Hi!

After my previous post about unordered_map now I want to explain hash functions.

std::hash.

C++ STL has one hash function in library <functional>. You can use it for this data types:

template<> struct hash<bool>;
template<> struct hash<char>;
template<> struct hash<signed char>;
template<> struct hash<unsigned char>;
template<> struct hash<char16_t>;
template<> struct hash<char32_t>;
template<> struct hash<wchar_t>;
template<> struct hash<short>;
template<> struct hash<unsigned short>;
template<> struct hash<int>;
template<> struct hash<unsigned int>;
template<> struct hash<long>;
template<> struct hash<long long>;
template<> struct hash<unsigned long>;
template<> struct hash<unsigned long long>;
template<> struct hash<float>;
template<> struct hash<double>;
template<> struct hash<long double>;

For example:

hash<int>hi;
int x=69;
cout<<hf(x)<<endl;
hash<long double>hld;
long double y=69.6969696969;
cout<<hld(y)<<endl;
hash<vector<int> >hv;
vector<int>v({69,69,69,69});//v→ 69,69,69,69
cout<<hv(v)<<endl;

Hand-made hash functions.

There are many ways to create hash function for your struct. For example:

struct reval{
  vector<int>v;
  int n;
  string s;
  size_t hash(){//size_t is alias of unsigned int
    hash<string>hs;
    hash<long long>hll;
    hash<int>hi;
    hash<vector>hv;
    long long ans=hs(s);
    ans<<=32;
    ans|=hi(n);
    ans=hll(ans);
    ans<<=32;
    ans|=hv(v);
    ans=hll(ans);
    return ans;
  }
}

A simple example:(be careful;it is only a sample and it isn't good hash function of course)

struct reval{
  vector<int>v;
  int n;
  string s;
  size_t hash(){//size_t is alias of unsigned int
    hash<string>hs;
    hash<int>hi;
    hash<vector>hv;
    return hs(s)+hv(v)+hi(n);
  }
}

A trick:

struct S{
  string first_name;
  string last_name;
};
namespace std{
    template<>struct hash<S>{
        size_t operator()(S const& s) const{
            size_t h1=hash<string>()(s.first_name);
            size_t h1=hash<string>()(s.last_name);
            return hash<long long>()( (h1<<32)^h2 );
        }
    };
}
int main(){
    S s;
    s.first_name="MohammadSina";
    s.last_name="PakSeresht";
    cout <<"hash(s) = "<<hash<S>()(s)<<endl;
}

Note1: You can use hash<type>()(variable), like this:

int x=69;
cout<<hash<int>()(x)<<endl;

Instead of:

int x=69;
hash<int>hasher;
cout<<hasher(x)<<endl;

Note2: please be careful about combining two hashes. There are two good ways:

1-x*P+y mod M(when P is prime number (like 43) and M is less than 2^32 and not a power of 2 (like 10^9+7)).

2-hash<long long>()((x<<32)^y) or hash<long long>()(x*1000000007LL+y).