Question

I am trying to use multimaps in C++. The main aim is that there is a map that stores the multimap pointer.

Now when I try to insert into the multimap I am getting the problem. Can someone tell me where am I going wrong.

typedef multimap<int,int> mm;
typedef map<int,mm*> v_map;

int main()
{
v_map map1;

v_map::iterator it;
it = map1.find(23);

mm *mm_map_pointer;

if( it == map1.end())
  {
   mm m_map1; 
   map1[23] = &m_map1;
   mm_map_pointer = &m_map1;
  }
else
 {
   mm_map_pointer = it->second; 
 }

mm_map_pointer->insert( pair<int, int>(1, 2));
return 0;
}

The problem is with mm_map_pointer->insert( pair(1, 2));
Can someone help?

Answer 1

Your m_map1 is a local variable, and you’re trying to store a pointer to that in your map. This results in a dangling pointer when the block containing that variable exits.

You have two ways to solve this:

• My preferred way is to store the multimap in the map directly (i.e., not using a pointer). As a bonus, this simplifies your code a lot:

  typedef multimap<int, int> mm;
  typedef map<int, mm> v_map;
  
  v_map map1;
  map1[23].insert(make_pair(1, 2));
  

  That’s it!

• Your other option is to use new to create a persistent copy of the multimap. In this case, your map should hold a shared_ptr<multimap<...> > so that you don’t have to deallocate the multimap manually.