c++ - Hashing function not working properly -

i've been trying week understand how use std::unordered_map , custom hash functions. after lot of research, tried implement own hash function glm::ivec3. problem here have syntax error of sort. don't i'm doing wrong, might omitted. here's header code:

namespace mctest3 {     class chunkmap     {     public:         struct keyhasher         {             std::size_t operator()(const glm::ivec3& key) const             {                 using std::size_t;                 using std::hash;                  return ((key.x * 5209) ^ (key.y * 1811)) ^ (key.z * 7297);             }         };           std::unordered_map<glm::ivec3, chunk, keyhasher> chunks;          chunkmap();          chunk* getchunkfrompos(const glm::vec3 &pos) const;         glm::ivec3 getchunkposfrompos(const glm::vec3 &pos) const;     }; } 

and bugged function need retrieve value unordered_map (or create one):

namespace mctest3 {     chunkmap::chunkmap()     {     }      chunk* chunkmap::getchunkfrompos(const glm::vec3 &pos) const     {         glm::ivec3 ipos = glm::ivec3((int)pos.x >> chunk::bit_size, (int)pos.y >> chunk::bit_size, (int)pos.z >> chunk::bit_size);         chunk* result = chunks[ipos]; // bug here         return result;     } } 

those 2 errors get:

chunkmap.cpp|12|error: passing 'const std::unordered_map<glm::detail::tvec3<int, (glm::precision)0u>, mctest3::chunk, mctest3::chunkmap::keyhasher>' 'this' argument of '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::tvec3<int, (glm::precision)0u>; _pair = std::pair<const glm::detail::tvec3<int, (glm::precision)0u>, mctest3::chunk>; _hashtable =|

chunkmap.cpp|12|error: cannot convert 'std::__detail::_map_base<glm::detail::tvec3<int, (glm::precision)0u>, std::pair<const glm::detail::tvec3<int, (glm::precision)0u>, mctest3::chunk>, std::_select1st<std::pair<const glm::detail::tvec3<int, (glm::precision)0u>, mctest3::chunk> >, true, std::_hashtable<glm::detail::tvec3<int, (glm::precision)0u>, std::pair<const glm::detail::tvec3<int, (glm::precision)0u>, mctest3::chunk>, std::allocator<std::pair<const glm::detail::tvec3<int, (glm::precision)0u>, mctest3::chunk> >, std:|

problem 1

chunk* result = chunks[ipos]; // bug here 

in above line trying assign chunks[ipos], of type chunk result of type chunk*.

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problem 2

when calling std::unordered_map<...>::operator[], such in chunks[ipos], value @ key (ipos) default-constructed unless there such key present.

in other words such operation might change container on used.

since member-function getchunkfrompos marked being const not allowed modify members of class, , compilers throws diagnostic in face - telling code ill-formed.