Question

I’m trying to write a custom cmp function for a map, a simple one that does the comparison over the second element of the map. I’d like to have the function as a template, but I can’t figure out how to pass the map’s .first and .second types along to my cmp function. My non-working code is below, which will clearly fail since the type of T1 and T2 are not passed along:

#include <map>
#include <vector>
#include <algorithm>

template<class T1, class T2>
bool pairCompare(const std::pair<T1,T2> & x,
                 const std::pair<T1,T2> & y) {
  return x.second < y.second; 
}

template<class T1>
typename T1::iterator map_max_element(const T1 & A) {

  // How do I pass the type to pairCompare?
  return std::max_element(A.begin(), A.end(), pairCompare<?????>);
}

int main() {
  std::map<std::vector<double>, int> A;
  map_max_element(A);

  return 0;
}

Answer 1

std::map has a nested type called value_type which is actually a typedef of std::pair<const K, V>. And std::pair has two nested types first_type and second_type. Use this info as:

template<class T>
typename T::const_iterator map_max_element(const T & A) 
{
   typedef typename T::value_type pair_type;
   typedef typename pair_type::first_type K;
   typedef typename pair_type::second_type V;
   return std::max_element(A.begin(), A.end(), pairCompare<K,V>);
}

Note that in your code the return type is wrong. It should be const_iterator, rather than iterator, because in the function A is const map. Hence what you can get from it is const_iterator. 🙂

---

Or you could simply write the compare function as:

template<class T>
bool pairCompare(const T & x, const T & y) {
  return x.second < y.second; 
}

And use it as:

return std::max_element(A.begin(), A.end(), pairCompare<typename T::value_type>);