Bitset & lt; & gt; default hash function for unordered container, bitsetunordered

Bitset <> default hash function for unordered container, bitsetunordered

Since c ++ 11, bitset is used for unordered container and will provide the default hash function.

In gcc, the related code is as follows:

01495   // DR 1182.01496   /// std::hash specialization for bitset.01497   template<size_t _Nb>01498     struct hash<_GLIBCXX_STD_D::bitset<_Nb>>01499     : public std::unary_function<_GLIBCXX_STD_D::bitset<_Nb>, size_t>01500     {01501       size_t01502       operator()(const _GLIBCXX_STD_D::bitset<_Nb>& __b) const01503       {01504     const size_t __clength = (_Nb + __CHAR_BIT__ - 1) / __CHAR_BIT__;01505     return std::_Fnv_hash::hash(__b._M_getdata(), __clength);01506       }01507     };01508 01509   template<>01510     struct hash<_GLIBCXX_STD_D::bitset<0>>01511     : public std::unary_function<_GLIBCXX_STD_D::bitset<0>, size_t>01512     {01513       size_t01514       operator()(const _GLIBCXX_STD_D::bitset<0>&) const01515       { return 0; }01516     };

In vs2015, the related code is as follows:

// TEMPLATE STRUCT SPECIALIZATION hashtemplate<size_t _Bits>    struct hash<bitset<_Bits> >    {    // hash functor for bitset<_Bits>    typedef bitset<_Bits> argument_type;    typedef size_t result_type;    size_t operator()(const argument_type& _Keyval) const        {    // hash _Keyval to size_t value by pseudorandomizing transform        return (_Keyval.hash());        }    };

I also wrote a small program to test the usage of bitset in unordered container:

#include <bitset>#include <unordered_set>#include <iostream>#include "print.hpp"using namespace std;int main(){    bitset<3> bitset00(0);    bitset<3> bitset01(1);    bitset<3> bitset02(2);    bitset<3> bitset03(3);    bitset<3> bitset04(4);    bitset<3> bitset05(5);    bitset<3> bitset06(6);    bitset<3> bitset07(7);        unordered_set<bitset<3>> coll = {bitset00, bitset01, bitset02, bitset03, bitset04, bitset05, bitset06, bitset07 };    PRINT_ELEMENTS(coll);    return 0;}

Note: print. hpp borrowed The code in The C ++ Standard Library of niclai M. josutis.

The output is as follows:

000 001 010 011 100 101 110 111