std::hash for __uint128_t

Question

I am facing an issue hashing __uint128_t. Following is my code:

#include <iostream>

int main () {
    __uint128_t var = 1;
    std::cout << std::hash<__uint128_t> () (var) << "\n";
    return 0;
}

I am getting the error as:

test.cpp: In function ‘int main()’:
test.cpp:5:40: error: use of deleted function ‘std::hash<__int128 unsigned>::hash()’
    5 |     size_t h = std::hash<__uint128_t> () (var);
      |                                        ^

How can I get the hash for __uint128_t? (Probably a very basic question but I have been stuck here for a while). Also, I would like to know the meaning of the error. Thanks in advance.

You can separate it into two 64-bit values and [combine](https://stackoverflow.com/questions/35985960/c-why-is-boosthash-combine-the-best-way-to-combine-hash-values) their hashes. — interjay, Apr 03 '21 at 12:38
`__uint128_t` is not a standard type so `std::hash` is not required to support it. — interjay, Apr 03 '21 at 14:52

score 6 · Answer 1 · answered Apr 03 '21 at 12:38

6

Taking a look at the docs on https://en.cppreference.com/w/cpp/utility/hash. You will have to write your own.

Here is some code for what a basic __uint128_t hash function might look like:

namespace std {
template<>
struct hash<__uint128_t> {
    size_t operator()(__uint128_t var) const {
        return std::hash<uint64_t>{}((uint64_t)var ^ (uint64_t)(var >> 64));
    }
};
}

Note not tested or compiled.

answered Apr 03 '21 at 12:38

ceorron

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Plus one. Could you further include the referenced hash_combine function instead of a simple xor? – davidhigh Apr 03 '21 at 12:50

std::hash for __uint128_t

1 Answers1