c++ - 无序地图的 std::distance 出错

Question

std::distacne此代码段有什么问题？

据我所知，对于std::mapor std::vector，可以调用std::distance. std::distance但是对于这个代码片段，如果被调用，程序就会崩溃。

// constructing unordered_multisets
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
std::unordered_multiset<std::string> sixth ( {"red","green","blue"} ); // range


auto itr = sixth.begin();
itr++;


int dis = std::distance(itr, sixth.begin()); //What's wrong?!
std::cout << ">>"<< dis << std::endl;

std::cout << "game over" << std::endl;

return 0;