предпринята попытка ссылки на удаленную функцию
The last few words say an attempt to refer to deleted function occurred.
I have tried a lot of ideas of mine and those found on the net. Still, there is no output. Could anybody please tell me how to solve this tricky problem?
Thanks in advance!

Answer 1

The solution is to arrange the street_hasher structure this way (it works but I am sure there is another way to make this work which would be more elegant and laconic):
struct street_hasher {
std::hash<std::string> name_hash;
std::hash<size_t> number_hash;
std::hash<size_t> houses_hash;
size_t operator()(const street& s) const {
const size_t coef = 2'946'901;
size_t hash_value = 0;
size_t add_hash = 0;
house_hasher hasher_for_house; // need this to hash houses
std::unordered_set < house, house_hasher > uset = s.getter_street_uset_houses();
std::unordered_set<house, house_hasher>::iterator uset_it = uset.begin();
std::unordered_set<house, house_hasher>::iterator end_it = uset.end();
size_t houses_it = s.getter_street_houses_n() + 2;
// there is 2 as we have y = 3 more attributes to hash in street
// but we start with the power of n - 1, where n = houses_n + y =>
// => n = s.getter_street_houses_n() + 3
// => n - 1 = s.getter_street_houses_n() + 2
hash_value = (pow(coef, houses_it) * name_hash(s.getter_street_name()) +
pow(coef, houses_it - 1) * number_hash(s.getter_street_number()) +
pow(coef, houses_it - 2) * houses_hash(s.getter_street_houses_n()) +
add_hash);
houses_it -= 3;
for (uset_it; uset_it != end_it; ++uset_it) {
add_hash += pow(coef, houses_it) * hasher_for_house(*uset_it);
--houses_it;
}
// I guess it is much safer to treat each house as an independent attribute
hash_value += add_hash;
return hash_value;
}
};
I have deleted std::hash<std::unordered_set<house, house_hasher>> uset_houses_hash; from the header and it stopped throwing the error. After one problem went away, another one came. The compiler was angry at how I initialized the loop saying list iterators incompatible. I have just rearranged the function a bit by setting so-called static house_hasher and iterators. As an addition, I came up with a thought to deal with each house in unordered_set as with a different attribute of street, so that we avoid collisions (I may have just said something stupid but still let it be this way). Thus, such way of hashing seems to me say more safer. Anyway, it works!
Cheers!