Why bool is not regarded as boost::true_type in C++? -

the following codes come example code illustrate how use boost::type_traits. use 2 methods swap 2 variables. easy understand when 2 variables integer (int) type traits correspond true_type. however, when 2 variables bool type not regarded true_type more. why happen? thanks.

#include <iostream> #include <typeinfo> #include <algorithm> #include <iterator> #include <vector> #include <memory>  #include <boost/test/included/prg_exec_monitor.hpp> #include <boost/type_traits.hpp>  using std::cout; using std::endl; using std::cin;  namespace opt{  // // iter_swap: // tests whether iterator proxying iterator or not, , // uses optimal form accordingly: // namespace detail{  template <typename i> static void do_swap(i one, two, const boost::false_type&) {    typedef typename std::iterator_traits<i>::value_type v_t;    v_t v = *one;    *one = *two;    *two = v; } template <typename i> static void do_swap(i one, two, const boost::true_type&) {    using std::swap;    swap(*one, *two); }  }  template <typename i1, typename i2> inline void iter_swap(i1 one, i2 two) {    //    // see both arguments non-proxying iterators,     // , if both iterator same type:    //    typedef typename std::iterator_traits<i1>::reference r1_t;    typedef typename std::iterator_traits<i2>::reference r2_t;     typedef boost::integral_constant<bool,       ::boost::is_reference<r1_t>::value       && ::boost::is_reference<r2_t>::value       && ::boost::is_same<r1_t, r2_t>::value> truth_type;     detail::do_swap(one, two, truth_type()); }   };   // namespace opt  int cpp_main(int argc, char* argv[]) {    //    // testing iter_swap    // check in fact compile...    std::vector<int> v1;    v1.push_back(0);    v1.push_back(1);    std::vector<bool> v2;    v2.push_back(0);    v2.push_back(1);    opt::iter_swap(v1.begin(), v1.begin()+1);    opt::iter_swap(v2.begin(), v2.begin()+1);     return 0; } 

you've got answer there in code (as comment):

see both arguments non-proxying iterators

vector<bool> has proxy iterators, because can't refer directly bit. if vector<bool> stored its' elements individual booleans (taking 1-4 bytes/entry, depending on system), iterators non-proxying. instead, vector<bool> stores 8 entries/byte , uses proxy iterators.