How to find the type of a pair from iterator?

I am newbee here as well as in template programming. I have a dictionary (means, it could be either std::map or std::vector<std::pair<type1, type2>> or std::set<std::pair<, >> ...) I wanted to write an algorithm, which act like standard library algorithms using the iterators of the passed container.

Following is the idea.

#include <iostream>
#include <algorithm>
#include <type_traits>
#include <vector>
#include <array>
#include <map>
#include <set>
#include <iterator>

// two different types
enum EnumA { one, two, three, four, five, six};
enum EnumB { one,      three, four,       six};

//                   TypeA TypeB
using map = std::map<EnumA, EnumB>;
           // or std::vector<std::pair<EnumA, EnumB>>
           // or std::set<std::pair<EnumA, EnumB>>
           // or std::array<std::pair<EnumA, EnumB>, 3>

const  map itemMap{       // the map
   {EnumA::one, EnumB::one}, 
   {EnumA::three, EnumB::three}, 
   {EnumA::six, EnumB::six}, 
};

template<typename Iterator, typename B>
/* type of KEY(first) of the map/container from the iterator???*/ AfromB(Iterator begin, Iterator end, B bObj)
{
   // static_assert(begin != end); // container should not be empty!
   using Type = typename std::iterator_traits<Iterator>::value_type;
   using AType = decltype( /* how to find the type of KEY(first) of the map/container? */);
   using BType = decltype(/* how to find the type of VALUE(second) of the map/container? */);

   auto iter = std::find_if(begin, end, [bObj](const Type& entry) { return entry.second == bObj;});
   return iter != end ? iter->first: begin->first; // if not found return the first element match
}
// will do BfromA(Iterator begin, Iterator end, B bObj) similarly afterwards

int main()
{
   EnumA aEnum = AfromB(itemMap.cbegin(), itemMap.cend(), EnumB::six);  // I can use it like
}

There you can see in the code, I do not know how to find the type of key/ first and value/second of the pair in the dictionary. After googling, I found I can find the type of key-value pair by

using Type = typename std::iterator_traits<Iterator>::value_type;

but not for the individuals of that pair. Is it possible to find? Im using C++11.

Sorry for the bad English. Thanks for the help.

Solution

You already have the maps value type:

using Type = typename std::iterator_traits<Iterator>::value_type;

A maps value type is a std::pair<first_type,second_type> and to get a pairs first and second type you can use its first_type and second_type:

using key_type = typename Type::first_type;
using mapped_type = typename Type::second_type;

To use the key_type as return type I'd probably use a small helper:

template <typename Iterator>
struct KeyAndMappedType {
    using value_type = typename std::iterator_traits<Iterator>::value_type;
    using const_key_type = typename value_type::first_type;
    using key_type = typename std::remove_const<const_key_type>::type;
    using mapped_type = typename value_type::second_type;
};

and then

template <typename Iterator, typename B>
typename KeyAndMappedType<Iterator>::key_type AfromB(Iterator begin, Iterator end, B bObj) {
    ...
}

Note that a maps key_type is always const. As sometimes you need that type also as non-const, I decided that my KeyAndMapType should provide both (maybe naming should have been reversed, ie key_type as const and a non_const_key_type, but I'll leave it up to you to decide on the details).