Bitset<> default hash function for unordered container

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

In GCC, the relevant code is as follows:

01495   //DR 1182.01496   ///Std::hash Specialization for Bitset.01497template<size_t _nb>01498     structHash<_glibcxx_std_d::bitset<_nb>>01499: PublicStd::unary_function<_glibcxx_std_d::bitset<_nb>, size_t>01500     {01501size_t01502       operator()(Const_glibcxx_std_d::bitset<_nb>& __b)Const01503       {01504     Constsize_t __clength = (_nb + __char_bit__-1) /__char_bit__;01505     returnStd::_fnv_hash::hash (__b._m_getdata (), __clength);01506       }01507     };01508 01509Template<>01510     structhash<_glibcxx_std_d::bitset<0>>01511: Publicstd::unary_function<_glibcxx_std_d::bitset<0, size_t>01512     {01513size_t01514       operator()(Const_glibcxx_std_d::bitset<0>&)Const01515{return 0; }01516};

In vs2015, the relevant 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;     operator () (constconst        {    //  hash _keyval to size_t value by Pseudorandomizing transform        return  (_keyval.hash ());        }    ;

I wrote a little program myself to test the use of Bitset in unordered container:

#include <bitset>#include<unordered_set>#include<iostream>#include"print.hpp"using namespacestd;intMain () {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: Where PRINT.HPP borrowed the code from the C + + standard Library of Nicolai M. Josuttis.

The output is as follows:

 the 001 010 011  - 101  the 111

Bitset<> default hash function for unordered container