glm::ivec2 as key in unordered map

Question

lately I am more a user of the Scala Programming Language than C++, And now I am frustrated in porting a very simple line of code

val map = new HashMap[Vec2i,Entity]

it simply refuses to compile in C++ with strange template errors. The equivalent of Vec2i in C++ is glm::ivec2, it is basicly a struct with two integers and some operators for math.

this is how far i got:

#include <iostream>
#include <unordered_map>
#include <glm/glm.hpp>

using namespace std;
using namespace glm;

struct KeyTraits {
size_t operator()(const ivec2& k) {
        return std::hash<int>()(k.x) ^ std::hash<int>()(k.y);
    }

bool operator()(const ivec2& a, const ivec2& b) {
        return a.x == b.x && a.y == b.y;
    }
};

typedef unordered_map<ivec2,int,KeyTraits,KeyTraits> MyMap;

int main(int argc, char **argv)
{
    MyMap map;

    map[ivec2(2,3)] = 7;
    map[ivec2(3,4)] = 8;

    for(auto it = map.begin(); it != map.end(); it++) {
        cout << it->second << endl;
    }
}

The current compile error is:

In file included from /usr/include/c++/4.5/bits/hashtable.h:35:0,
             from /usr/include/c++/4.5/unordered_map:45,
             from /home/arne/codelite/LanguageTest/main.cpp:2:
/usr/include/c++/4.5/bits/hashtable_policy.h: In member function ‘std::__detail::_Hash_code_base<_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false>::_Hash_code_type std::__detail::_Hash_code_base<_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false>::_M_hash_code(const _Key&) const [with _Key = glm::detail::tvec2<int>, _Value = std::pair<const glm::detail::tvec2<int>, int>, _ExtractKey = std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, _Equal = KeyTraits, _H1 = KeyTraits, _H2 = std::__detail::_Mod_range_hashing, std::__detail::_Hash_code_base<_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false>::_Hash_code_type = long unsigned int]’:
/usr/include/c++/4.5/bits/hashtable_policy.h:535:74:   instantiated from ‘std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type& std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::operator[](const _Key&) [with _Key = glm::detail::tvec2<int>, _Pair = std::pair<const glm::detail::tvec2<int>, int>, _Hashtable = std::_Hashtable<glm::detail::tvec2<int>, std::pair<const glm::detail::tvec2<int>, int>, std::allocator<std::pair<const glm::detail::tvec2<int>, int> >, std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, KeyTraits, KeyTraits, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>, std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type = int]’
/home/arne/codelite/LanguageTest/main.cpp:24:16:   instantiated from here
/usr/include/c++/4.5/bits/hashtable_policy.h:727:25: error: passing ‘const KeyTraits’ as ‘this’ argument of ‘size_t KeyTraits::operator()(const glm::core::type::ivec2&)’ discards qualifiers
/usr/include/c++/4.5/bits/hashtable_policy.h: In member function ‘bool std::__detail::_Hash_code_base<_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false>::_M_compare(const _Key&, std::__detail::_Hash_code_base<_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false>::_Hash_code_type, std::__detail::_Hash_node<_Value, false>*) const [with _Key = glm::detail::tvec2<int>, _Value = std::pair<const glm::detail::tvec2<int>, int>, _ExtractKey = std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, _Equal = KeyTraits, _H1 = KeyTraits, _H2 = std::__detail::_Mod_range_hashing, std::__detail::_Hash_code_base<_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false>::_Hash_code_type = long unsigned int]’:
/usr/include/c++/4.5/bits/hashtable.h:879:2:   instantiated from ‘std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_Node* std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_M_find_node(std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_Node*, const key_type&, typename std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_Hash_code_type) const [with _Key = glm::detail::tvec2<int>, _Value = std::pair<const glm::detail::tvec2<int>, int>, _Allocator = std::allocator<std::pair<const glm::detail::tvec2<int>, int> >, _ExtractKey = std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, _Equal = KeyTraits, _H1 = KeyTraits, _H2 = std::__detail::_Mod_range_hashing, _Hash = std::__detail::_Default_ranged_hash, _RehashPolicy = std::__detail::_Prime_rehash_policy, bool __cache_hash_code = false, bool __constant_iterators = false, bool __unique_keys = true, std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_Node = std::__detail::_Hash_node<std::pair<const glm::detail::tvec2<int>, int>, false>, key_type = glm::detail::tvec2<int>, typename std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_Hash_code_type = long unsigned int]’
/usr/include/c++/4.5/bits/hashtable_policy.h:540:53:   instantiated from ‘std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type& std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::operator[](const _Key&) [with _Key = glm::detail::tvec2<int>, _Pair = std::pair<const glm::detail::tvec2<int>, int>, _Hashtable = std::_Hashtable<glm::detail::tvec2<int>, std::pair<const glm::detail::tvec2<int>, int>, std::allocator<std::pair<const glm::detail::tvec2<int>, int> >, std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, KeyTraits, KeyTraits, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>, std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type = int]’
/home/arne/codelite/LanguageTest/main.cpp:24:16:   instantiated from here
/usr/include/c++/4.5/bits/hashtable_policy.h:742:48: error: passing ‘const KeyTraits’ as ‘this’ argument of ‘bool KeyTraits::operator()(const glm::core::type::ivec2&, const glm::core::type::ivec2&)’ discards qualifiers
/usr/include/c++/4.5/bits/hashtable_policy.h: In member function ‘size_t std::__detail::_Hash_code_base<_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false>::_M_bucket_index(const std::__detail::_Hash_node<_Value, false>*, size_t) const [with _Key = glm::detail::tvec2<int>, _Value = std::pair<const glm::detail::tvec2<int>, int>, _ExtractKey = std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, _Equal = KeyTraits, _H1 = KeyTraits, _H2 = std::__detail::_Mod_range_hashing, size_t = long unsigned int]’:
/usr/include/c++/4.5/bits/hashtable.h:1170:59:   instantiated from ‘void std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_M_rehash(std::_Hashtable::size_type) [with _Key = glm::detail::tvec2<int>, _Value = std::pair<const glm::detail::tvec2<int>, int>, _Allocator = std::allocator<std::pair<const glm::detail::tvec2<int>, int> >, _ExtractKey = std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, _Equal = KeyTraits, _H1 = KeyTraits, _H2 = std::__detail::_Mod_range_hashing, _Hash = std::__detail::_Default_ranged_hash, _RehashPolicy = std::__detail::_Prime_rehash_policy, bool __cache_hash_code = false, bool __constant_iterators = false, bool __unique_keys = true, std::_Hashtable::size_type = long unsigned int]’
/usr/include/c++/4.5/bits/hashtable.h:911:8:   instantiated from ‘std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::iterator std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_M_insert_bucket(const value_type&, std::_Hashtable::size_type, typename std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_Hash_code_type) [with _Key = glm::detail::tvec2<int>, _Value = std::pair<const glm::detail::tvec2<int>, int>, _Allocator = std::allocator<std::pair<const glm::detail::tvec2<int>, int> >, _ExtractKey = std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, _Equal = KeyTraits, _H1 = KeyTraits, _H2 = std::__detail::_Mod_range_hashing, _Hash = std::__detail::_Default_ranged_hash, _RehashPolicy = std::__detail::_Prime_rehash_policy, bool __cache_hash_code = false, bool __constant_iterators = false, bool __unique_keys = true, std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::iterator = std::__detail::_Hashtable_iterator<std::pair<const glm::detail::tvec2<int>, int>, false, false>, value_type = std::pair<const glm::detail::tvec2<int>, int>, std::_Hashtable::size_type = long unsigned int, typename std::_Hashtable<_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys>::_Hash_code_type = long unsigned int]’
/usr/include/c++/4.5/bits/hashtable_policy.h:543:24:   instantiated from ‘std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type& std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::operator[](const _Key&) [with _Key = glm::detail::tvec2<int>, _Pair = std::pair<const glm::detail::tvec2<int>, int>, _Hashtable = std::_Hashtable<glm::detail::tvec2<int>, std::pair<const glm::detail::tvec2<int>, int>, std::allocator<std::pair<const glm::detail::tvec2<int>, int> >, std::_Select1st<std::pair<const glm::detail::tvec2<int>, int> >, KeyTraits, KeyTraits, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>, std::__detail::_Map_base<_Key, _Pair, std::_Select1st<_Pair>, true, _Hashtable>::mapped_type = int]’
/home/arne/codelite/LanguageTest/main.cpp:24:16:   instantiated from here
/usr/include/c++/4.5/bits/hashtable_policy.h:737:55: error: passing ‘const KeyTraits’ as ‘this’ argument of ‘size_t KeyTraits::operator()(const glm::core::type::ivec2&)’ discards qualifiers

Answer 1

You need to specify a hash class and a comparator class in your typedef. See template params Hash and KeyEqual here: http://en.cppreference.com/w/cpp/container/unordered_map

it will look like that (consider the const qualifiers at the end of the method signatures):

struct KeyFuncs
{
    size_t operator()(const ivec2& k)const
    {
        return std::hash<int>()(k.x) ^ std::hash<int>()(k.y);
    }

    bool operator()(const ivec2& a, const ivec2& b)const
    {
            return a.x == b.x && a.y == b.y;
    }
};


typedef unordered_map<ivec2,int,KeyFuncs,KeyFuncs> MyMap;

Answer 2

Just look at the linker error, it tells you what you should implement or provide in the list of template arguments:

std::hash<glm::detail::tvec2<int> >::operator()(glm::detail::tvec2<int>) const

The program doesn't know how to create a hash based on the vector object. You'd have to calculate your own hash, so the map code is able to differentiate between vectors.

Edit: I'd tend to use pointers to vectors, as this might screw up if you add some element and change it later on (so you should add const objects).

---

Edit 2: With the updated code/error message, it seems like you forgot to make the methods inside KeyTaits const, so their this pointer is of type KeyTraits*, but the value passed is meant to be const KeyTraits*.

Answer 3

hashes are built in to the glm library, however they are part of the exensions so just do this after you include glm and then you won't need to write your own hash:

#define GLM_ENABLE_EXPERIMENTAL
#include "glm/gtx/hash.hpp"