怎么删除hash_map 中的KEY

 
 // hash_map_erase.cpp 
 // compile with: /EHsc 
 #define _DEFINE_DEPRECATED_HASH_CLASSES 0 
 #include  <hash_map> 
 #include  <iostream> 

 int main() 
 { 
   using namespace std; 
   using namespace stdext; 
   hash_map <int, int> hm1, hm2, hm3; 
   hash_map <int, int> :: iterator pIter, Iter1, Iter2; 
   int i; 
   hash_map <int, int>::size_type n; 
   typedef pair <int, int> Int_Pair; 

   for (i = 1; i  < 5; i++) 
   { 
     hm1.insert(Int_Pair (i, i)); 
     hm2.insert(Int_Pair (i, i*i)); 
     hm3.insert(Int_Pair (i, i-1)); 
   } 

   // The 1st member function removes an element at a given position 
   Iter1 = ++hm1.begin(); 
   hm1.erase(Iter1); 

   cout  < < "After the 2nd element is deleted, the hash_map hm1 is:"; 
   for (pIter = hm1.begin(); pIter != hm1.end(); pIter++) 
     cout  < < " "  < < pIter -> second; 
   cout  < < "."  < < endl; 

   // The 2nd member function removes elements 
   // in the range [_First, _Last) 
   Iter1 = ++hm2.begin(); 
   Iter2 = --hm2.end(); 
   hm2.erase(Iter1, Iter2); 

   cout  < < "After the middle two elements are deleted, " 
       < < "the hash_map hm2 is:"; 
   for (pIter = hm2.begin(); pIter != hm2.end(); pIter++) 
     cout  < < " "  < < pIter -> second; 
   cout  < < "."  < < endl; 

   // The 3rd member function removes elements with a given _Key 
   n = hm3.erase(2); 

   cout  < < "After the element with a key of 2 is deleted,\n" 
       < < "the hash_map hm3 is:"; 
   for (pIter = hm3.begin(); pIter != hm3.end(); pIter++) 
     cout  < < " "  < < pIter -> second; 
   cout  < < "."  < < endl; 

   // The 3rd member function returns the number of elements removed 
   cout  < < "The number of elements removed from hm3 is: " 
       < < n  < < "."  < < endl; 

   // The dereferenced iterator can also be used to specify a key 
   Iter1 = ++hm3.begin(); 
   hm3.erase(Iter1); 

   cout  < < "After another element with a key equal to that" 
       < < endl; 
   cout   < < "of the 2nd element is deleted, " 
      < < "the hash_map hm3 is:"; 
   for (pIter = hm3.begin(); pIter != hm3.end(); pIter++) 
     cout  < < " "  < < pIter -> second; 
   cout  < < "."  < < endl; 
 }