Question

    bool takeInt(int a,int b) 
    {
      cout << a << " " << b << endl;   return true;
    } 

    int main()
    { 
      map<int,int> m;
      m.insert(make_pair(1,2));
      m.insert(make_pair(2,5));
      m.insert(make_pair(4,8));
      m.insert(make_pair(5,6));
for_each(m.begin(),m.end(),boost::bind(&takeInt,42,boost::bind(&pair<int,int>::first,_1)));
     return 0;
    }

I want an output of

42 1
42 2
42 4
42 5

Fails compilation with loong errors.
Any clue on what I am missing?

Answer 1

The type of map<T, U>::value_type is pair<const T, U>. Use this:

for_each(m.begin(),m.end(),
         boost::bind(&takeInt, 42, boost::bind(&pair<const int,int>::first,_1)));
                                                     ^^^^^