c++11 - How to correctly use an unordered_map C++ -

i wanted create std::unordered_map custom hash function. having trouble figuring out declare/how use unordered_map though.

here situation. have class called object. it's super simple , merely contains integer id. here header file:

// object.hpp class object {     public:         object();         ~object(){};         int   id();         void setid(int i);     private:         int id; }; 

i have class called dataset acts container hold millions of these objects. simplicities sake, want able construct dataset, add object dataset, delete object id dataset, clear dataset, , size of dataset.

the structure want (and required to) use in dataset class std::unordered_map. map, want key integer id associated object, , actual object* value. have hash function want use unordered_map. here have in dataset header file:

// dataset.hpp struct hashkey {     unsigned int hash(unsigned int x) {         x = ((x >> 16) ^ x) * 0x45d9f3b;         x = ((x >> 16) ^ x) * 0x45d9f3b;         x = ((x >> 16) ^ x);         return x;     } };  class dataset{  public:     std::unordered_map<int,object*,hashkey> objects;      dataset();     ~dataset();      int addobject(object *object);     void clear();     int deleteobject(int id);     int getsize(); }; 

as of right i'm trying figure out how create addobject in dataset.cpp. here (broken) try:

int dataset::addobject(object *object) {     objects.emplace(object->id(),object);     return 1; } 

when compiled, end error:

type 'const hashkey' not provide call operator         {return static_cast<const _hash&>(*this)(__x.__cc.first);} 

what want able in file called driver.cpp, have loop add millions of objects. this:

dataset container; for(int = 0; < 10000000; ++i) {     object *object = new object();     object->setid(i);     container.addobject(object); } 

is there way make unordered_map can accomplish this? side note, required use current dataset class , current object class is. need make std::unordered_map it.

you need define hash function const call operator (i.e. operator()) takes object of key type , returns size_t:

size_t operator()(int x) const { ... }