STL: Maps instance with the operator=

Hello,

    I'm trying to understand why the following syntax is not allowed while
learning about maps. I've provided a snippet of the code I'm working
with. My gcc compiler does not like m_pos =m.begin();

m_pos is StringIntMap::iterator;
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#include <iostream>
#include <vector>
#include <string>
#include <map>

using std::vector;
using std::map;
using std::string;
using std::iterator;
using std::cout;

typedef map<string,int> StringIntMap;

vector< map<string,int>::const_iterator > val_count(
        const map<string,int>&m, int val );

int main( void )
{
   StringIntMap myMap;
   vector< StringIntMap::const_iterator > myVec;

   // inserting some elements
   myMap["USA"] = 100;
   myMap["Canada"] = 200;
   myMap["Russia"] = 300;
   myMap["Japan"] = 400;
   myMap["Germany"]= 500;
   myMap["China"] = 600;
   myMap["India"] = 700;
   myMap["Italy"] = 800;

   myVec =val_count( myMap, 500 );

   return 0;
}

vector< map<string,int>::const_iterator > val_count(
                const map<string,int> & m, int val )
{
   vector< map<string,int>::const_iterator >v_pos;
   StringIntMap::iterator m_pos;

   // why can't I do this?
   m_pos =m.begin();

   // will do some other stuff
   return v_pos;
}
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Regards,
Alden